diff options
Diffstat (limited to 'lib')
| -rw-r--r-- | lib/ntdb/doc/TDB_porting.txt | 408 | ||||
| -rw-r--r-- | lib/ntdb/doc/design-1.3.txt | 1049 | ||||
| -rw-r--r-- | lib/ntdb/doc/design.lyx | 72 | ||||
| -rw-r--r-- | lib/ntdb/doc/design.lyx,v | 4679 | ||||
| -rw-r--r-- | lib/ntdb/doc/design.pdf | bin | 240440 -> 191943 bytes | |||
| -rw-r--r-- | lib/ntdb/doc/design.txt | 118 |
6 files changed, 505 insertions, 5821 deletions
diff --git a/lib/ntdb/doc/TDB_porting.txt b/lib/ntdb/doc/TDB_porting.txt index 34e536ffcb..8b0ca2fec8 100644 --- a/lib/ntdb/doc/TDB_porting.txt +++ b/lib/ntdb/doc/TDB_porting.txt @@ -6,59 +6,421 @@ Interface differences between TDB and NTDB. otherwise you'll get a compile error when tdb.h re-defined struct TDB_DATA. + Example: + #include <tdb.h> + #include <ntdb.h> + - ntdb functions return NTDB_SUCCESS (ie 0) on success, and a negative error on failure, whereas tdb functions returned 0 on success, and -1 on failure. tdb then used tdb_error() to determine the error; this API is nasty if we ever want to support threads, so is not supported. + Example: + #include <tdb.h> + #include <ntdb.h> + + void tdb_example(struct tdb_context *tdb, TDB_DATA key, TDB_DATA d) + { + if (tdb_store(tdb, key, d) == -1) { + printf("store failed: %s\n", tdb_errorstr(tdb)); + } + } + + void ntdb_example(struct ntdb_context *ntdb, NTDB_DATA key, NTDB_DATA d) + { + enum NTDB_ERROR e; + + e = ntdb_store(ntdb, key, d); + if (e) { + printf("store failed: %s\n", ntdb_errorstr(e)); + } + } + - ntdb's ntdb_fetch() returns an error, tdb's returned the data directly (or tdb_null, and you were supposed to check tdb_error() to find out why). + Example: + #include <tdb.h> + #include <ntdb.h> + + void tdb_example(struct tdb_context *tdb, TDB_DATA key) + { + TDB_DATA data; + + data = tdb_fetch(tdb, key); + if (!data.dptr) { + printf("fetch failed: %s\n", tdb_errorstr(tdb)); + } + } + + void ntdb_example(struct ntdb_context *ntdb, NTDB_DATA key) + { + NTDB_DATA data; + enum NTDB_ERROR e; + + e = ntdb_fetch(ntdb, key, &data); + if (e) { + printf("fetch failed: %s\n", ntdb_errorstr(e)); + } + } + - ntdb's ntdb_nextkey() frees the old key's dptr, in tdb you needed to do this manually. -- tdb's tdb_open/tdb_open_ex took an explicit hash size. ntdb's hash table - resizes as required. + Example: + #include <tdb.h> + #include <ntdb.h> + + void tdb_example(struct tdb_context *tdb) + { + TDB_DATA key, next, data; + + for (key = tdb_firstkey(tdb); key.dptr; key = next) { + printf("Got key!\n"); + next = tdb_nextkey(tdb, key); + free(key.dptr); + } + } + + + void ntdb_example(struct ntdb_context *ntdb) + { + NTDB_DATA k, data; + enum NTDB_ERROR e; + + for (e = ntdb_firstkey(ntdb,&k); !e; e = ntdb_nextkey(ntdb,&k)) + printf("Got key!\n"); + } + +- Unlike tdb_open/tdb_open_ex, ntdb_open does not allow NULL names, + even for NTDB_INTERNAL dbs, and thus ntdb_name() never returns NULL. + + Example: + #include <tdb.h> + #include <ntdb.h> + + struct tdb_context *tdb_example(void) + { + return tdb_open(NULL, 0, TDB_INTERNAL, O_RDWR, 0); + } + + struct ntdb_context *ntdb_example(void) + { + return ntdb_open("example", NTDB_INTERNAL, O_RDWR, 0); + } - ntdb uses a linked list of attribute structures to implement logging and alternate hashes. tdb used tdb_open_ex, which was not extensible. + Example: + #include <tdb.h> + #include <ntdb.h> + + /* Custom hash function */ + static unsigned int my_tdb_hash_func(TDB_DATA *key) + { + return key->dsize; + } + + struct tdb_context *tdb_example(void) + { + return tdb_open_ex("example.tdb", 0, TDB_DEFAULT, + O_CREAT|O_RDWR, 0600, NULL, my_hash_func); + } + + /* Custom hash function */ + static unsigned int my_ntdb_hash_func(const void *key, size_t len, + uint32_t seed, void *data) + { + return len; + } + + struct ntdb_context *ntdb_example(void) + { + union ntdb_attribute hash; + + hash.base.attr = NTDB_ATTRIBUTE_HASH; + hash.base.next = NULL; + hash.hash.fn = my_ntdb_hash_func; + return ntdb_open("example.ntdb", NTDB_DEFAULT, + O_CREAT|O_RDWR, 0600, &hash); + } + +- tdb's tdb_open/tdb_open_ex took an explicit hash size, defaulting to + 131. ntdb's uses an attribute for this, defaulting to 8192. + + Example: + #include <tdb.h> + #include <ntdb.h> + + struct tdb_context *tdb_example(void) + { + return tdb_open("example.tdb", 10007, TDB_DEFAULT, + O_CREAT|O_RDWR, 0600); + } + + struct ntdb_context *ntdb_example(void) + { + union ntdb_attribute hashsize; + + hashsize.base.attr = NTDB_ATTRIBUTE_HASHSIZE; + hashsize.base.next = NULL; + hashsize.hashsize.size = 16384; + return ntdb_open("example.ntdb", NTDB_DEFAULT, + O_CREAT|O_RDWR, 0600, &hashsize); + } + - ntdb does locking on read-only databases (ie. O_RDONLY passed to ntdb_open). tdb did not: use the NTDB_NOLOCK flag if you want to suppress locking. -- ntdb's log function is simpler than tdb's log function. The string is - already formatted, and it takes an enum ntdb_log_level not a tdb_debug_level, - and which has only three values: NTDB_LOG_ERROR, NTDB_LOG_USE_ERROR and - NTDB_LOG_WARNING. + Example: + #include <tdb.h> + #include <ntdb.h> + + struct tdb_context *tdb_example(void) + { + return tdb_open("example.tdb", 0, TDB_DEFAULT, O_RDONLY, 0); + } + + struct ntdb_context *ntdb_example(void) + { + return ntdb_open("example.ntdb", NTDB_NOLOCK, O_RDONLY, NULL); + } + +- ntdb's log function is simpler than tdb's log function. The string + is already formatted, is not terminated by a '\n', and it takes an + enum ntdb_log_level not a tdb_debug_level, and which has only three + values: NTDB_LOG_ERROR, NTDB_LOG_USE_ERROR and NTDB_LOG_WARNING. + + #include <tdb.h> + #include <ntdb.h> + + static void tdb_log(struct tdb_context *tdb, + enum tdb_debug_level level, const char *fmt, ...) + { + va_list ap; + const char *name; + + switch (level) { + case TDB_DEBUG_FATAL: + fprintf(stderr, "FATAL: "); + break; + case TDB_DEBUG_ERROR: + fprintf(stderr, "ERROR: "); + break; + case TDB_DEBUG_WARNING: + fprintf(stderr, "WARNING: "); + break; + case TDB_DEBUG_TRACE: + /* Don't print out tracing. */ + return; + } + + name = tdb_name(tdb); + if (!name) { + name = "unnamed"; + } + + fprintf(stderr, "tdb(%s):", name); + + va_start(ap, fmt); + vfprintf(stderr, fmt, ap); + va_end(ap); + } + + struct tdb_context *tdb_example(void) + { + struct tdb_logging_context lctx; + + lctx.log_fn = tdb_log; + return tdb_open_ex("example.tdb", 0, TDB_DEFAULT, + O_CREAT|O_RDWR, 0600, &lctx, NULL); + } + + static void ntdb_log(struct ntdb_context *ntdb, + enum ntdb_log_level level, + enum NTDB_ERROR ecode, + const char *message, + void *data) + { + switch (level) { + case NTDB_LOG_ERROR: + fprintf(stderr, "ERROR: "); + break; + case NTDB_LOG_USE_ERROR: + /* We made a mistake, so abort. */ + abort(); + break; + case NTDB_LOG_WARNING: + fprintf(stderr, "WARNING: "); + break; + } + + fprintf(stderr, "ntdb(%s):%s:%s\n", + ntdb_name(ntdb), ntdb_errorstr(ecode), message); + } + + struct ntdb_context *ntdb_example(void) + { + union ntdb_attribute log; + + log.base.attr = NTDB_ATTRIBUTE_LOG; + log.base.next = NULL; + log.log.fn = ntdb_log; + return ntdb_open("example.ntdb", NTDB_DEFAULT, + O_CREAT|O_RDWR, 0600, &log); + } - ntdb provides ntdb_deq() for comparing two NTDB_DATA, and ntdb_mkdata() for creating an NTDB_DATA. -- ntdb's ntdb_name() returns a copy of the name even for NTDB_INTERNAL dbs. + #include <tdb.h> + #include <ntdb.h> -- ntdb does not need tdb_reopen() or tdb_reopen_all(). If you call - fork() after during certain operations the child should close the - tdb, or complete the operations before continuing to use the tdb: + void tdb_example(struct tdb_context *tdb) + { + TDB_DATA data, key; - ntdb_transaction_start(): child must ntdb_transaction_cancel() - ntdb_lockall(): child must call ntdb_unlockall() - ntdb_lockall_read(): child must call ntdb_unlockall_read() - ntdb_chainlock(): child must call ntdb_chainunlock() - ntdb_parse() callback: child must return from ntdb_parse() + key.dsize = strlen("hello"); + key.dptr = "hello"; + data = tdb_fetch(tdb, key); + if (data.dsize == key.dsize + && !memcmp(data.dptr, key.dptr, key.dsize)) + printf("key is same as data\n"); + } + free(data.dptr); + } -- ntdb will not open a non-tdb file, even if O_CREAT is specified. + void ntdb_example(struct ntdb_context *ntdb) + { + NTDB_DATA data, key; -- There is no ntdb_traverse_read. For operating on TDB files, you can - simulate it by ntdb_add_flag(tdb, NTDB_RDONLY); ntdb_traverse(); - ntdb_remove_flag(tdb, NTDB_RDONLY). This may be desirable because - traverse on TDB files use a write lock on the entire database - unless it's read-only. + key = ntdb_mkdata("hello", strlen("hello")); + if (ntdb_fetch(ntdb, key, &data) == NTDB_SUCCESS) { + if (ntdb_deq(key, data)) { + printf("key is same as data\n"); + } + free(data.dptr); + } + } - Failure inside a transaction (such as a lock function failing) does not implicitly cancel the transaction; you still need to call ntdb_transaction_cancel(). + #include <tdb.h> + #include <ntdb.h> + + void tdb_example(struct tdb_context *tdb, TDB_DATA key, TDB_DATA d) + { + if (tdb_transaction_start(tdb) == -1) { + printf("transaction failed: %s\n", tdb_errorstr(tdb)); + return; + } + + if (tdb_store(tdb, key, d) == -1) { + printf("store failed: %s\n", tdb_errorstr(tdb)); + return; + } + if (tdb_transaction_commit(tdb) == -1) { + printf("commit failed: %s\n", tdb_errorstr(tdb)); + } + } + + void ntdb_example(struct ntdb_context *ntdb, NTDB_DATA key, NTDB_DATA d) + { + enum NTDB_ERROR e; + + e = ntdb_transaction_start(ntdb); + if (e) { + printf("transaction failed: %s\n", ntdb_errorstr(e)); + return; + } + + e = ntdb_store(ntdb, key, d); + if (e) { + printf("store failed: %s\n", ntdb_errorstr(e)); + ntdb_transaction_cancel(ntdb); + } + + e = ntdb_transaction_commit(ntdb); + if (e) { + printf("commit failed: %s\n", ntdb_errorstr(e)); + } + } + - There is no NTDB_CLEAR_IF_FIRST flag; it has severe scalability and API problems. If necessary, you can emulate this by using the open - hook and placing a 1-byte lock at offset 4. If your program forks, - you will need to place this lock again in the child. + hook and placing a 1-byte lock at offset 4. If your program forks + and exits, you will need to place this lock again in the child before + the parent exits. + + Example: + + #include <tdb.h> + #include <ntdb.h> + + struct tdb_context *tdb_example(void) + { + return tdb_open("example.tdb", 0, TDB_CLEAR_IF_FIRST, + O_CREAT|O_RDWR, 0600); + } + + static enum NTDB_ERROR clear_if_first(int fd, void *unused) + { + /* We hold a lock offset 4 always, so we can tell if + * anyone else is. */ + struct flock fl; + + fl.l_type = F_WRLCK; + fl.l_whence = SEEK_SET; + fl.l_start = 4; /* ACTIVE_LOCK */ + fl.l_len = 1; + + if (fcntl(fd, F_SETLK, &fl) == 0) { + /* We must be first ones to open it! Clear it. */ + if (ftruncate(fd, 0) != 0) { + return NTDB_ERR_IO; + } + } + fl.l_type = F_RDLCK; + if (fcntl(fd, F_SETLKW, &fl) != 0) { + return NTDB_ERR_IO; + } + return NTDB_SUCCESS; + } + + struct ntdb_context *ntdb_example(void) + { + union ntdb_attribute open_attr; + + open_attr.openhook.base.attr = NTDB_ATTRIBUTE_OPENHOOK; + open_attr.openhook.base.next = NULL; + open_attr.openhook.fn = clear_if_first; + + return ntdb_open("example.ntdb", NTDB_DEFAULT, + O_CREAT|O_RDWR, 0600, &open_attr); + } + +- ntdb traversals are not reliable if the database is changed during + the traversal, ie your traversal may not cover all elements, or may + cover elements multiple times. As a special exception, deleting the + current record within ntdb_traverse() is reliable. + +- There is no ntdb_traverse_read, since ntdb_traverse does not hold + a lock across the entire traversal anyway. If you want to make sure + that your traversal function does not write to the database, you can + set and clear the NTDB_RDONLY flag around the traversal. + +- ntdb does not need tdb_reopen() or tdb_reopen_all(). If you call + fork() after during certain operations the child should close the + ntdb, or complete the operations before continuing to use the tdb: + + ntdb_transaction_start(): child must ntdb_transaction_cancel() + ntdb_lockall(): child must call ntdb_unlockall() + ntdb_lockall_read(): child must call ntdb_unlockall_read() + ntdb_chainlock(): child must call ntdb_chainunlock() + ntdb_parse() callback: child must return from ntdb_parse() + +- ntdb will not open a non-ntdb file, even if O_CREAT is specified. tdb + will overwrite an unknown file in that case. diff --git a/lib/ntdb/doc/design-1.3.txt b/lib/ntdb/doc/design-1.3.txt deleted file mode 100644 index f81ecf7885..0000000000 --- a/lib/ntdb/doc/design-1.3.txt +++ /dev/null @@ -1,1049 +0,0 @@ -TDB2: A Redesigning The Trivial DataBase - -Rusty Russell, IBM Corporation - -27-April-2010 - -Abstract - -The Trivial DataBase on-disk format is 32 bits; with usage cases -heading towards the 4G limit, that must change. This required -breakage provides an opportunity to revisit TDB's other design -decisions and reassess them. - -1 Introduction - -The Trivial DataBase was originally written by Andrew Tridgell as -a simple key/data pair storage system with the same API as dbm, -but allowing multiple readers and writers while being small -enough (< 1000 lines of C) to include in SAMBA. The simple design -created in 1999 has proven surprisingly robust and performant, -used in Samba versions 3 and 4 as well as numerous other -projects. Its useful life was greatly increased by the -(backwards-compatible!) addition of transaction support in 2005. - -The wider variety and greater demands of TDB-using code has lead -to some organic growth of the API, as well as some compromises on -the implementation. None of these, by themselves, are seen as -show-stoppers, but the cumulative effect is to a loss of elegance -over the initial, simple TDB implementation. Here is a table of -the approximate number of lines of implementation code and number -of API functions at the end of each year: - - -+-----------+----------------+--------------------------------+ -| Year End | API Functions | Lines of C Code Implementation | -+-----------+----------------+--------------------------------+ -+-----------+----------------+--------------------------------+ -| 1999 | 13 | 1195 | -+-----------+----------------+--------------------------------+ -| 2000 | 24 | 1725 | -+-----------+----------------+--------------------------------+ -| 2001 | 32 | 2228 | -+-----------+----------------+--------------------------------+ -| 2002 | 35 | 2481 | -+-----------+----------------+--------------------------------+ -| 2003 | 35 | 2552 | -+-----------+----------------+--------------------------------+ -| 2004 | 40 | 2584 | -+-----------+----------------+--------------------------------+ -| 2005 | 38 | 2647 | -+-----------+----------------+--------------------------------+ -| 2006 | 52 | 3754 | -+-----------+----------------+--------------------------------+ -| 2007 | 66 | 4398 | -+-----------+----------------+--------------------------------+ -| 2008 | 71 | 4768 | -+-----------+----------------+--------------------------------+ -| 2009 | 73 | 5715 | -+-----------+----------------+--------------------------------+ - - -This review is an attempt to catalog and address all the known -issues with TDB and create solutions which address the problems -without significantly increasing complexity; all involved are far -too aware of the dangers of second system syndrome in rewriting a -successful project like this. - -2 API Issues - -2.1 tdb_open_ex Is Not Expandable - -The tdb_open() call was expanded to tdb_open_ex(), which added an -optional hashing function and an optional logging function -argument. Additional arguments to open would require the -introduction of a tdb_open_ex2 call etc. - -2.1.1 Proposed Solution - -tdb_open() will take a linked-list of attributes: - -enum tdb_attribute { - - TDB_ATTRIBUTE_LOG = 0, - - TDB_ATTRIBUTE_HASH = 1 - -}; - -struct tdb_attribute_base { - - enum tdb_attribute attr; - - union tdb_attribute *next; - -}; - -struct tdb_attribute_log { - - struct tdb_attribute_base base; /* .attr = TDB_ATTRIBUTE_LOG -*/ - - tdb_log_func log_fn; - - void *log_private; - -}; - -struct tdb_attribute_hash { - - struct tdb_attribute_base base; /* .attr = TDB_ATTRIBUTE_HASH -*/ - - tdb_hash_func hash_fn; - - void *hash_private; - -}; - -union tdb_attribute { - - struct tdb_attribute_base base; - - struct tdb_attribute_log log; - - struct tdb_attribute_hash hash; - -}; - -This allows future attributes to be added, even if this expands -the size of the union. - -2.2 tdb_traverse Makes Impossible Guarantees - -tdb_traverse (and tdb_firstkey/tdb_nextkey) predate transactions, -and it was thought that it was important to guarantee that all -records which exist at the start and end of the traversal would -be included, and no record would be included twice. - -This adds complexity (see[Reliable-Traversal-Adds]) and does not -work anyway for records which are altered (in particular, those -which are expanded may be effectively deleted and re-added behind -the traversal). - -2.2.1 <traverse-Proposed-Solution>Proposed Solution - -Abandon the guarantee. You will see every record if no changes -occur during your traversal, otherwise you will see some subset. -You can prevent changes by using a transaction or the locking -API. - -2.3 Nesting of Transactions Is Fraught - -TDB has alternated between allowing nested transactions and not -allowing them. Various paths in the Samba codebase assume that -transactions will nest, and in a sense they can: the operation is -only committed to disk when the outer transaction is committed. -There are two problems, however: - -1. Canceling the inner transaction will cause the outer - transaction commit to fail, and will not undo any operations - since the inner transaction began. This problem is soluble with - some additional internal code. - -2. An inner transaction commit can be cancelled by the outer - transaction. This is desirable in the way which Samba's - database initialization code uses transactions, but could be a - surprise to any users expecting a successful transaction commit - to expose changes to others. - -The current solution is to specify the behavior at tdb_open(), -with the default currently that nested transactions are allowed. -This flag can also be changed at runtime. - -2.3.1 Proposed Solution - -Given the usage patterns, it seems that the “least-surprise” -behavior of disallowing nested transactions should become the -default. Additionally, it seems the outer transaction is the only -code which knows whether inner transactions should be allowed, so -a flag to indicate this could be added to tdb_transaction_start. -However, this behavior can be simulated with a wrapper which uses -tdb_add_flags() and tdb_remove_flags(), so the API should not be -expanded for this relatively-obscure case. - -2.4 Incorrect Hash Function is Not Detected - -tdb_open_ex() allows the calling code to specify a different hash -function to use, but does not check that all other processes -accessing this tdb are using the same hash function. The result -is that records are missing from tdb_fetch(). - -2.4.1 Proposed Solution - -The header should contain an example hash result (eg. the hash of -0xdeadbeef), and tdb_open_ex() should check that the given hash -function produces the same answer, or fail the tdb_open call. - -2.5 tdb_set_max_dead/TDB_VOLATILE Expose Implementation - -In response to scalability issues with the free list ([TDB-Freelist-Is] -) two API workarounds have been incorporated in TDB: -tdb_set_max_dead() and the TDB_VOLATILE flag to tdb_open. The -latter actually calls the former with an argument of “5”. - -This code allows deleted records to accumulate without putting -them in the free list. On delete we iterate through each chain -and free them in a batch if there are more than max_dead entries. -These are never otherwise recycled except as a side-effect of a -tdb_repack. - -2.5.1 Proposed Solution - -With the scalability problems of the freelist solved, this API -can be removed. The TDB_VOLATILE flag may still be useful as a -hint that store and delete of records will be at least as common -as fetch in order to allow some internal tuning, but initially -will become a no-op. - -2.6 <TDB-Files-Cannot>TDB Files Cannot Be Opened Multiple Times - In The Same Process - -No process can open the same TDB twice; we check and disallow it. -This is an unfortunate side-effect of fcntl locks, which operate -on a per-file rather than per-file-descriptor basis, and do not -nest. Thus, closing any file descriptor on a file clears all the -locks obtained by this process, even if they were placed using a -different file descriptor! - -Note that even if this were solved, deadlock could occur if -operations were nested: this is a more manageable programming -error in most cases. - -2.6.1 Proposed Solution - -We could lobby POSIX to fix the perverse rules, or at least lobby -Linux to violate them so that the most common implementation does -not have this restriction. This would be a generally good idea -for other fcntl lock users. - -Samba uses a wrapper which hands out the same tdb_context to -multiple callers if this happens, and does simple reference -counting. We should do this inside the tdb library, which already -emulates lock nesting internally; it would need to recognize when -deadlock occurs within a single process. This would create a new -failure mode for tdb operations (while we currently handle -locking failures, they are impossible in normal use and a process -encountering them can do little but give up). - -I do not see benefit in an additional tdb_open flag to indicate -whether re-opening is allowed, as though there may be some -benefit to adding a call to detect when a tdb_context is shared, -to allow other to create such an API. - -2.7 TDB API Is Not POSIX Thread-safe - -The TDB API uses an error code which can be queried after an -operation to determine what went wrong. This programming model -does not work with threads, unless specific additional guarantees -are given by the implementation. In addition, even -otherwise-independent threads cannot open the same TDB (as in [TDB-Files-Cannot] -). - -2.7.1 Proposed Solution - -Reachitecting the API to include a tdb_errcode pointer would be a -great deal of churn; we are better to guarantee that the -tdb_errcode is per-thread so the current programming model can be -maintained. - -This requires dynamic per-thread allocations, which is awkward -with POSIX threads (pthread_key_create space is limited and we -cannot simply allocate a key for every TDB). - -Internal locking is required to make sure that fcntl locks do not -overlap between threads, and also that the global list of tdbs is -maintained. - -The aim is that building tdb with -DTDB_PTHREAD will result in a -pthread-safe version of the library, and otherwise no overhead -will exist. - -2.8 *_nonblock Functions And *_mark Functions Expose - Implementation - -CTDB[footnote: -Clustered TDB, see http://ctdb.samba.org -] wishes to operate on TDB in a non-blocking manner. This is -currently done as follows: - -1. Call the _nonblock variant of an API function (eg. - tdb_lockall_nonblock). If this fails: - -2. Fork a child process, and wait for it to call the normal - variant (eg. tdb_lockall). - -3. If the child succeeds, call the _mark variant to indicate we - already have the locks (eg. tdb_lockall_mark). - -4. Upon completion, tell the child to release the locks (eg. - tdb_unlockall). - -5. Indicate to tdb that it should consider the locks removed (eg. - tdb_unlockall_mark). - -There are several issues with this approach. Firstly, adding two -new variants of each function clutters the API for an obscure -use, and so not all functions have three variants. Secondly, it -assumes that all paths of the functions ask for the same locks, -otherwise the parent process will have to get a lock which the -child doesn't have under some circumstances. I don't believe this -is currently the case, but it constrains the implementation. - -2.8.1 <Proposed-Solution-locking-hook>Proposed Solution - -Implement a hook for locking methods, so that the caller can -control the calls to create and remove fcntl locks. In this -scenario, ctdbd would operate as follows: - -1. Call the normal API function, eg tdb_lockall(). - -2. When the lock callback comes in, check if the child has the - lock. Initially, this is always false. If so, return 0. - Otherwise, try to obtain it in non-blocking mode. If that - fails, return EWOULDBLOCK. - -3. Release locks in the unlock callback as normal. - -4. If tdb_lockall() fails, see if we recorded a lock failure; if - so, call the child to repeat the operation. - -5. The child records what locks it obtains, and returns that - information to the parent. - -6. When the child has succeeded, goto 1. - -This is flexible enough to handle any potential locking scenario, -even when lock requirements change. It can be optimized so that -the parent does not release locks, just tells the child which -locks it doesn't need to obtain. - -It also keeps the complexity out of the API, and in ctdbd where -it is needed. - -2.9 tdb_chainlock Functions Expose Implementation - -tdb_chainlock locks some number of records, including the record -indicated by the given key. This gave atomicity guarantees; -no-one can start a transaction, alter, read or delete that key -while the lock is held. - -It also makes the same guarantee for any other key in the chain, -which is an internal implementation detail and potentially a -cause for deadlock. - -2.9.1 Proposed Solution - -None. It would be nice to have an explicit single entry lock -which effected no other keys. Unfortunately, this won't work for -an entry which doesn't exist. Thus while chainlock may be -implemented more efficiently for the existing case, it will still -have overlap issues with the non-existing case. So it is best to -keep the current (lack of) guarantee about which records will be -effected to avoid constraining our implementation. - -2.10 Signal Handling is Not Race-Free - -The tdb_setalarm_sigptr() call allows the caller's signal handler -to indicate that the tdb locking code should return with a -failure, rather than trying again when a signal is received (and -errno == EAGAIN). This is usually used to implement timeouts. - -Unfortunately, this does not work in the case where the signal is -received before the tdb code enters the fcntl() call to place the -lock: the code will sleep within the fcntl() code, unaware that -the signal wants it to exit. In the case of long timeouts, this -does not happen in practice. - -2.10.1 Proposed Solution - -The locking hooks proposed in[Proposed-Solution-locking-hook] -would allow the user to decide on whether to fail the lock -acquisition on a signal. This allows the caller to choose their -own compromise: they could narrow the race by checking -immediately before the fcntl call.[footnote: -It may be possible to make this race-free in some implementations -by having the signal handler alter the struct flock to make it -invalid. This will cause the fcntl() lock call to fail with -EINVAL if the signal occurs before the kernel is entered, -otherwise EAGAIN. -] - -2.11 The API Uses Gratuitous Typedefs, Capitals - -typedefs are useful for providing source compatibility when types -can differ across implementations, or arguably in the case of -function pointer definitions which are hard for humans to parse. -Otherwise it is simply obfuscation and pollutes the namespace. - -Capitalization is usually reserved for compile-time constants and -macros. - - TDB_CONTEXT There is no reason to use this over 'struct - tdb_context'; the definition isn't visible to the API user - anyway. - - TDB_DATA There is no reason to use this over struct TDB_DATA; - the struct needs to be understood by the API user. - - struct TDB_DATA This would normally be called 'struct - tdb_data'. - - enum TDB_ERROR Similarly, this would normally be enum - tdb_error. - -2.11.1 Proposed Solution - -None. Introducing lower case variants would please pedants like -myself, but if it were done the existing ones should be kept. -There is little point forcing a purely cosmetic change upon tdb -users. - -2.12 <tdb_log_func-Doesnt-Take>tdb_log_func Doesn't Take The - Private Pointer - -For API compatibility reasons, the logging function needs to call -tdb_get_logging_private() to retrieve the pointer registered by -the tdb_open_ex for logging. - -2.12.1 Proposed Solution - -It should simply take an extra argument, since we are prepared to -break the API/ABI. - -2.13 Various Callback Functions Are Not Typesafe - -The callback functions in tdb_set_logging_function (after [tdb_log_func-Doesnt-Take] - is resolved), tdb_parse_record, tdb_traverse, tdb_traverse_read -and tdb_check all take void * and must internally convert it to -the argument type they were expecting. - -If this type changes, the compiler will not produce warnings on -the callers, since it only sees void *. - -2.13.1 Proposed Solution - -With careful use of macros, we can create callback functions -which give a warning when used on gcc and the types of the -callback and its private argument differ. Unsupported compilers -will not give a warning, which is no worse than now. In addition, -the callbacks become clearer, as they need not use void * for -their parameter. - -See CCAN's typesafe_cb module at -http://ccan.ozlabs.org/info/typesafe_cb.html - -2.14 TDB_CLEAR_IF_FIRST Must Be Specified On All Opens, - tdb_reopen_all Problematic - -The TDB_CLEAR_IF_FIRST flag to tdb_open indicates that the TDB -file should be cleared if the caller discovers it is the only -process with the TDB open. However, if any caller does not -specify TDB_CLEAR_IF_FIRST it will not be detected, so will have -the TDB erased underneath them (usually resulting in a crash). - -There is a similar issue on fork(); if the parent exits (or -otherwise closes the tdb) before the child calls tdb_reopen_all() -to establish the lock used to indicate the TDB is opened by -someone, a TDB_CLEAR_IF_FIRST opener at that moment will believe -it alone has opened the TDB and will erase it. - -2.14.1 Proposed Solution - -Remove TDB_CLEAR_IF_FIRST. Other workarounds are possible, but -see [TDB_CLEAR_IF_FIRST-Imposes-Performance]. - -3 Performance And Scalability Issues - -3.1 <TDB_CLEAR_IF_FIRST-Imposes-Performance>TDB_CLEAR_IF_FIRST - Imposes Performance Penalty - -When TDB_CLEAR_IF_FIRST is specified, a 1-byte read lock is -placed at offset 4 (aka. the ACTIVE_LOCK). While these locks -never conflict in normal tdb usage, they do add substantial -overhead for most fcntl lock implementations when the kernel -scans to detect if a lock conflict exists. This is often a single -linked list, making the time to acquire and release a fcntl lock -O(N) where N is the number of processes with the TDB open, not -the number actually doing work. - -In a Samba server it is common to have huge numbers of clients -sitting idle, and thus they have weaned themselves off the -TDB_CLEAR_IF_FIRST flag.[footnote: -There is a flag to tdb_reopen_all() which is used for this -optimization: if the parent process will outlive the child, the -child does not need the ACTIVE_LOCK. This is a workaround for -this very performance issue. -] - -3.1.1 Proposed Solution - -Remove the flag. It was a neat idea, but even trivial servers -tend to know when they are initializing for the first time and -can simply unlink the old tdb at that point. - -3.2 TDB Files Have a 4G Limit - -This seems to be becoming an issue (so much for “trivial”!), -particularly for ldb. - -3.2.1 Proposed Solution - -A new, incompatible TDB format which uses 64 bit offsets -internally rather than 32 bit as now. For simplicity of endian -conversion (which TDB does on the fly if required), all values -will be 64 bit on disk. In practice, some upper bits may be used -for other purposes, but at least 56 bits will be available for -file offsets. - -tdb_open() will automatically detect the old version, and even -create them if TDB_VERSION6 is specified to tdb_open. - -32 bit processes will still be able to access TDBs larger than 4G -(assuming that their off_t allows them to seek to 64 bits), they -will gracefully fall back as they fail to mmap. This can happen -already with large TDBs. - -Old versions of tdb will fail to open the new TDB files (since 28 -August 2009, commit 398d0c29290: prior to that any unrecognized -file format would be erased and initialized as a fresh tdb!) - -3.3 TDB Records Have a 4G Limit - -This has not been a reported problem, and the API uses size_t -which can be 64 bit on 64 bit platforms. However, other limits -may have made such an issue moot. - -3.3.1 Proposed Solution - -Record sizes will be 64 bit, with an error returned on 32 bit -platforms which try to access such records (the current -implementation would return TDB_ERR_OOM in a similar case). It -seems unlikely that 32 bit keys will be a limitation, so the -implementation may not support this (see [sub:Records-Incur-A]). - -3.4 Hash Size Is Determined At TDB Creation Time - -TDB contains a number of hash chains in the header; the number is -specified at creation time, and defaults to 131. This is such a -bottleneck on large databases (as each hash chain gets quite -long), that LDB uses 10,000 for this hash. In general it is -impossible to know what the 'right' answer is at database -creation time. - -3.4.1 Proposed Solution - -After comprehensive performance testing on various scalable hash -variants[footnote: -http://rusty.ozlabs.org/?p=89 and http://rusty.ozlabs.org/?p=94 -This was annoying because I was previously convinced that an -expanding tree of hashes would be very close to optimal. -], it became clear that it is hard to beat a straight linear hash -table which doubles in size when it reaches saturation. There are -three details which become important: - -1. On encountering a full bucket, we use the next bucket. - -2. Extra hash bits are stored with the offset, to reduce - comparisons. - -3. A marker entry is used on deleting an entry. - -The doubling of the table must be done under a transaction; we -will not reduce it on deletion, so it will be an unusual case. It -will either be placed at the head (other entries will be moved -out the way so we can expand). We could have a pointer in the -header to the current hashtable location, but that pointer would -have to be read frequently to check for hashtable moves. - -The locking for this is slightly more complex than the chained -case; we currently have one lock per bucket, and that means we -would need to expand the lock if we overflow to the next bucket. -The frequency of such collisions will effect our locking -heuristics: we can always lock more buckets than we need. - -One possible optimization is to only re-check the hash size on an -insert or a lookup miss. - -3.5 <TDB-Freelist-Is>TDB Freelist Is Highly Contended - -TDB uses a single linked list for the free list. Allocation -occurs as follows, using heuristics which have evolved over time: - -1. Get the free list lock for this whole operation. - -2. Multiply length by 1.25, so we always over-allocate by 25%. - -3. Set the slack multiplier to 1. - -4. Examine the current freelist entry: if it is > length but < - the current best case, remember it as the best case. - -5. Multiply the slack multiplier by 1.05. - -6. If our best fit so far is less than length * slack multiplier, - return it. The slack will be turned into a new free record if - it's large enough. - -7. Otherwise, go onto the next freelist entry. - -Deleting a record occurs as follows: - -1. Lock the hash chain for this whole operation. - -2. Walk the chain to find the record, keeping the prev pointer - offset. - -3. If max_dead is non-zero: - - (a) Walk the hash chain again and count the dead records. - - (b) If it's more than max_dead, bulk free all the dead ones - (similar to steps 4 and below, but the lock is only obtained - once). - - (c) Simply mark this record as dead and return. - -4. Get the free list lock for the remainder of this operation. - -5. <right-merging>Examine the following block to see if it is - free; if so, enlarge the current block and remove that block - from the free list. This was disabled, as removal from the free - list was O(entries-in-free-list). - -6. Examine the preceeding block to see if it is free: for this - reason, each block has a 32-bit tailer which indicates its - length. If it is free, expand it to cover our new block and - return. - -7. Otherwise, prepend ourselves to the free list. - -Disabling right-merging (step [right-merging]) causes -fragmentation; the other heuristics proved insufficient to -address this, so the final answer to this was that when we expand -the TDB file inside a transaction commit, we repack the entire -tdb. - -The single list lock limits our allocation rate; due to the other -issues this is not currently seen as a bottleneck. - -3.5.1 Proposed Solution - -The first step is to remove all the current heuristics, as they -obviously interact, then examine them once the lock contention is -addressed. - -The free list must be split to reduce contention. Assuming -perfect free merging, we can at most have 1 free list entry for -each entry. This implies that the number of free lists is related -to the size of the hash table, but as it is rare to walk a large -number of free list entries we can use far fewer, say 1/32 of the -number of hash buckets. - -There are various benefits in using per-size free lists (see [sub:TDB-Becomes-Fragmented] -) but it's not clear this would reduce contention in the common -case where all processes are allocating/freeing the same size. -Thus we almost certainly need to divide in other ways: the most -obvious is to divide the file into zones, and using a free list -(or set of free lists) for each. This approximates address -ordering. - -Note that this means we need to split the free lists when we -expand the file; this is probably acceptable when we double the -hash table size, since that is such an expensive operation -already. In the case of increasing the file size, there is an -optimization we can use: if we use M in the formula above as the -file size rounded up to the next power of 2, we only need -reshuffle free lists when the file size crosses a power of 2 -boundary, and reshuffling the free lists is trivial: we simply -merge every consecutive pair of free lists. - -The basic algorithm is as follows. Freeing is simple: - -1. Identify the correct zone. - -2. Lock the corresponding list. - -3. Re-check the zone (we didn't have a lock, sizes could have - changed): relock if necessary. - -4. Place the freed entry in the list for that zone. - -Allocation is a little more complicated, as we perform delayed -coalescing at this point: - -1. Pick a zone either the zone we last freed into, or based on a “ - random” number. - -2. Lock the corresponding list. - -3. Re-check the zone: relock if necessary. - -4. If the top entry is -large enough, remove it from the list and - return it. - -5. Otherwise, coalesce entries in the list. - - (a) - - (b) - - (c) - - (d) - -6. If there was no entry large enough, unlock the list and try - the next zone. - -7. - -8. - -9. If no zone satisfies, expand the file. - -This optimizes rapid insert/delete of free list entries by not -coalescing them all the time.. First-fit address ordering -ordering seems to be fairly good for keeping fragmentation low -(see [sub:TDB-Becomes-Fragmented]). Note that address ordering -does not need a tailer to coalesce, though if we needed one we -could have one cheaply: see [sub:Records-Incur-A]. - - - -I anticipate that the number of entries in each free zone would -be small, but it might be worth using one free entry to hold -pointers to the others for cache efficiency. - -3.6 <sub:TDB-Becomes-Fragmented>TDB Becomes Fragmented - -Much of this is a result of allocation strategy[footnote: -The Memory Fragmentation Problem: Solved? Johnstone & Wilson 1995 -ftp://ftp.cs.utexas.edu/pub/garbage/malloc/ismm98.ps -] and deliberate hobbling of coalescing; internal fragmentation -(aka overallocation) is deliberately set at 25%, and external -fragmentation is only cured by the decision to repack the entire -db when a transaction commit needs to enlarge the file. - -3.6.1 Proposed Solution - -The 25% overhead on allocation works in practice for ldb because -indexes tend to expand by one record at a time. This internal -fragmentation can be resolved by having an “expanded” bit in the -header to note entries that have previously expanded, and -allocating more space for them. - -There are is a spectrum of possible solutions for external -fragmentation: one is to use a fragmentation-avoiding allocation -strategy such as best-fit address-order allocator. The other end -of the spectrum would be to use a bump allocator (very fast and -simple) and simply repack the file when we reach the end. - -There are three problems with efficient fragmentation-avoiding -allocators: they are non-trivial, they tend to use a single free -list for each size, and there's no evidence that tdb allocation -patterns will match those recorded for general allocators (though -it seems likely). - -Thus we don't spend too much effort on external fragmentation; we -will be no worse than the current code if we need to repack on -occasion. More effort is spent on reducing freelist contention, -and reducing overhead. - -3.7 <sub:Records-Incur-A>Records Incur A 28-Byte Overhead - -Each TDB record has a header as follows: - -struct tdb_record { - - tdb_off_t next; /* offset of the next record in the list -*/ - - tdb_len_t rec_len; /* total byte length of record */ - - tdb_len_t key_len; /* byte length of key */ - - tdb_len_t data_len; /* byte length of data */ - - uint32_t full_hash; /* the full 32 bit hash of the key */ - - uint32_t magic; /* try to catch errors */ - - /* the following union is implied: - - union { - - char record[rec_len]; - - struct { - - char key[key_len]; - - char data[data_len]; - - } - - uint32_t totalsize; (tailer) - - } - - */ - -}; - -Naively, this would double to a 56-byte overhead on a 64 bit -implementation. - -3.7.1 Proposed Solution - -We can use various techniques to reduce this for an allocated -block: - -1. The 'next' pointer is not required, as we are using a flat - hash table. - -2. 'rec_len' can instead be expressed as an addition to key_len - and data_len (it accounts for wasted or overallocated length in - the record). Since the record length is always a multiple of 8, - we can conveniently fit it in 32 bits (representing up to 35 - bits). - -3. 'key_len' and 'data_len' can be reduced. I'm unwilling to - restrict 'data_len' to 32 bits, but instead we can combine the - two into one 64-bit field and using a 5 bit value which - indicates at what bit to divide the two. Keys are unlikely to - scale as fast as data, so I'm assuming a maximum key size of 32 - bits. - -4. 'full_hash' is used to avoid a memcmp on the “miss” case, but - this is diminishing returns after a handful of bits (at 10 - bits, it reduces 99.9% of false memcmp). As an aside, as the - lower bits are already incorporated in the hash table - resolution, the upper bits should be used here. - -5. 'magic' does not need to be enlarged: it currently reflects - one of 5 values (used, free, dead, recovery, and - unused_recovery). It is useful for quick sanity checking - however, and should not be eliminated. - -6. 'tailer' is only used to coalesce free blocks (so a block to - the right can find the header to check if this block is free). - This can be replaced by a single 'free' bit in the header of - the following block (and the tailer only exists in free - blocks).[footnote: -This technique from Thomas Standish. Data Structure Techniques. -Addison-Wesley, Reading, Massachusetts, 1980. -] The current proposed coalescing algorithm doesn't need this, - however. - -This produces a 16 byte used header like this: - -struct tdb_used_record { - - uint32_t magic : 16, - - prev_is_free: 1, - - key_data_divide: 5, - - top_hash: 10; - - uint32_t extra_octets; - - uint64_t key_and_data_len; - -}; - -And a free record like this: - -struct tdb_free_record { - - uint32_t free_magic; - - uint64_t total_length; - - ... - - uint64_t tailer; - -}; - - - -3.8 Transaction Commit Requires 4 fdatasync - -The current transaction algorithm is: - -1. write_recovery_data(); - -2. sync(); - -3. write_recovery_header(); - -4. sync(); - -5. overwrite_with_new_data(); - -6. sync(); - -7. remove_recovery_header(); - -8. sync(); - -On current ext3, each sync flushes all data to disk, so the next -3 syncs are relatively expensive. But this could become a -performance bottleneck on other filesystems such as ext4. - -3.8.1 Proposed Solution - - - - - - - - - -Neil Brown points out that this is overzealous, and only one sync -is needed: - -1. Bundle the recovery data, a transaction counter and a strong - checksum of the new data. - -2. Strong checksum that whole bundle. - -3. Store the bundle in the database. - -4. Overwrite the oldest of the two recovery pointers in the - header (identified using the transaction counter) with the - offset of this bundle. - -5. sync. - -6. Write the new data to the file. - -Checking for recovery means identifying the latest bundle with a -valid checksum and using the new data checksum to ensure that it -has been applied. This is more expensive than the current check, -but need only be done at open. For running databases, a separate -header field can be used to indicate a transaction in progress; -we need only check for recovery if this is set. - -3.9 TDB Does Not Have Snapshot Support - -3.9.1 Proposed Solution - -None. At some point you say “use a real database”. - -But as a thought experiment, if we implemented transactions to -only overwrite free entries (this is tricky: there must not be a -header in each entry which indicates whether it is free, but use -of presence in metadata elsewhere), and a pointer to the hash -table, we could create an entirely new commit without destroying -existing data. Then it would be easy to implement snapshots in a -similar way. - -This would not allow arbitrary changes to the database, such as -tdb_repack does, and would require more space (since we have to -preserve the current and future entries at once). If we used hash -trees rather than one big hash table, we might only have to -rewrite some sections of the hash, too. - -We could then implement snapshots using a similar method, using -multiple different hash tables/free tables. - -3.10 Transactions Cannot Operate in Parallel - -This would be useless for ldb, as it hits the index records with -just about every update. It would add significant complexity in -resolving clashes, and cause the all transaction callers to write -their code to loop in the case where the transactions spuriously -failed. - -3.10.1 Proposed Solution - -We could solve a small part of the problem by providing read-only -transactions. These would allow one write transaction to begin, -but it could not commit until all r/o transactions are done. This -would require a new RO_TRANSACTION_LOCK, which would be upgraded -on commit. - -3.11 Default Hash Function Is Suboptimal - -The Knuth-inspired multiplicative hash used by tdb is fairly slow -(especially if we expand it to 64 bits), and works best when the -hash bucket size is a prime number (which also means a slow -modulus). In addition, it is highly predictable which could -potentially lead to a Denial of Service attack in some TDB uses. - -3.11.1 Proposed Solution - -The Jenkins lookup3 hash[footnote: -http://burtleburtle.net/bob/c/lookup3.c -] is a fast and superbly-mixing hash. It's used by the Linux -kernel and almost everything else. This has the particular -properties that it takes an initial seed, and produces two 32 bit -hash numbers, which we can combine into a 64-bit hash. - -The seed should be created at tdb-creation time from some random -source, and placed in the header. This is far from foolproof, but -adds a little bit of protection against hash bombing. - -3.12 <Reliable-Traversal-Adds>Reliable Traversal Adds Complexity - -We lock a record during traversal iteration, and try to grab that -lock in the delete code. If that grab on delete fails, we simply -mark it deleted and continue onwards; traversal checks for this -condition and does the delete when it moves off the record. - -If traversal terminates, the dead record may be left -indefinitely. - -3.12.1 Proposed Solution - -Remove reliability guarantees; see [traverse-Proposed-Solution]. - -3.13 Fcntl Locking Adds Overhead - -Placing a fcntl lock means a system call, as does removing one. -This is actually one reason why transactions can be faster -(everything is locked once at transaction start). In the -uncontended case, this overhead can theoretically be eliminated. - -3.13.1 Proposed Solution - -None. - -We tried this before with spinlock support, in the early days of -TDB, and it didn't make much difference except in manufactured -benchmarks. - -We could use spinlocks (with futex kernel support under Linux), -but it means that we lose automatic cleanup when a process dies -with a lock. There is a method of auto-cleanup under Linux, but -it's not supported by other operating systems. We could -reintroduce a clear-if-first-style lock and sweep for dead -futexes on open, but that wouldn't help the normal case of one -concurrent opener dying. Increasingly elaborate repair schemes -could be considered, but they require an ABI change (everyone -must use them) anyway, so there's no need to do this at the same -time as everything else. diff --git a/lib/ntdb/doc/design.lyx b/lib/ntdb/doc/design.lyx index 0a1d6a14bc..5a10ee35c3 100644 --- a/lib/ntdb/doc/design.lyx +++ b/lib/ntdb/doc/design.lyx @@ -1,48 +1,66 @@ -#LyX 1.6.7 created this file. For more info see http://www.lyx.org/ -\lyxformat 345 +#LyX 2.0 created this file. For more info see http://www.lyx.org/ +\lyxformat 413 \begin_document \begin_header \textclass article \use_default_options true +\maintain_unincluded_children false \language english +\language_package default \inputencoding auto +\fontencoding global \font_roman default \font_sans default \font_typewriter default \font_default_family default +\use_non_tex_fonts false \font_sc false \font_osf false \font_sf_scale 100 \font_tt_scale 100 \graphics default +\default_output_format default +\output_sync 0 +\bibtex_command default +\index_command default \paperfontsize default \use_hyperref false \papersize default \use_geometry false \use_amsmath 1 \use_esint 1 +\use_mhchem 1 +\use_mathdots 1 \cite_engine basic \use_bibtopic false +\use_indices false \paperorientation portrait +\suppress_date false +\use_refstyle 0 +\index Index +\shortcut idx +\color #008000 +\end_index \secnumdepth 3 \tocdepth 3 \paragraph_separation indent -\defskip medskip +\paragraph_indentation default \quotes_language english \papercolumns 1 \papersides 1 \paperpagestyle default \tracking_changes true \output_changes true -\author "" -\author "" +\html_math_output 0 +\html_css_as_file 0 +\html_be_strict false \end_header \begin_body \begin_layout Title -TDB2: A Redesigning The Trivial DataBase +NTDB: Redesigning The Trivial DataBase \end_layout \begin_layout Author @@ -50,7 +68,7 @@ Rusty Russell, IBM Corporation \end_layout \begin_layout Date -17-March-2011 +19 June 2012 \end_layout \begin_layout Abstract @@ -87,7 +105,7 @@ The wider variety and greater demands of TDB-using code has lead to some \begin_layout Standard \begin_inset Tabular <lyxtabular version="3" rows="12" columns="3"> -<features> +<features tabularvalignment="middle"> <column alignment="center" valignment="top" width="0"> <column alignment="center" valignment="top" width="0"> <column alignment="center" valignment="top" width="0"> @@ -453,6 +471,20 @@ This review is an attempt to catalog and address all the known issues with second system syndrome in rewriting a successful project like this. \end_layout +\begin_layout Standard +Note: the final decision was to make ntdb a separate library, with a separarate + 'ntdb' namespace so both can potentially be linked together. + This document still refers to +\begin_inset Quotes eld +\end_inset + +tdb +\begin_inset Quotes erd +\end_inset + + everywhere, for simplicity. +\end_layout + \begin_layout Section API Issues \end_layout @@ -960,7 +992,6 @@ There are several issues with this approach. have under some circumstances. I don't believe this is currently the case, but it constrains the implementatio n. - \end_layout \begin_layout Subsubsection @@ -1025,7 +1056,7 @@ Status \end_layout \begin_layout Standard -Incomplete. +Complete. \end_layout \begin_layout Subsection @@ -1114,7 +1145,7 @@ Status \end_layout \begin_layout Standard -Incomplete. +Complete. \end_layout \begin_layout Subsection @@ -1291,6 +1322,7 @@ Status \begin_layout Standard Complete. + An open hook is provided to replicate this functionality if required. \end_layout \begin_layout Subsection @@ -1433,7 +1465,7 @@ Status \end_layout \begin_layout Standard -Deferred. +Complete, using the NTDB_ATTRIBUTE_ALLOCATOR attribute. \end_layout \begin_layout Section @@ -1661,7 +1693,12 @@ Status \end_layout \begin_layout Standard -Complete. +Ignore. + Scaling the hash automatically proved inefficient at small hash sizes; + we default to a 8192-element hash (changable via NTDB_ATTRIBUTE_HASHSIZE), + and when buckets clash we expand to an array of hash entries. + This scales slightly better than the tdb chain (due to the 8 top bits containin +g extra hash). \end_layout \begin_layout Subsection @@ -1738,7 +1775,6 @@ If it's more than max_dead, bulk free all the dead ones (similar to steps \begin_layout Enumerate Simply mark this record as dead and return. - \end_layout \end_deeper @@ -1920,7 +1956,6 @@ reference "sub:Records-Incur-A" \end_inset . - \end_layout \begin_layout Standard @@ -2357,7 +2392,11 @@ TDB Does Not Have Snapshot Support \end_layout \begin_layout Subsubsection -Proposed SolutionNone. +Proposed Solution +\end_layout + +\begin_layout Standard +None. At some point you say \begin_inset Quotes eld \end_inset @@ -2666,7 +2705,6 @@ name "replay-attribute" \begin_layout Standard Tridge points out that an attribute can be later added to tdb_open (see - \begin_inset CommandInset ref LatexCommand ref reference "attributes" diff --git a/lib/ntdb/doc/design.lyx,v b/lib/ntdb/doc/design.lyx,v deleted file mode 100644 index 13e6387f7f..0000000000 --- a/lib/ntdb/doc/design.lyx,v +++ /dev/null @@ -1,4679 +0,0 @@ -head 1.13; -access; -symbols; -locks; strict; -comment @# @; - - -1.13 -date 2011.03.01.11.46.54; author rusty; state Exp; -branches; -next 1.12; - -1.12 -date 2010.12.01.12.20.49; author rusty; state Exp; -branches; -next 1.11; - -1.11 -date 2010.12.01.11.55.20; author rusty; state Exp; -branches; -next 1.10; - -1.10 -date 2010.09.14.00.33.57; author rusty; state Exp; -branches; -next 1.9; - -1.9 -date 2010.09.09.07.25.12; author rusty; state Exp; -branches; -next 1.8; - -1.8 -date 2010.09.02.02.29.05; author rusty; state Exp; -branches; -next 1.7; - -1.7 -date 2010.09.01.10.58.12; author rusty; state Exp; -branches; -next 1.6; - -1.6 -date 2010.08.02.00.21.43; author rusty; state Exp; -branches; -next 1.5; - -1.5 -date 2010.08.02.00.21.16; author rusty; state Exp; -branches; -next 1.4; - -1.4 -date 2010.05.10.13.09.11; author rusty; state Exp; -branches; -next 1.3; - -1.3 -date 2010.05.10.11.58.37; author rusty; state Exp; -branches; -next 1.2; - -1.2 -date 2010.05.10.05.35.13; author rusty; state Exp; -branches; -next 1.1; - -1.1 -date 2010.05.04.02.29.16; author rusty; state Exp; -branches; -next ; - - -desc -@First draft -@ - - -1.13 -log -@Thread-safe API -@ -text -@#LyX 1.6.7 created this file. For more info see http://www.lyx.org/ -\lyxformat 345 -\begin_document -\begin_header -\textclass article -\use_default_options true -\language english -\inputencoding auto -\font_roman default -\font_sans default -\font_typewriter default -\font_default_family default -\font_sc false -\font_osf false -\font_sf_scale 100 -\font_tt_scale 100 - -\graphics default -\paperfontsize default -\use_hyperref false -\papersize default -\use_geometry false -\use_amsmath 1 -\use_esint 1 -\cite_engine basic -\use_bibtopic false -\paperorientation portrait -\secnumdepth 3 -\tocdepth 3 -\paragraph_separation indent -\defskip medskip -\quotes_language english -\papercolumns 1 -\papersides 1 -\paperpagestyle default -\tracking_changes true -\output_changes true -\author "Rusty Russell,,," -\author "" -\end_header - -\begin_body - -\begin_layout Title -TDB2: A Redesigning The Trivial DataBase -\end_layout - -\begin_layout Author -Rusty Russell, IBM Corporation -\end_layout - -\begin_layout Date -1-December-2010 -\end_layout - -\begin_layout Abstract -The Trivial DataBase on-disk format is 32 bits; with usage cases heading - towards the 4G limit, that must change. - This required breakage provides an opportunity to revisit TDB's other design - decisions and reassess them. -\end_layout - -\begin_layout Section -Introduction -\end_layout - -\begin_layout Standard -The Trivial DataBase was originally written by Andrew Tridgell as a simple - key/data pair storage system with the same API as dbm, but allowing multiple - readers and writers while being small enough (< 1000 lines of C) to include - in SAMBA. - The simple design created in 1999 has proven surprisingly robust and performant -, used in Samba versions 3 and 4 as well as numerous other projects. - Its useful life was greatly increased by the (backwards-compatible!) addition - of transaction support in 2005. -\end_layout - -\begin_layout Standard -The wider variety and greater demands of TDB-using code has lead to some - organic growth of the API, as well as some compromises on the implementation. - None of these, by themselves, are seen as show-stoppers, but the cumulative - effect is to a loss of elegance over the initial, simple TDB implementation. - Here is a table of the approximate number of lines of implementation code - and number of API functions at the end of each year: -\end_layout - -\begin_layout Standard -\begin_inset Tabular -<lyxtabular version="3" rows="12" columns="3"> -<features> -<column alignment="center" valignment="top" width="0"> -<column alignment="center" valignment="top" width="0"> -<column alignment="center" valignment="top" width="0"> -<row> -<cell alignment="center" valignment="top" topline="true" bottomline="true" leftline="true" usebox="none"> -\begin_inset Text - -\begin_layout Plain Layout -Year End -\end_layout - -\end_inset -</cell> -<cell alignment="center" valignment="top" topline="true" bottomline="true" leftline="true" usebox="none"> -\begin_inset Text - -\begin_layout Plain Layout -API Functions -\end_layout - -\end_inset -</cell> -<cell alignment="center" valignment="top" topline="true" bottomline="true" leftline="true" rightline="true" usebox="none"> -\begin_inset Text - -\begin_layout Plain Layout -Lines of C Code Implementation -\end_layout - -\end_inset -</cell> -</row> -<row> -<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none"> -\begin_inset Text - -\begin_layout Plain Layout -1999 -\end_layout - -\end_inset -</cell> -<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none"> -\begin_inset Text - -\begin_layout Plain Layout -13 -\end_layout - -\end_inset -</cell> -<cell alignment="center" valignment="top" topline="true" leftline="true" rightline="true" usebox="none"> -\begin_inset Text - -\begin_layout Plain Layout -1195 -\end_layout - -\end_inset -</cell> -</row> -<row> -<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none"> -\begin_inset Text - -\begin_layout Plain Layout -2000 -\end_layout - -\end_inset -</cell> -<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none"> -\begin_inset Text - -\begin_layout Plain Layout -24 -\end_layout - -\end_inset -</cell> -<cell alignment="center" valignment="top" topline="true" leftline="true" rightline="true" usebox="none"> -\begin_inset Text - -\begin_layout Plain Layout -1725 -\end_layout - -\end_inset -</cell> -</row> -<row> -<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none"> -\begin_inset Text - -\begin_layout Plain Layout -2001 -\end_layout - -\end_inset -</cell> -<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none"> -\begin_inset Text - -\begin_layout Plain Layout -32 -\end_layout - -\end_inset -</cell> -<cell alignment="center" valignment="top" topline="true" leftline="true" rightline="true" usebox="none"> -\begin_inset Text - -\begin_layout Plain Layout -2228 -\end_layout - -\end_inset -</cell> -</row> -<row> -<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none"> -\begin_inset Text - -\begin_layout Plain Layout -2002 -\end_layout - -\end_inset -</cell> -<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none"> -\begin_inset Text - -\begin_layout Plain Layout -35 -\end_layout - -\end_inset -</cell> -<cell alignment="center" valignment="top" topline="true" leftline="true" rightline="true" usebox="none"> -\begin_inset Text - -\begin_layout Plain Layout -2481 -\end_layout - -\end_inset -</cell> -</row> -<row> -<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none"> -\begin_inset Text - -\begin_layout Plain Layout -2003 -\end_layout - -\end_inset -</cell> -<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none"> -\begin_inset Text - -\begin_layout Plain Layout -35 -\end_layout - -\end_inset -</cell> -<cell alignment="center" valignment="top" topline="true" leftline="true" rightline="true" usebox="none"> -\begin_inset Text - -\begin_layout Plain Layout -2552 -\end_layout - -\end_inset -</cell> -</row> -<row> -<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none"> -\begin_inset Text - -\begin_layout Plain Layout -2004 -\end_layout - -\end_inset -</cell> -<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none"> -\begin_inset Text - -\begin_layout Plain Layout -40 -\end_layout - -\end_inset -</cell> -<cell alignment="center" valignment="top" topline="true" leftline="true" rightline="true" usebox="none"> -\begin_inset Text - -\begin_layout Plain Layout -2584 -\end_layout - -\end_inset -</cell> -</row> -<row> -<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none"> -\begin_inset Text - -\begin_layout Plain Layout -2005 -\end_layout - -\end_inset -</cell> -<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none"> -\begin_inset Text - -\begin_layout Plain Layout -38 -\end_layout - -\end_inset -</cell> -<cell alignment="center" valignment="top" topline="true" leftline="true" rightline="true" usebox="none"> -\begin_inset Text - -\begin_layout Plain Layout -2647 -\end_layout - -\end_inset -</cell> -</row> -<row> -<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none"> -\begin_inset Text - -\begin_layout Plain Layout -2006 -\end_layout - -\end_inset -</cell> -<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none"> -\begin_inset Text - -\begin_layout Plain Layout -52 -\end_layout - -\end_inset -</cell> -<cell alignment="center" valignment="top" topline="true" leftline="true" rightline="true" usebox="none"> -\begin_inset Text - -\begin_layout Plain Layout -3754 -\end_layout - -\end_inset -</cell> -</row> -<row> -<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none"> -\begin_inset Text - -\begin_layout Plain Layout -2007 -\end_layout - -\end_inset -</cell> -<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none"> -\begin_inset Text - -\begin_layout Plain Layout -66 -\end_layout - -\end_inset -</cell> -<cell alignment="center" valignment="top" topline="true" leftline="true" rightline="true" usebox="none"> -\begin_inset Text - -\begin_layout Plain Layout -4398 -\end_layout - -\end_inset -</cell> -</row> -<row> -<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none"> -\begin_inset Text - -\begin_layout Plain Layout -2008 -\end_layout - -\end_inset -</cell> -<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none"> -\begin_inset Text - -\begin_layout Plain Layout -71 -\end_layout - -\end_inset -</cell> -<cell alignment="center" valignment="top" topline="true" leftline="true" rightline="true" usebox="none"> -\begin_inset Text - -\begin_layout Plain Layout -4768 -\end_layout - -\end_inset -</cell> -</row> -<row> -<cell alignment="center" valignment="top" topline="true" bottomline="true" leftline="true" usebox="none"> -\begin_inset Text - -\begin_layout Plain Layout -2009 -\end_layout - -\end_inset -</cell> -<cell alignment="center" valignment="top" topline="true" bottomline="true" leftline="true" usebox="none"> -\begin_inset Text - -\begin_layout Plain Layout -73 -\end_layout - -\end_inset -</cell> -<cell alignment="center" valignment="top" topline="true" bottomline="true" leftline="true" rightline="true" usebox="none"> -\begin_inset Text - -\begin_layout Plain Layout -5715 -\end_layout - -\end_inset -</cell> -</row> -</lyxtabular> - -\end_inset - - -\end_layout - -\begin_layout Standard -This review is an attempt to catalog and address all the known issues with - TDB and create solutions which address the problems without significantly - increasing complexity; all involved are far too aware of the dangers of - second system syndrome in rewriting a successful project like this. -\end_layout - -\begin_layout Section -API Issues -\end_layout - -\begin_layout Subsection -tdb_open_ex Is Not Expandable -\end_layout - -\begin_layout Standard -The tdb_open() call was expanded to tdb_open_ex(), which added an optional - hashing function and an optional logging function argument. - Additional arguments to open would require the introduction of a tdb_open_ex2 - call etc. -\end_layout - -\begin_layout Subsubsection -Proposed Solution -\begin_inset CommandInset label -LatexCommand label -name "attributes" - -\end_inset - - -\end_layout - -\begin_layout Standard -tdb_open() will take a linked-list of attributes: -\end_layout - -\begin_layout LyX-Code -enum tdb_attribute { -\end_layout - -\begin_layout LyX-Code - TDB_ATTRIBUTE_LOG = 0, -\end_layout - -\begin_layout LyX-Code - TDB_ATTRIBUTE_HASH = 1 -\end_layout - -\begin_layout LyX-Code -}; -\end_layout - -\begin_layout LyX-Code -struct tdb_attribute_base { -\end_layout - -\begin_layout LyX-Code - enum tdb_attribute attr; -\end_layout - -\begin_layout LyX-Code - union tdb_attribute *next; -\end_layout - -\begin_layout LyX-Code -}; -\end_layout - -\begin_layout LyX-Code -struct tdb_attribute_log { -\end_layout - -\begin_layout LyX-Code - struct tdb_attribute_base base; /* .attr = TDB_ATTRIBUTE_LOG */ -\end_layout - -\begin_layout LyX-Code - tdb_log_func log_fn; -\end_layout - -\begin_layout LyX-Code - void *log_private; -\end_layout - -\begin_layout LyX-Code -}; -\end_layout - -\begin_layout LyX-Code -struct tdb_attribute_hash { -\end_layout - -\begin_layout LyX-Code - struct tdb_attribute_base base; /* .attr = TDB_ATTRIBUTE_HASH */ -\end_layout - -\begin_layout LyX-Code - tdb_hash_func hash_fn; -\end_layout - -\begin_layout LyX-Code - void *hash_private; -\end_layout - -\begin_layout LyX-Code -}; -\end_layout - -\begin_layout LyX-Code -union tdb_attribute { -\end_layout - -\begin_layout LyX-Code - struct tdb_attribute_base base; -\end_layout - -\begin_layout LyX-Code - struct tdb_attribute_log log; -\end_layout - -\begin_layout LyX-Code - struct tdb_attribute_hash hash; -\end_layout - -\begin_layout LyX-Code -}; -\end_layout - -\begin_layout Standard -This allows future attributes to be added, even if this expands the size - of the union. -\end_layout - -\begin_layout Subsubsection -Status -\end_layout - -\begin_layout Standard -Complete. -\end_layout - -\begin_layout Subsection -tdb_traverse Makes Impossible Guarantees -\end_layout - -\begin_layout Standard -tdb_traverse (and tdb_firstkey/tdb_nextkey) predate transactions, and it - was thought that it was important to guarantee that all records which exist - at the start and end of the traversal would be included, and no record - would be included twice. -\end_layout - -\begin_layout Standard -This adds complexity (see -\begin_inset CommandInset ref -LatexCommand ref -reference "Reliable-Traversal-Adds" - -\end_inset - -) and does not work anyway for records which are altered (in particular, - those which are expanded may be effectively deleted and re-added behind - the traversal). -\end_layout - -\begin_layout Subsubsection -\begin_inset CommandInset label -LatexCommand label -name "traverse-Proposed-Solution" - -\end_inset - -Proposed Solution -\end_layout - -\begin_layout Standard -Abandon the guarantee. - You will see every record if no changes occur during your traversal, otherwise - you will see some subset. - You can prevent changes by using a transaction or the locking API. -\end_layout - -\begin_layout Subsubsection -Status -\end_layout - -\begin_layout Standard -Complete. - Delete-during-traverse will still delete every record, too (assuming no - other changes). -\end_layout - -\begin_layout Subsection -Nesting of Transactions Is Fraught -\end_layout - -\begin_layout Standard -TDB has alternated between allowing nested transactions and not allowing - them. - Various paths in the Samba codebase assume that transactions will nest, - and in a sense they can: the operation is only committed to disk when the - outer transaction is committed. - There are two problems, however: -\end_layout - -\begin_layout Enumerate -Canceling the inner transaction will cause the outer transaction commit - to fail, and will not undo any operations since the inner transaction began. - This problem is soluble with some additional internal code. -\end_layout - -\begin_layout Enumerate -An inner transaction commit can be cancelled by the outer transaction. - This is desirable in the way which Samba's database initialization code - uses transactions, but could be a surprise to any users expecting a successful - transaction commit to expose changes to others. -\end_layout - -\begin_layout Standard -The current solution is to specify the behavior at tdb_open(), with the - default currently that nested transactions are allowed. - This flag can also be changed at runtime. -\end_layout - -\begin_layout Subsubsection -Proposed Solution -\end_layout - -\begin_layout Standard -Given the usage patterns, it seems that the -\begin_inset Quotes eld -\end_inset - -least-surprise -\begin_inset Quotes erd -\end_inset - - behavior of disallowing nested transactions should become the default. - Additionally, it seems the outer transaction is the only code which knows - whether inner transactions should be allowed, so a flag to indicate this - could be added to tdb_transaction_start. - However, this behavior can be simulated with a wrapper which uses tdb_add_flags -() and tdb_remove_flags(), so the API should not be expanded for this relatively --obscure case. -\end_layout - -\begin_layout Subsubsection -Status -\end_layout - -\begin_layout Standard - -\change_deleted 0 1298979572 -Incomplete; nesting flag is still defined as per tdb1. -\change_inserted 0 1298979584 -Complete; the nesting flag has been removed. -\change_unchanged - -\end_layout - -\begin_layout Subsection -Incorrect Hash Function is Not Detected -\end_layout - -\begin_layout Standard -tdb_open_ex() allows the calling code to specify a different hash function - to use, but does not check that all other processes accessing this tdb - are using the same hash function. - The result is that records are missing from tdb_fetch(). -\end_layout - -\begin_layout Subsubsection -Proposed Solution -\end_layout - -\begin_layout Standard -The header should contain an example hash result (eg. - the hash of 0xdeadbeef), and tdb_open_ex() should check that the given - hash function produces the same answer, or fail the tdb_open call. -\end_layout - -\begin_layout Subsubsection -Status -\end_layout - -\begin_layout Standard -Complete. -\end_layout - -\begin_layout Subsection -tdb_set_max_dead/TDB_VOLATILE Expose Implementation -\end_layout - -\begin_layout Standard -In response to scalability issues with the free list ( -\begin_inset CommandInset ref -LatexCommand ref -reference "TDB-Freelist-Is" - -\end_inset - -) two API workarounds have been incorporated in TDB: tdb_set_max_dead() - and the TDB_VOLATILE flag to tdb_open. - The latter actually calls the former with an argument of -\begin_inset Quotes eld -\end_inset - -5 -\begin_inset Quotes erd -\end_inset - -. -\end_layout - -\begin_layout Standard -This code allows deleted records to accumulate without putting them in the - free list. - On delete we iterate through each chain and free them in a batch if there - are more than max_dead entries. - These are never otherwise recycled except as a side-effect of a tdb_repack. -\end_layout - -\begin_layout Subsubsection -Proposed Solution -\end_layout - -\begin_layout Standard -With the scalability problems of the freelist solved, this API can be removed. - The TDB_VOLATILE flag may still be useful as a hint that store and delete - of records will be at least as common as fetch in order to allow some internal - tuning, but initially will become a no-op. -\end_layout - -\begin_layout Subsubsection -Status -\end_layout - -\begin_layout Standard -Incomplete. - TDB_VOLATILE still defined, but implementation should fail on unknown flags - to be future-proof. -\end_layout - -\begin_layout Subsection -\begin_inset CommandInset label -LatexCommand label -name "TDB-Files-Cannot" - -\end_inset - -TDB Files Cannot Be Opened Multiple Times In The Same Process -\end_layout - -\begin_layout Standard -No process can open the same TDB twice; we check and disallow it. - This is an unfortunate side-effect of fcntl locks, which operate on a per-file - rather than per-file-descriptor basis, and do not nest. - Thus, closing any file descriptor on a file clears all the locks obtained - by this process, even if they were placed using a different file descriptor! -\end_layout - -\begin_layout Standard -Note that even if this were solved, deadlock could occur if operations were - nested: this is a more manageable programming error in most cases. -\end_layout - -\begin_layout Subsubsection -Proposed Solution -\end_layout - -\begin_layout Standard -We could lobby POSIX to fix the perverse rules, or at least lobby Linux - to violate them so that the most common implementation does not have this - restriction. - This would be a generally good idea for other fcntl lock users. -\end_layout - -\begin_layout Standard -Samba uses a wrapper which hands out the same tdb_context to multiple callers - if this happens, and does simple reference counting. - We should do this inside the tdb library, which already emulates lock nesting - internally; it would need to recognize when deadlock occurs within a single - process. - This would create a new failure mode for tdb operations (while we currently - handle locking failures, they are impossible in normal use and a process - encountering them can do little but give up). -\end_layout - -\begin_layout Standard -I do not see benefit in an additional tdb_open flag to indicate whether - re-opening is allowed, as though there may be some benefit to adding a - call to detect when a tdb_context is shared, to allow other to create such - an API. -\end_layout - -\begin_layout Subsubsection -Status -\end_layout - -\begin_layout Standard -Incomplete. -\end_layout - -\begin_layout Subsection -TDB API Is Not POSIX Thread-safe -\end_layout - -\begin_layout Standard -The TDB API uses an error code which can be queried after an operation to - determine what went wrong. - This programming model does not work with threads, unless specific additional - guarantees are given by the implementation. - In addition, even otherwise-independent threads cannot open the same TDB - (as in -\begin_inset CommandInset ref -LatexCommand ref -reference "TDB-Files-Cannot" - -\end_inset - -). -\end_layout - -\begin_layout Subsubsection -Proposed Solution -\end_layout - -\begin_layout Standard -Reachitecting the API to include a tdb_errcode pointer would be a great - deal of churn -\change_inserted 0 1298979557 -, but fortunately most functions return 0 on success and -1 on error: we - can change these to return 0 on success and a negative error code on error, - and the API remains similar to previous. - The tdb_fetch, tdb_firstkey and tdb_nextkey functions need to take a TDB_DATA - pointer and return an error code. - It is also simpler to have tdb_nextkey replace its key argument in place, - freeing up any old .dptr. -\end_layout - -\begin_layout Standard - -\change_deleted 0 1298979438 -; we are better to guarantee that the tdb_errcode is per-thread so the current - programming model can be maintained. -\end_layout - -\begin_layout Standard - -\change_deleted 0 1298979438 -This requires dynamic per-thread allocations, which is awkward with POSIX - threads (pthread_key_create space is limited and we cannot simply allocate - a key for every TDB). -\change_unchanged - -\end_layout - -\begin_layout Standard -Internal locking is required to make sure that fcntl locks do not overlap - between threads, and also that the global list of tdbs is maintained. -\end_layout - -\begin_layout Standard -The aim is that building tdb with -DTDB_PTHREAD will result in a pthread-safe - version of the library, and otherwise no overhead will exist. - Alternatively, a hooking mechanism similar to that proposed for -\begin_inset CommandInset ref -LatexCommand ref -reference "Proposed-Solution-locking-hook" - -\end_inset - - could be used to enable pthread locking at runtime. -\end_layout - -\begin_layout Subsubsection -Status -\end_layout - -\begin_layout Standard -Incomplete -\change_inserted 0 1298979681 -; API has been changed but thread safety has not been implemented. -\change_deleted 0 1298979669 -. -\change_unchanged - -\end_layout - -\begin_layout Subsection -*_nonblock Functions And *_mark Functions Expose Implementation -\end_layout - -\begin_layout Standard -CTDB -\begin_inset Foot -status collapsed - -\begin_layout Plain Layout -Clustered TDB, see http://ctdb.samba.org -\end_layout - -\end_inset - - wishes to operate on TDB in a non-blocking manner. - This is currently done as follows: -\end_layout - -\begin_layout Enumerate -Call the _nonblock variant of an API function (eg. - tdb_lockall_nonblock). - If this fails: -\end_layout - -\begin_layout Enumerate -Fork a child process, and wait for it to call the normal variant (eg. - tdb_lockall). -\end_layout - -\begin_layout Enumerate -If the child succeeds, call the _mark variant to indicate we already have - the locks (eg. - tdb_lockall_mark). -\end_layout - -\begin_layout Enumerate -Upon completion, tell the child to release the locks (eg. - tdb_unlockall). -\end_layout - -\begin_layout Enumerate -Indicate to tdb that it should consider the locks removed (eg. - tdb_unlockall_mark). -\end_layout - -\begin_layout Standard -There are several issues with this approach. - Firstly, adding two new variants of each function clutters the API for - an obscure use, and so not all functions have three variants. - Secondly, it assumes that all paths of the functions ask for the same locks, - otherwise the parent process will have to get a lock which the child doesn't - have under some circumstances. - I don't believe this is currently the case, but it constrains the implementatio -n. - -\end_layout - -\begin_layout Subsubsection -\begin_inset CommandInset label -LatexCommand label -name "Proposed-Solution-locking-hook" - -\end_inset - -Proposed Solution -\end_layout - -\begin_layout Standard -Implement a hook for locking methods, so that the caller can control the - calls to create and remove fcntl locks. - In this scenario, ctdbd would operate as follows: -\end_layout - -\begin_layout Enumerate -Call the normal API function, eg tdb_lockall(). -\end_layout - -\begin_layout Enumerate -When the lock callback comes in, check if the child has the lock. - Initially, this is always false. - If so, return 0. - Otherwise, try to obtain it in non-blocking mode. - If that fails, return EWOULDBLOCK. -\end_layout - -\begin_layout Enumerate -Release locks in the unlock callback as normal. -\end_layout - -\begin_layout Enumerate -If tdb_lockall() fails, see if we recorded a lock failure; if so, call the - child to repeat the operation. -\end_layout - -\begin_layout Enumerate -The child records what locks it obtains, and returns that information to - the parent. -\end_layout - -\begin_layout Enumerate -When the child has succeeded, goto 1. -\end_layout - -\begin_layout Standard -This is flexible enough to handle any potential locking scenario, even when - lock requirements change. - It can be optimized so that the parent does not release locks, just tells - the child which locks it doesn't need to obtain. -\end_layout - -\begin_layout Standard -It also keeps the complexity out of the API, and in ctdbd where it is needed. -\end_layout - -\begin_layout Subsubsection -Status -\end_layout - -\begin_layout Standard -Incomplete. -\end_layout - -\begin_layout Subsection -tdb_chainlock Functions Expose Implementation -\end_layout - -\begin_layout Standard -tdb_chainlock locks some number of records, including the record indicated - by the given key. - This gave atomicity guarantees; no-one can start a transaction, alter, - read or delete that key while the lock is held. -\end_layout - -\begin_layout Standard -It also makes the same guarantee for any other key in the chain, which is - an internal implementation detail and potentially a cause for deadlock. -\end_layout - -\begin_layout Subsubsection -Proposed Solution -\end_layout - -\begin_layout Standard -None. - It would be nice to have an explicit single entry lock which effected no - other keys. - Unfortunately, this won't work for an entry which doesn't exist. - Thus while chainlock may be implemented more efficiently for the existing - case, it will still have overlap issues with the non-existing case. - So it is best to keep the current (lack of) guarantee about which records - will be effected to avoid constraining our implementation. -\end_layout - -\begin_layout Subsection -Signal Handling is Not Race-Free -\end_layout - -\begin_layout Standard -The tdb_setalarm_sigptr() call allows the caller's signal handler to indicate - that the tdb locking code should return with a failure, rather than trying - again when a signal is received (and errno == EAGAIN). - This is usually used to implement timeouts. -\end_layout - -\begin_layout Standard -Unfortunately, this does not work in the case where the signal is received - before the tdb code enters the fcntl() call to place the lock: the code - will sleep within the fcntl() code, unaware that the signal wants it to - exit. - In the case of long timeouts, this does not happen in practice. -\end_layout - -\begin_layout Subsubsection -Proposed Solution -\end_layout - -\begin_layout Standard -The locking hooks proposed in -\begin_inset CommandInset ref -LatexCommand ref -reference "Proposed-Solution-locking-hook" - -\end_inset - - would allow the user to decide on whether to fail the lock acquisition - on a signal. - This allows the caller to choose their own compromise: they could narrow - the race by checking immediately before the fcntl call. -\begin_inset Foot -status collapsed - -\begin_layout Plain Layout -It may be possible to make this race-free in some implementations by having - the signal handler alter the struct flock to make it invalid. - This will cause the fcntl() lock call to fail with EINVAL if the signal - occurs before the kernel is entered, otherwise EAGAIN. -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Subsubsection -Status -\end_layout - -\begin_layout Standard -Incomplete. -\end_layout - -\begin_layout Subsection -The API Uses Gratuitous Typedefs, Capitals -\end_layout - -\begin_layout Standard -typedefs are useful for providing source compatibility when types can differ - across implementations, or arguably in the case of function pointer definitions - which are hard for humans to parse. - Otherwise it is simply obfuscation and pollutes the namespace. -\end_layout - -\begin_layout Standard -Capitalization is usually reserved for compile-time constants and macros. -\end_layout - -\begin_layout Description -TDB_CONTEXT There is no reason to use this over 'struct tdb_context'; the - definition isn't visible to the API user anyway. -\end_layout - -\begin_layout Description -TDB_DATA There is no reason to use this over struct TDB_DATA; the struct - needs to be understood by the API user. -\end_layout - -\begin_layout Description -struct -\begin_inset space ~ -\end_inset - -TDB_DATA This would normally be called 'struct tdb_data'. -\end_layout - -\begin_layout Description -enum -\begin_inset space ~ -\end_inset - -TDB_ERROR Similarly, this would normally be enum tdb_error. -\end_layout - -\begin_layout Subsubsection -Proposed Solution -\end_layout - -\begin_layout Standard -None. - Introducing lower case variants would please pedants like myself, but if - it were done the existing ones should be kept. - There is little point forcing a purely cosmetic change upon tdb users. -\end_layout - -\begin_layout Subsection -\begin_inset CommandInset label -LatexCommand label -name "tdb_log_func-Doesnt-Take" - -\end_inset - -tdb_log_func Doesn't Take The Private Pointer -\end_layout - -\begin_layout Standard -For API compatibility reasons, the logging function needs to call tdb_get_loggin -g_private() to retrieve the pointer registered by the tdb_open_ex for logging. -\end_layout - -\begin_layout Subsubsection -Proposed Solution -\end_layout - -\begin_layout Standard -It should simply take an extra argument, since we are prepared to break - the API/ABI. -\end_layout - -\begin_layout Subsubsection -Status -\end_layout - -\begin_layout Standard -Complete. -\end_layout - -\begin_layout Subsection -Various Callback Functions Are Not Typesafe -\end_layout - -\begin_layout Standard -The callback functions in tdb_set_logging_function (after -\begin_inset CommandInset ref -LatexCommand ref -reference "tdb_log_func-Doesnt-Take" - -\end_inset - - is resolved), tdb_parse_record, tdb_traverse, tdb_traverse_read and tdb_check - all take void * and must internally convert it to the argument type they - were expecting. -\end_layout - -\begin_layout Standard -If this type changes, the compiler will not produce warnings on the callers, - since it only sees void *. -\end_layout - -\begin_layout Subsubsection -Proposed Solution -\end_layout - -\begin_layout Standard -With careful use of macros, we can create callback functions which give - a warning when used on gcc and the types of the callback and its private - argument differ. - Unsupported compilers will not give a warning, which is no worse than now. - In addition, the callbacks become clearer, as they need not use void * - for their parameter. -\end_layout - -\begin_layout Standard -See CCAN's typesafe_cb module at http://ccan.ozlabs.org/info/typesafe_cb.html -\end_layout - -\begin_layout Subsubsection -Status -\end_layout - -\begin_layout Standard -Incomplete. -\end_layout - -\begin_layout Subsection -TDB_CLEAR_IF_FIRST Must Be Specified On All Opens, tdb_reopen_all Problematic -\end_layout - -\begin_layout Standard -The TDB_CLEAR_IF_FIRST flag to tdb_open indicates that the TDB file should - be cleared if the caller discovers it is the only process with the TDB - open. - However, if any caller does not specify TDB_CLEAR_IF_FIRST it will not - be detected, so will have the TDB erased underneath them (usually resulting - in a crash). -\end_layout - -\begin_layout Standard -There is a similar issue on fork(); if the parent exits (or otherwise closes - the tdb) before the child calls tdb_reopen_all() to establish the lock - used to indicate the TDB is opened by someone, a TDB_CLEAR_IF_FIRST opener - at that moment will believe it alone has opened the TDB and will erase - it. -\end_layout - -\begin_layout Subsubsection -Proposed Solution -\end_layout - -\begin_layout Standard -Remove TDB_CLEAR_IF_FIRST. - Other workarounds are possible, but see -\begin_inset CommandInset ref -LatexCommand ref -reference "TDB_CLEAR_IF_FIRST-Imposes-Performance" - -\end_inset - -. -\end_layout - -\begin_layout Subsubsection -Status -\end_layout - -\begin_layout Standard - -\change_deleted 0 1298979699 -Incomplete, TDB_CLEAR_IF_FIRST still defined, but not implemented. -\change_inserted 0 1298979700 -Complete. -\change_unchanged - -\end_layout - -\begin_layout Subsection -Extending The Header Is Difficult -\end_layout - -\begin_layout Standard -We have reserved (zeroed) words in the TDB header, which can be used for - future features. - If the future features are compulsory, the version number must be updated - to prevent old code from accessing the database. - But if the future feature is optional, we have no way of telling if older - code is accessing the database or not. -\end_layout - -\begin_layout Subsubsection -Proposed Solution -\end_layout - -\begin_layout Standard -The header should contain a -\begin_inset Quotes eld -\end_inset - -format variant -\begin_inset Quotes erd -\end_inset - - value (64-bit). - This is divided into two 32-bit parts: -\end_layout - -\begin_layout Enumerate -The lower part reflects the format variant understood by code accessing - the database. -\end_layout - -\begin_layout Enumerate -The upper part reflects the format variant you must understand to write - to the database (otherwise you can only open for reading). -\end_layout - -\begin_layout Standard -The latter field can only be written at creation time, the former should - be written under the OPEN_LOCK when opening the database for writing, if - the variant of the code is lower than the current lowest variant. -\end_layout - -\begin_layout Standard -This should allow backwards-compatible features to be added, and detection - if older code (which doesn't understand the feature) writes to the database. -\end_layout - -\begin_layout Subsubsection -Status -\end_layout - -\begin_layout Standard -Incomplete. -\end_layout - -\begin_layout Subsection -Record Headers Are Not Expandible -\end_layout - -\begin_layout Standard -If we later want to add (say) checksums on keys and data, it would require - another format change, which we'd like to avoid. -\end_layout - -\begin_layout Subsubsection -Proposed Solution -\end_layout - -\begin_layout Standard -We often have extra padding at the tail of a record. - If we ensure that the first byte (if any) of this padding is zero, we will - have a way for future changes to detect code which doesn't understand a - new format: the new code would write (say) a 1 at the tail, and thus if - there is no tail or the first byte is 0, we would know the extension is - not present on that record. -\end_layout - -\begin_layout Subsubsection -Status -\end_layout - -\begin_layout Standard -Incomplete. -\end_layout - -\begin_layout Subsection -TDB Does Not Use Talloc -\end_layout - -\begin_layout Standard -Many users of TDB (particularly Samba) use the talloc allocator, and thus - have to wrap TDB in a talloc context to use it conveniently. -\end_layout - -\begin_layout Subsubsection -Proposed Solution -\end_layout - -\begin_layout Standard -The allocation within TDB is not complicated enough to justify the use of - talloc, and I am reluctant to force another (excellent) library on TDB - users. - Nonetheless a compromise is possible. - An attribute (see -\begin_inset CommandInset ref -LatexCommand ref -reference "attributes" - -\end_inset - -) can be added later to tdb_open() to provide an alternate allocation mechanism, - specifically for talloc but usable by any other allocator (which would - ignore the -\begin_inset Quotes eld -\end_inset - -context -\begin_inset Quotes erd -\end_inset - - argument). -\end_layout - -\begin_layout Standard -This would form a talloc heirarchy as expected, but the caller would still - have to attach a destructor to the tdb context returned from tdb_open to - close it. - All TDB_DATA fields would be children of the tdb_context, and the caller - would still have to manage them (using talloc_free() or talloc_steal()). -\end_layout - -\begin_layout Subsubsection -Status -\end_layout - -\begin_layout Standard -Deferred. -\end_layout - -\begin_layout Section -Performance And Scalability Issues -\end_layout - -\begin_layout Subsection -\begin_inset CommandInset label -LatexCommand label -name "TDB_CLEAR_IF_FIRST-Imposes-Performance" - -\end_inset - -TDB_CLEAR_IF_FIRST Imposes Performance Penalty -\end_layout - -\begin_layout Standard -When TDB_CLEAR_IF_FIRST is specified, a 1-byte read lock is placed at offset - 4 (aka. - the ACTIVE_LOCK). - While these locks never conflict in normal tdb usage, they do add substantial - overhead for most fcntl lock implementations when the kernel scans to detect - if a lock conflict exists. - This is often a single linked list, making the time to acquire and release - a fcntl lock O(N) where N is the number of processes with the TDB open, - not the number actually doing work. -\end_layout - -\begin_layout Standard -In a Samba server it is common to have huge numbers of clients sitting idle, - and thus they have weaned themselves off the TDB_CLEAR_IF_FIRST flag. -\begin_inset Foot -status collapsed - -\begin_layout Plain Layout -There is a flag to tdb_reopen_all() which is used for this optimization: - if the parent process will outlive the child, the child does not need the - ACTIVE_LOCK. - This is a workaround for this very performance issue. -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Subsubsection -Proposed Solution -\end_layout - -\begin_layout Standard -Remove the flag. - It was a neat idea, but even trivial servers tend to know when they are - initializing for the first time and can simply unlink the old tdb at that - point. -\end_layout - -\begin_layout Subsubsection -Status -\end_layout - -\begin_layout Standard - -\change_deleted 0 1298979837 -Incomplete; TDB_CLEAR_IF_FIRST still defined, but does nothing. -\change_inserted 0 1298979837 -Complete. -\change_unchanged - -\end_layout - -\begin_layout Subsection -TDB Files Have a 4G Limit -\end_layout - -\begin_layout Standard -This seems to be becoming an issue (so much for -\begin_inset Quotes eld -\end_inset - -trivial -\begin_inset Quotes erd -\end_inset - -!), particularly for ldb. -\end_layout - -\begin_layout Subsubsection -Proposed Solution -\end_layout - -\begin_layout Standard -A new, incompatible TDB format which uses 64 bit offsets internally rather - than 32 bit as now. - For simplicity of endian conversion (which TDB does on the fly if required), - all values will be 64 bit on disk. - In practice, some upper bits may be used for other purposes, but at least - 56 bits will be available for file offsets. -\end_layout - -\begin_layout Standard -tdb_open() will automatically detect the old version, and even create them - if TDB_VERSION6 is specified to tdb_open. -\end_layout - -\begin_layout Standard -32 bit processes will still be able to access TDBs larger than 4G (assuming - that their off_t allows them to seek to 64 bits), they will gracefully - fall back as they fail to mmap. - This can happen already with large TDBs. -\end_layout - -\begin_layout Standard -Old versions of tdb will fail to open the new TDB files (since 28 August - 2009, commit 398d0c29290: prior to that any unrecognized file format would - be erased and initialized as a fresh tdb!) -\end_layout - -\begin_layout Subsubsection -Status -\end_layout - -\begin_layout Standard -Complete. -\end_layout - -\begin_layout Subsection -TDB Records Have a 4G Limit -\end_layout - -\begin_layout Standard -This has not been a reported problem, and the API uses size_t which can - be 64 bit on 64 bit platforms. - However, other limits may have made such an issue moot. -\end_layout - -\begin_layout Subsubsection -Proposed Solution -\end_layout - -\begin_layout Standard -Record sizes will be 64 bit, with an error returned on 32 bit platforms - which try to access such records (the current implementation would return - TDB_ERR_OOM in a similar case). - It seems unlikely that 32 bit keys will be a limitation, so the implementation - may not support this (see -\begin_inset CommandInset ref -LatexCommand ref -reference "sub:Records-Incur-A" - -\end_inset - -). -\end_layout - -\begin_layout Subsubsection -Status -\end_layout - -\begin_layout Standard -Complete. -\end_layout - -\begin_layout Subsection -Hash Size Is Determined At TDB Creation Time -\end_layout - -\begin_layout Standard -TDB contains a number of hash chains in the header; the number is specified - at creation time, and defaults to 131. - This is such a bottleneck on large databases (as each hash chain gets quite - long), that LDB uses 10,000 for this hash. - In general it is impossible to know what the 'right' answer is at database - creation time. -\end_layout - -\begin_layout Subsubsection -\begin_inset CommandInset label -LatexCommand label -name "sub:Hash-Size-Solution" - -\end_inset - -Proposed Solution -\end_layout - -\begin_layout Standard -After comprehensive performance testing on various scalable hash variants -\begin_inset Foot -status collapsed - -\begin_layout Plain Layout -http://rusty.ozlabs.org/?p=89 and http://rusty.ozlabs.org/?p=94 This was annoying - because I was previously convinced that an expanding tree of hashes would - be very close to optimal. -\end_layout - -\end_inset - -, it became clear that it is hard to beat a straight linear hash table which - doubles in size when it reaches saturation. - Unfortunately, altering the hash table introduces serious locking complications -: the entire hash table needs to be locked to enlarge the hash table, and - others might be holding locks. - Particularly insidious are insertions done under tdb_chainlock. -\end_layout - -\begin_layout Standard -Thus an expanding layered hash will be used: an array of hash groups, with - each hash group exploding into pointers to lower hash groups once it fills, - turning into a hash tree. - This has implications for locking: we must lock the entire group in case - we need to expand it, yet we don't know how deep the tree is at that point. -\end_layout - -\begin_layout Standard -Note that bits from the hash table entries should be stolen to hold more - hash bits to reduce the penalty of collisions. - We can use the otherwise-unused lower 3 bits. - If we limit the size of the database to 64 exabytes, we can use the top - 8 bits of the hash entry as well. - These 11 bits would reduce false positives down to 1 in 2000 which is more - than we need: we can use one of the bits to indicate that the extra hash - bits are valid. - This means we can choose not to re-hash all entries when we expand a hash - group; simply use the next bits we need and mark them invalid. -\end_layout - -\begin_layout Subsubsection -Status -\end_layout - -\begin_layout Standard -Complete. -\end_layout - -\begin_layout Subsection -\begin_inset CommandInset label -LatexCommand label -name "TDB-Freelist-Is" - -\end_inset - -TDB Freelist Is Highly Contended -\end_layout - -\begin_layout Standard -TDB uses a single linked list for the free list. - Allocation occurs as follows, using heuristics which have evolved over - time: -\end_layout - -\begin_layout Enumerate -Get the free list lock for this whole operation. -\end_layout - -\begin_layout Enumerate -Multiply length by 1.25, so we always over-allocate by 25%. -\end_layout - -\begin_layout Enumerate -Set the slack multiplier to 1. -\end_layout - -\begin_layout Enumerate -Examine the current freelist entry: if it is > length but < the current - best case, remember it as the best case. -\end_layout - -\begin_layout Enumerate -Multiply the slack multiplier by 1.05. -\end_layout - -\begin_layout Enumerate -If our best fit so far is less than length * slack multiplier, return it. - The slack will be turned into a new free record if it's large enough. -\end_layout - -\begin_layout Enumerate -Otherwise, go onto the next freelist entry. -\end_layout - -\begin_layout Standard -Deleting a record occurs as follows: -\end_layout - -\begin_layout Enumerate -Lock the hash chain for this whole operation. -\end_layout - -\begin_layout Enumerate -Walk the chain to find the record, keeping the prev pointer offset. -\end_layout - -\begin_layout Enumerate -If max_dead is non-zero: -\end_layout - -\begin_deeper -\begin_layout Enumerate -Walk the hash chain again and count the dead records. -\end_layout - -\begin_layout Enumerate -If it's more than max_dead, bulk free all the dead ones (similar to steps - 4 and below, but the lock is only obtained once). -\end_layout - -\begin_layout Enumerate -Simply mark this record as dead and return. - -\end_layout - -\end_deeper -\begin_layout Enumerate -Get the free list lock for the remainder of this operation. -\end_layout - -\begin_layout Enumerate -\begin_inset CommandInset label -LatexCommand label -name "right-merging" - -\end_inset - -Examine the following block to see if it is free; if so, enlarge the current - block and remove that block from the free list. - This was disabled, as removal from the free list was O(entries-in-free-list). -\end_layout - -\begin_layout Enumerate -Examine the preceeding block to see if it is free: for this reason, each - block has a 32-bit tailer which indicates its length. - If it is free, expand it to cover our new block and return. -\end_layout - -\begin_layout Enumerate -Otherwise, prepend ourselves to the free list. -\end_layout - -\begin_layout Standard -Disabling right-merging (step -\begin_inset CommandInset ref -LatexCommand ref -reference "right-merging" - -\end_inset - -) causes fragmentation; the other heuristics proved insufficient to address - this, so the final answer to this was that when we expand the TDB file - inside a transaction commit, we repack the entire tdb. -\end_layout - -\begin_layout Standard -The single list lock limits our allocation rate; due to the other issues - this is not currently seen as a bottleneck. -\end_layout - -\begin_layout Subsubsection -Proposed Solution -\end_layout - -\begin_layout Standard -The first step is to remove all the current heuristics, as they obviously - interact, then examine them once the lock contention is addressed. -\end_layout - -\begin_layout Standard -The free list must be split to reduce contention. - Assuming perfect free merging, we can at most have 1 free list entry for - each entry. - This implies that the number of free lists is related to the size of the - hash table, but as it is rare to walk a large number of free list entries - we can use far fewer, say 1/32 of the number of hash buckets. -\end_layout - -\begin_layout Standard -It seems tempting to try to reuse the hash implementation which we use for - records here, but we have two ways of searching for free entries: for allocatio -n we search by size (and possibly zone) which produces too many clashes - for our hash table to handle well, and for coalescing we search by address. - Thus an array of doubly-linked free lists seems preferable. -\end_layout - -\begin_layout Standard -There are various benefits in using per-size free lists (see -\begin_inset CommandInset ref -LatexCommand ref -reference "sub:TDB-Becomes-Fragmented" - -\end_inset - -) but it's not clear this would reduce contention in the common case where - all processes are allocating/freeing the same size. - Thus we almost certainly need to divide in other ways: the most obvious - is to divide the file into zones, and using a free list (or table of free - lists) for each. - This approximates address ordering. -\end_layout - -\begin_layout Standard -Unfortunately it is difficult to know what heuristics should be used to - determine zone sizes, and our transaction code relies on being able to - create a -\begin_inset Quotes eld -\end_inset - -recovery area -\begin_inset Quotes erd -\end_inset - - by simply appending to the file (difficult if it would need to create a - new zone header). - Thus we use a linked-list of free tables; currently we only ever create - one, but if there is more than one we choose one at random to use. - In future we may use heuristics to add new free tables on contention. - We only expand the file when all free tables are exhausted. -\end_layout - -\begin_layout Standard -The basic algorithm is as follows. - Freeing is simple: -\end_layout - -\begin_layout Enumerate -Identify the correct free list. -\end_layout - -\begin_layout Enumerate -Lock the corresponding list. -\end_layout - -\begin_layout Enumerate -Re-check the list (we didn't have a lock, sizes could have changed): relock - if necessary. -\end_layout - -\begin_layout Enumerate -Place the freed entry in the list. -\end_layout - -\begin_layout Standard -Allocation is a little more complicated, as we perform delayed coalescing - at this point: -\end_layout - -\begin_layout Enumerate -Pick a free table; usually the previous one. -\end_layout - -\begin_layout Enumerate -Lock the corresponding list. -\end_layout - -\begin_layout Enumerate -If the top entry is -large enough, remove it from the list and return it. -\end_layout - -\begin_layout Enumerate -Otherwise, coalesce entries in the list.If there was no entry large enough, - unlock the list and try the next largest list -\end_layout - -\begin_layout Enumerate -If no list has an entry which meets our needs, try the next free table. -\end_layout - -\begin_layout Enumerate -If no zone satisfies, expand the file. -\end_layout - -\begin_layout Standard -This optimizes rapid insert/delete of free list entries by not coalescing - them all the time.. - First-fit address ordering ordering seems to be fairly good for keeping - fragmentation low (see -\begin_inset CommandInset ref -LatexCommand ref -reference "sub:TDB-Becomes-Fragmented" - -\end_inset - -). - Note that address ordering does not need a tailer to coalesce, though if - we needed one we could have one cheaply: see -\begin_inset CommandInset ref -LatexCommand ref -reference "sub:Records-Incur-A" - -\end_inset - -. - -\end_layout - -\begin_layout Standard -Each free entry has the free table number in the header: less than 255. - It also contains a doubly-linked list for easy deletion. -\end_layout - -\begin_layout Subsection -\begin_inset CommandInset label -LatexCommand label -name "sub:TDB-Becomes-Fragmented" - -\end_inset - -TDB Becomes Fragmented -\end_layout - -\begin_layout Standard -Much of this is a result of allocation strategy -\begin_inset Foot -status collapsed - -\begin_layout Plain Layout -The Memory Fragmentation Problem: Solved? Johnstone & Wilson 1995 ftp://ftp.cs.ute -xas.edu/pub/garbage/malloc/ismm98.ps -\end_layout - -\end_inset - - and deliberate hobbling of coalescing; internal fragmentation (aka overallocati -on) is deliberately set at 25%, and external fragmentation is only cured - by the decision to repack the entire db when a transaction commit needs - to enlarge the file. -\end_layout - -\begin_layout Subsubsection -Proposed Solution -\end_layout - -\begin_layout Standard -The 25% overhead on allocation works in practice for ldb because indexes - tend to expand by one record at a time. - This internal fragmentation can be resolved by having an -\begin_inset Quotes eld -\end_inset - -expanded -\begin_inset Quotes erd -\end_inset - - bit in the header to note entries that have previously expanded, and allocating - more space for them. -\end_layout - -\begin_layout Standard -There are is a spectrum of possible solutions for external fragmentation: - one is to use a fragmentation-avoiding allocation strategy such as best-fit - address-order allocator. - The other end of the spectrum would be to use a bump allocator (very fast - and simple) and simply repack the file when we reach the end. -\end_layout - -\begin_layout Standard -There are three problems with efficient fragmentation-avoiding allocators: - they are non-trivial, they tend to use a single free list for each size, - and there's no evidence that tdb allocation patterns will match those recorded - for general allocators (though it seems likely). -\end_layout - -\begin_layout Standard -Thus we don't spend too much effort on external fragmentation; we will be - no worse than the current code if we need to repack on occasion. - More effort is spent on reducing freelist contention, and reducing overhead. -\end_layout - -\begin_layout Subsection -\begin_inset CommandInset label -LatexCommand label -name "sub:Records-Incur-A" - -\end_inset - -Records Incur A 28-Byte Overhead -\end_layout - -\begin_layout Standard -Each TDB record has a header as follows: -\end_layout - -\begin_layout LyX-Code -struct tdb_record { -\end_layout - -\begin_layout LyX-Code - tdb_off_t next; /* offset of the next record in the list */ -\end_layout - -\begin_layout LyX-Code - tdb_len_t rec_len; /* total byte length of record */ -\end_layout - -\begin_layout LyX-Code - tdb_len_t key_len; /* byte length of key */ -\end_layout - -\begin_layout LyX-Code - tdb_len_t data_len; /* byte length of data */ -\end_layout - -\begin_layout LyX-Code - uint32_t full_hash; /* the full 32 bit hash of the key */ -\end_layout - -\begin_layout LyX-Code - uint32_t magic; /* try to catch errors */ -\end_layout - -\begin_layout LyX-Code - /* the following union is implied: -\end_layout - -\begin_layout LyX-Code - union { -\end_layout - -\begin_layout LyX-Code - char record[rec_len]; -\end_layout - -\begin_layout LyX-Code - struct { -\end_layout - -\begin_layout LyX-Code - char key[key_len]; -\end_layout - -\begin_layout LyX-Code - char data[data_len]; -\end_layout - -\begin_layout LyX-Code - } -\end_layout - -\begin_layout LyX-Code - uint32_t totalsize; (tailer) -\end_layout - -\begin_layout LyX-Code - } -\end_layout - -\begin_layout LyX-Code - */ -\end_layout - -\begin_layout LyX-Code -}; -\end_layout - -\begin_layout Standard -Naively, this would double to a 56-byte overhead on a 64 bit implementation. -\end_layout - -\begin_layout Subsubsection -Proposed Solution -\end_layout - -\begin_layout Standard -We can use various techniques to reduce this for an allocated block: -\end_layout - -\begin_layout Enumerate -The 'next' pointer is not required, as we are using a flat hash table. -\end_layout - -\begin_layout Enumerate -'rec_len' can instead be expressed as an addition to key_len and data_len - (it accounts for wasted or overallocated length in the record). - Since the record length is always a multiple of 8, we can conveniently - fit it in 32 bits (representing up to 35 bits). -\end_layout - -\begin_layout Enumerate -'key_len' and 'data_len' can be reduced. - I'm unwilling to restrict 'data_len' to 32 bits, but instead we can combine - the two into one 64-bit field and using a 5 bit value which indicates at - what bit to divide the two. - Keys are unlikely to scale as fast as data, so I'm assuming a maximum key - size of 32 bits. -\end_layout - -\begin_layout Enumerate -'full_hash' is used to avoid a memcmp on the -\begin_inset Quotes eld -\end_inset - -miss -\begin_inset Quotes erd -\end_inset - - case, but this is diminishing returns after a handful of bits (at 10 bits, - it reduces 99.9% of false memcmp). - As an aside, as the lower bits are already incorporated in the hash table - resolution, the upper bits should be used here. - Note that it's not clear that these bits will be a win, given the extra - bits in the hash table itself (see -\begin_inset CommandInset ref -LatexCommand ref -reference "sub:Hash-Size-Solution" - -\end_inset - -). -\end_layout - -\begin_layout Enumerate -'magic' does not need to be enlarged: it currently reflects one of 5 values - (used, free, dead, recovery, and unused_recovery). - It is useful for quick sanity checking however, and should not be eliminated. -\end_layout - -\begin_layout Enumerate -'tailer' is only used to coalesce free blocks (so a block to the right can - find the header to check if this block is free). - This can be replaced by a single 'free' bit in the header of the following - block (and the tailer only exists in free blocks). -\begin_inset Foot -status collapsed - -\begin_layout Plain Layout -This technique from Thomas Standish. - Data Structure Techniques. - Addison-Wesley, Reading, Massachusetts, 1980. -\end_layout - -\end_inset - - The current proposed coalescing algorithm doesn't need this, however. -\end_layout - -\begin_layout Standard -This produces a 16 byte used header like this: -\end_layout - -\begin_layout LyX-Code -struct tdb_used_record { -\end_layout - -\begin_layout LyX-Code - uint32_t used_magic : 16, -\end_layout - -\begin_layout LyX-Code - -\end_layout - -\begin_layout LyX-Code - key_data_divide: 5, -\end_layout - -\begin_layout LyX-Code - top_hash: 11; -\end_layout - -\begin_layout LyX-Code - uint32_t extra_octets; -\end_layout - -\begin_layout LyX-Code - uint64_t key_and_data_len; -\end_layout - -\begin_layout LyX-Code -}; -\end_layout - -\begin_layout Standard -And a free record like this: -\end_layout - -\begin_layout LyX-Code -struct tdb_free_record { -\end_layout - -\begin_layout LyX-Code - uint64_t free_magic: 8, -\end_layout - -\begin_layout LyX-Code - prev : 56; -\end_layout - -\begin_layout LyX-Code - -\end_layout - -\begin_layout LyX-Code - uint64_t free_table: 8, -\end_layout - -\begin_layout LyX-Code - total_length : 56 -\end_layout - -\begin_layout LyX-Code - uint64_t next;; -\end_layout - -\begin_layout LyX-Code -}; -\end_layout - -\begin_layout Standard - -\change_deleted 0 1291206079 - -\change_unchanged -Note that by limiting valid offsets to 56 bits, we can pack everything we - need into 3 64-byte words, meaning our minimum record size is 8 bytes. -\end_layout - -\begin_layout Subsubsection -Status -\end_layout - -\begin_layout Standard -Complete. -\end_layout - -\begin_layout Subsection -Transaction Commit Requires 4 fdatasync -\end_layout - -\begin_layout Standard -The current transaction algorithm is: -\end_layout - -\begin_layout Enumerate -write_recovery_data(); -\end_layout - -\begin_layout Enumerate -sync(); -\end_layout - -\begin_layout Enumerate -write_recovery_header(); -\end_layout - -\begin_layout Enumerate -sync(); -\end_layout - -\begin_layout Enumerate -overwrite_with_new_data(); -\end_layout - -\begin_layout Enumerate -sync(); -\end_layout - -\begin_layout Enumerate -remove_recovery_header(); -\end_layout - -\begin_layout Enumerate -sync(); -\end_layout - -\begin_layout Standard -On current ext3, each sync flushes all data to disk, so the next 3 syncs - are relatively expensive. - But this could become a performance bottleneck on other filesystems such - as ext4. -\end_layout - -\begin_layout Subsubsection -Proposed Solution -\end_layout - -\begin_layout Standard -Neil Brown points out that this is overzealous, and only one sync is needed: -\end_layout - -\begin_layout Enumerate -Bundle the recovery data, a transaction counter and a strong checksum of - the new data. -\end_layout - -\begin_layout Enumerate -Strong checksum that whole bundle. -\end_layout - -\begin_layout Enumerate -Store the bundle in the database. -\end_layout - -\begin_layout Enumerate -Overwrite the oldest of the two recovery pointers in the header (identified - using the transaction counter) with the offset of this bundle. -\end_layout - -\begin_layout Enumerate -sync. -\end_layout - -\begin_layout Enumerate -Write the new data to the file. -\end_layout - -\begin_layout Standard -Checking for recovery means identifying the latest bundle with a valid checksum - and using the new data checksum to ensure that it has been applied. - This is more expensive than the current check, but need only be done at - open. - For running databases, a separate header field can be used to indicate - a transaction in progress; we need only check for recovery if this is set. -\end_layout - -\begin_layout Subsubsection -Status -\end_layout - -\begin_layout Standard -Deferred. -\end_layout - -\begin_layout Subsection -\begin_inset CommandInset label -LatexCommand label -name "sub:TDB-Does-Not" - -\end_inset - -TDB Does Not Have Snapshot Support -\end_layout - -\begin_layout Subsubsection -Proposed SolutionNone. - At some point you say -\begin_inset Quotes eld -\end_inset - -use a real database -\begin_inset Quotes erd -\end_inset - - (but see -\begin_inset CommandInset ref -LatexCommand ref -reference "replay-attribute" - -\end_inset - -). -\end_layout - -\begin_layout Standard -But as a thought experiment, if we implemented transactions to only overwrite - free entries (this is tricky: there must not be a header in each entry - which indicates whether it is free, but use of presence in metadata elsewhere), - and a pointer to the hash table, we could create an entirely new commit - without destroying existing data. - Then it would be easy to implement snapshots in a similar way. -\end_layout - -\begin_layout Standard -This would not allow arbitrary changes to the database, such as tdb_repack - does, and would require more space (since we have to preserve the current - and future entries at once). - If we used hash trees rather than one big hash table, we might only have - to rewrite some sections of the hash, too. -\end_layout - -\begin_layout Standard -We could then implement snapshots using a similar method, using multiple - different hash tables/free tables. -\end_layout - -\begin_layout Subsubsection -Status -\end_layout - -\begin_layout Standard -Deferred. -\end_layout - -\begin_layout Subsection -Transactions Cannot Operate in Parallel -\end_layout - -\begin_layout Standard -This would be useless for ldb, as it hits the index records with just about - every update. - It would add significant complexity in resolving clashes, and cause the - all transaction callers to write their code to loop in the case where the - transactions spuriously failed. -\end_layout - -\begin_layout Subsubsection -Proposed Solution -\end_layout - -\begin_layout Standard -None (but see -\begin_inset CommandInset ref -LatexCommand ref -reference "replay-attribute" - -\end_inset - -). - We could solve a small part of the problem by providing read-only transactions. - These would allow one write transaction to begin, but it could not commit - until all r/o transactions are done. - This would require a new RO_TRANSACTION_LOCK, which would be upgraded on - commit. -\end_layout - -\begin_layout Subsubsection -Status -\end_layout - -\begin_layout Standard -Deferred. -\end_layout - -\begin_layout Subsection -Default Hash Function Is Suboptimal -\end_layout - -\begin_layout Standard -The Knuth-inspired multiplicative hash used by tdb is fairly slow (especially - if we expand it to 64 bits), and works best when the hash bucket size is - a prime number (which also means a slow modulus). - In addition, it is highly predictable which could potentially lead to a - Denial of Service attack in some TDB uses. -\end_layout - -\begin_layout Subsubsection -Proposed Solution -\end_layout - -\begin_layout Standard -The Jenkins lookup3 hash -\begin_inset Foot -status open - -\begin_layout Plain Layout -http://burtleburtle.net/bob/c/lookup3.c -\end_layout - -\end_inset - - is a fast and superbly-mixing hash. - It's used by the Linux kernel and almost everything else. - This has the particular properties that it takes an initial seed, and produces - two 32 bit hash numbers, which we can combine into a 64-bit hash. -\end_layout - -\begin_layout Standard -The seed should be created at tdb-creation time from some random source, - and placed in the header. - This is far from foolproof, but adds a little bit of protection against - hash bombing. -\end_layout - -\begin_layout Subsubsection -Status -\end_layout - -\begin_layout Standard -Complete. -\end_layout - -\begin_layout Subsection -\begin_inset CommandInset label -LatexCommand label -name "Reliable-Traversal-Adds" - -\end_inset - -Reliable Traversal Adds Complexity -\end_layout - -\begin_layout Standard -We lock a record during traversal iteration, and try to grab that lock in - the delete code. - If that grab on delete fails, we simply mark it deleted and continue onwards; - traversal checks for this condition and does the delete when it moves off - the record. -\end_layout - -\begin_layout Standard -If traversal terminates, the dead record may be left indefinitely. -\end_layout - -\begin_layout Subsubsection -Proposed Solution -\end_layout - -\begin_layout Standard -Remove reliability guarantees; see -\begin_inset CommandInset ref -LatexCommand ref -reference "traverse-Proposed-Solution" - -\end_inset - -. -\end_layout - -\begin_layout Subsubsection -Status -\end_layout - -\begin_layout Standard -Complete. -\end_layout - -\begin_layout Subsection -Fcntl Locking Adds Overhead -\end_layout - -\begin_layout Standard -Placing a fcntl lock means a system call, as does removing one. - This is actually one reason why transactions can be faster (everything - is locked once at transaction start). - In the uncontended case, this overhead can theoretically be eliminated. -\end_layout - -\begin_layout Subsubsection -Proposed Solution -\end_layout - -\begin_layout Standard -None. -\end_layout - -\begin_layout Standard -We tried this before with spinlock support, in the early days of TDB, and - it didn't make much difference except in manufactured benchmarks. -\end_layout - -\begin_layout Standard -We could use spinlocks (with futex kernel support under Linux), but it means - that we lose automatic cleanup when a process dies with a lock. - There is a method of auto-cleanup under Linux, but it's not supported by - other operating systems. - We could reintroduce a clear-if-first-style lock and sweep for dead futexes - on open, but that wouldn't help the normal case of one concurrent opener - dying. - Increasingly elaborate repair schemes could be considered, but they require - an ABI change (everyone must use them) anyway, so there's no need to do - this at the same time as everything else. -\end_layout - -\begin_layout Subsection -Some Transactions Don't Require Durability -\end_layout - -\begin_layout Standard -Volker points out that gencache uses a CLEAR_IF_FIRST tdb for normal (fast) - usage, and occasionally empties the results into a transactional TDB. - This kind of usage prioritizes performance over durability: as long as - we are consistent, data can be lost. -\end_layout - -\begin_layout Standard -This would be more neatly implemented inside tdb: a -\begin_inset Quotes eld -\end_inset - -soft -\begin_inset Quotes erd -\end_inset - - transaction commit (ie. - syncless) which meant that data may be reverted on a crash. -\end_layout - -\begin_layout Subsubsection -Proposed Solution -\end_layout - -\begin_layout Standard -None. -\end_layout - -\begin_layout Standard -Unfortunately any transaction scheme which overwrites old data requires - a sync before that overwrite to avoid the possibility of corruption. -\end_layout - -\begin_layout Standard -It seems possible to use a scheme similar to that described in -\begin_inset CommandInset ref -LatexCommand ref -reference "sub:TDB-Does-Not" - -\end_inset - -,where transactions are committed without overwriting existing data, and - an array of top-level pointers were available in the header. - If the transaction is -\begin_inset Quotes eld -\end_inset - -soft -\begin_inset Quotes erd -\end_inset - - then we would not need a sync at all: existing processes would pick up - the new hash table and free list and work with that. -\end_layout - -\begin_layout Standard -At some later point, a sync would allow recovery of the old data into the - free lists (perhaps when the array of top-level pointers filled). - On crash, tdb_open() would examine the array of top levels, and apply the - transactions until it encountered an invalid checksum. -\end_layout - -\begin_layout Subsection -Tracing Is Fragile, Replay Is External -\end_layout - -\begin_layout Standard -The current TDB has compile-time-enabled tracing code, but it often breaks - as it is not enabled by default. - In a similar way, the ctdb code has an external wrapper which does replay - tracing so it can coordinate cluster-wide transactions. -\end_layout - -\begin_layout Subsubsection -Proposed Solution -\begin_inset CommandInset label -LatexCommand label -name "replay-attribute" - -\end_inset - - -\end_layout - -\begin_layout Standard -Tridge points out that an attribute can be later added to tdb_open (see - -\begin_inset CommandInset ref -LatexCommand ref -reference "attributes" - -\end_inset - -) to provide replay/trace hooks, which could become the basis for this and - future parallel transactions and snapshot support. -\end_layout - -\begin_layout Subsubsection -Status -\end_layout - -\begin_layout Standard -Deferred. -\end_layout - -\end_body -\end_document -@ - - -1.12 -log -@Add status, some fixes, linked freelists. -@ -text -@d53 1 -a53 7 - -\change_deleted 0 1291204535 -14-September -\change_inserted 0 1291204533 -1-December -\change_unchanged --2010 -a580 2 -\change_inserted 0 1291204563 - -a583 2 - -\change_inserted 0 1291204572 -a587 2 - -\change_inserted 0 1291204573 -a588 2 -\change_unchanged - -a629 2 -\change_inserted 0 1291204588 - -a632 2 - -\change_inserted 0 1291204588 -a636 2 - -\change_inserted 0 1291204631 -a639 2 -\change_unchanged - -a693 2 -\change_inserted 0 1291204639 - -a696 2 - -\change_inserted 0 1291204640 -d702 1 -a702 1 -\change_inserted 0 1291204665 -d704 2 -a728 2 -\change_inserted 0 1291204671 - -a731 2 - -\change_inserted 0 1291204671 -a735 2 - -\change_inserted 0 1291204673 -a736 2 -\change_unchanged - -a780 2 -\change_inserted 0 1291204731 - -a783 2 - -\change_inserted 0 1291204732 -a787 2 - -\change_inserted 0 1291204779 -a790 2 -\change_unchanged - -a842 2 -\change_inserted 0 1291204830 - -a845 2 - -\change_inserted 0 1291204831 -a849 2 - -\change_inserted 0 1291204834 -a850 2 -\change_unchanged - -d879 9 -a887 2 - deal of churn; we are better to guarantee that the tdb_errcode is per-thread - so the current programming model can be maintained. -d891 9 -d903 2 -a922 2 -\change_inserted 0 1291204847 - -a925 2 - -\change_inserted 0 1291204847 -d930 5 -a934 3 - -\change_inserted 0 1291204852 -Incomplete. -a1051 2 -\change_inserted 0 1291204881 - -a1054 2 - -\change_inserted 0 1291204881 -a1058 2 - -\change_inserted 0 1291204885 -a1059 2 -\change_unchanged - -a1140 2 -\change_inserted 0 1291204898 - -a1143 2 - -\change_inserted 0 1291204898 -a1147 2 - -\change_inserted 0 1291204901 -a1148 2 -\change_unchanged - -a1224 2 -\change_inserted 0 1291204908 - -a1227 2 - -\change_inserted 0 1291204908 -a1231 2 - -\change_inserted 0 1291204908 -a1232 2 -\change_unchanged - -a1271 2 -\change_inserted 0 1291204917 - -a1274 2 - -\change_inserted 0 1291204917 -a1278 2 - -\change_inserted 0 1291204920 -a1279 2 -\change_unchanged - -a1316 2 -\change_inserted 0 1291204927 - -a1319 2 - -\change_inserted 0 1291204928 -d1325 1 -a1325 1 -\change_inserted 0 1291204942 -d1327 2 -a1381 2 -\change_inserted 0 1291205003 - -a1384 2 - -\change_inserted 0 1291205004 -a1388 2 - -\change_inserted 0 1291205007 -a1411 2 -\change_inserted 0 1291205019 - -a1414 2 - -\change_inserted 0 1291205019 -a1418 2 - -\change_inserted 0 1291205023 -a1419 2 -\change_unchanged - -a1465 2 -\change_inserted 0 1291205029 - -a1468 2 - -\change_inserted 0 1291205029 -a1472 2 - -\change_inserted 0 1291206020 -a1473 2 -\change_unchanged - -a1528 2 -\change_inserted 0 1291205043 - -a1531 2 - -\change_inserted 0 1291205043 -d1537 1 -a1537 1 -\change_inserted 0 1291205057 -d1539 2 -a1589 2 -\change_inserted 0 1291205062 - -a1592 2 - -\change_inserted 0 1291205062 -a1596 2 - -\change_inserted 0 1291205062 -a1597 2 -\change_unchanged - -a1626 2 -\change_inserted 0 1291205072 - -a1629 2 - -\change_inserted 0 1291205073 -a1633 2 - -\change_inserted 0 1291205073 -a1634 2 -\change_unchanged - -a1674 4 - -\change_deleted 0 1291204504 - -\change_unchanged -a1699 2 -\change_inserted 0 1291205079 - -a1702 2 - -\change_inserted 0 1291205080 -a1706 2 - -\change_inserted 0 1291205080 -a1707 2 -\change_unchanged - -a1833 2 -\change_inserted 0 1291205090 - -d1869 2 -a1870 7 - is to divide the file into zones, and using a free list (or -\change_inserted 0 1291205498 -table -\change_deleted 0 1291205497 -set -\change_unchanged - of free lists) for each. -a1871 2 -\change_inserted 0 1291205203 - -a1874 2 - -\change_inserted 0 1291205358 -a1890 21 -\change_unchanged - -\end_layout - -\begin_layout Standard - -\change_deleted 0 1291205198 -Note that this means we need to split the free lists when we expand the - file; this is probably acceptable when we double the hash table size, since - that is such an expensive operation already. - In the case of increasing the file size, there is an optimization we can - use: if we use M in the formula above as the file size rounded up to the - next power of 2, we only need reshuffle free lists when the file size crosses - a power of 2 boundary, -\emph on -and -\emph default -reshuffling the free lists is trivial: we simply merge every consecutive - pair of free lists. -\change_unchanged - -d1899 1 -a1899 7 -Identify the correct -\change_inserted 0 1291205366 -free list -\change_deleted 0 1291205364 -zone -\change_unchanged -. -d1907 2 -a1908 7 -Re-check the -\change_inserted 0 1291205372 -list -\change_deleted 0 1291205371 -zone -\change_unchanged - (we didn't have a lock, sizes could have changed): relock if necessary. -d1912 1 -a1912 5 -Place the freed entry in the list -\change_deleted 0 1291205382 - for that zone -\change_unchanged -. -d1921 1 -a1921 15 -Pick a -\change_deleted 0 1291205403 -zone either the zone we last freed into, or based on a -\begin_inset Quotes eld -\end_inset - -random -\begin_inset Quotes erd -\end_inset - - number. -\change_inserted 0 1291205411 -free table; usually the previous one. -\change_unchanged - -a1925 10 -\change_deleted 0 1291205432 - -\end_layout - -\begin_layout Enumerate - -\change_deleted 0 1291205428 -Re-check the zone: relock if necessary. -\change_unchanged - -d1934 1 -a1934 7 - unlock the list and try the next -\change_inserted 0 1291205455 -largest list -\change_deleted 0 1291205452 -zone. -\change_inserted 0 1291205457 - -a1937 2 - -\change_inserted 0 1291205476 -a1938 2 -\change_unchanged - -a1966 2 -\change_inserted 0 1291205542 - -a1969 2 - -\change_inserted 0 1291205591 -a1971 70 -\change_unchanged - -\end_layout - -\begin_layout Standard - -\change_deleted 0 1291205539 -I anticipate that the number of entries in each free zone would be small, - but it might be worth using one free entry to hold pointers to the others - for cache efficiency. -\change_unchanged - -\end_layout - -\begin_layout Standard - -\change_deleted 0 1291205534 -\begin_inset CommandInset label -LatexCommand label -name "freelist-in-zone" - -\end_inset - -If we want to avoid locking complexity (enlarging the free lists when we - enlarge the file) we could place the array of free lists at the beginning - of each zone. - This means existing array lists never move, but means that a record cannot - be larger than a zone. - That in turn implies that zones should be variable sized (say, power of - 2), which makes the question -\begin_inset Quotes eld -\end_inset - -what zone is this record in? -\begin_inset Quotes erd -\end_inset - - much harder (and -\begin_inset Quotes eld -\end_inset - -pick a random zone -\begin_inset Quotes erd -\end_inset - -, but that's less common). - It could be done with as few as 4 bits from the record header. -\begin_inset Foot -status collapsed - -\begin_layout Plain Layout -Using -\begin_inset Formula $2^{16+N*3}$ -\end_inset - -means 0 gives a minimal 65536-byte zone, 15 gives the maximal -\begin_inset Formula $2^{61}$ -\end_inset - - byte zone. - Zones range in factor of 8 steps. - Given the zone size for the zone the current record is in, we can determine - the start of the zone. -\end_layout - -\end_inset - - -\change_inserted 0 1291205139 - -d2218 1 -a2218 5 - uint32_t -\change_inserted 0 1291205758 -used_ -\change_unchanged -magic : 16, -a2222 4 -\change_deleted 0 1291205693 - prev_is_free: 1, -\change_unchanged - -d2230 1 -a2230 7 - top_hash: 1 -\change_inserted 0 1291205704 -1 -\change_deleted 0 1291205704 -0 -\change_unchanged -; -d2254 1 -a2254 9 - uint -\change_inserted 0 1291205725 -64 -\change_deleted 0 1291205723 -32 -\change_unchanged -_t -\change_inserted 0 1291205753 -free_magic: 8, -a2257 2 - -\change_inserted 0 1291205746 -a2262 24 -\change_deleted 0 1291205749 -free_magic; -\change_unchanged - -\end_layout - -\begin_layout LyX-Code - uint64_t -\change_inserted 0 1291205786 -free_table: 8, -\end_layout - -\begin_layout LyX-Code - -\change_inserted 0 1291205788 - -\change_unchanged -total_length -\change_inserted 0 1291205792 - : 56 -\change_deleted 0 1291205790 -; -\change_unchanged - -d2266 1 -a2266 7 - uint64_t -\change_deleted 0 1291205801 -prev, -\change_unchanged -next; -\change_deleted 0 1291205811 - -d2270 1 -a2270 3 - -\change_deleted 0 1291205811 - ... -d2274 1 -a2274 5 - -\change_deleted 0 1291205808 - uint64_t tailer -\change_unchanged -; -d2283 5 -a2287 16 -\change_deleted 0 1291205827 -We might want to take some bits from the used record's top_hash (and the - free record which has 32 bits of padding to spare anyway) if we use variable - sized zones. - See -\begin_inset CommandInset ref -LatexCommand ref -reference "freelist-in-zone" - -\end_inset - -. - -\change_inserted 0 1291205885 - Note that by limiting valid offsets to 56 bits, we can pack everything - we need into 3 64-byte words, meaning our minimum record size is 8 bytes. -a2290 2 - -\change_inserted 0 1291205886 -a2294 2 - -\change_inserted 0 1291205886 -a2295 2 -\change_unchanged - -a2385 2 -\change_inserted 0 1291205894 - -a2388 2 - -\change_inserted 0 1291205894 -a2392 2 - -\change_inserted 0 1291205902 -a2393 2 -\change_unchanged - -a2415 4 - -\change_deleted 0 1291204504 - -\change_unchanged -a2445 2 -\change_inserted 0 1291205910 - -a2448 2 - -\change_inserted 0 1291205910 -a2452 2 - -\change_inserted 0 1291205914 -a2453 2 -\change_unchanged - -a2485 2 -\change_inserted 0 1291205919 - -a2488 2 - -\change_inserted 0 1291205919 -a2492 2 - -\change_inserted 0 1291205922 -a2493 2 -\change_unchanged - -a2533 2 -\change_inserted 0 1291205929 - -a2536 2 - -\change_inserted 0 1291205929 -a2540 2 - -\change_inserted 0 1291205929 -a2541 2 -\change_unchanged - -a2578 2 -\change_inserted 0 1291205932 - -a2581 2 - -\change_inserted 0 1291205933 -a2585 2 - -\change_inserted 0 1291205933 -a2586 2 -\change_unchanged - -a2724 2 -\change_inserted 0 1291205944 - -a2727 2 - -\change_inserted 0 1291205945 -a2731 2 - -\change_inserted 0 1291205948 -a2732 2 -\change_unchanged - -@ - - -1.11 -log -@Merge changes -@ -text -@d53 7 -a59 1 -14-September-2010 -d587 16 -d644 18 -d716 16 -d753 16 -d813 18 -d883 16 -d953 16 -d1084 16 -d1181 16 -d1273 16 -d1328 16 -d1381 16 -d1447 19 -a1465 2 - if older code (which doesn't understand the feature) writes to the database.Reco -rd Headers Are Not Expandible -d1484 16 -d1546 16 -d1617 16 -d1680 16 -d1725 16 -d1810 16 -d1951 8 -a1958 3 -Proposed SolutionThe first step is to remove all the current heuristics, - as they obviously interact, then examine them once the lock contention - is addressed. -d1989 7 -a1995 2 - is to divide the file into zones, and using a free list (or set of free - lists) for each. -d1997 2 -d2002 25 -d2039 2 -d2049 7 -a2055 1 -Identify the correct zone. -d2063 7 -a2069 2 -Re-check the zone (we didn't have a lock, sizes could have changed): relock - if necessary. -d2073 5 -a2077 1 -Place the freed entry in the list for that zone. -d2086 3 -a2088 1 -Pick a zone either the zone we last freed into, or based on a -d2097 4 -d2105 2 -d2110 2 -d2113 2 -d2123 15 -a2137 1 - unlock the list and try the next zone. -d2166 11 -d2180 2 -d2185 2 -d2190 2 -d2223 1 -a2223 1 -status open -d2243 2 -d2491 5 -a2495 1 - uint32_t magic : 16, -d2499 2 -d2502 2 -d2511 7 -a2517 1 - top_hash: 10; -d2541 29 -a2569 1 - uint32_t free_magic; -d2573 11 -a2583 1 - uint64_t total_length; -d2587 7 -a2593 1 - uint64_t prev, next; -d2597 2 -d2603 5 -a2607 1 - uint64_t tailer; -d2615 2 -d2628 18 -d2736 16 -d2808 16 -d2856 16 -d2912 16 -d2965 16 -d3119 16 -@ - - -1.10 -log -@Tracing attribute, talloc support. -@ -text -@d1 1 -a1 1 -#LyX 1.6.5 created this file. For more info see http://www.lyx.org/ -d53 1 -a53 7 - -\change_deleted 0 1283307542 -26-July -\change_inserted 0 1284423485 -14-September -\change_unchanged --2010 -a472 2 -\change_inserted 0 1284422789 - -a479 2 -\change_unchanged - -a838 2 - -\change_inserted 0 1284016998 -a846 2 -\change_unchanged - -a1194 2 -\change_inserted 0 1284015637 - -a1197 2 - -\change_inserted 0 1284015716 -a1201 2 - -\change_inserted 0 1284015906 -a1210 2 - -\change_inserted 0 1284015637 -a1214 2 - -\change_inserted 0 1284016114 -a1227 2 - -\change_inserted 0 1284016149 -a1232 2 - -\change_inserted 0 1284016639 -a1237 2 - -\change_inserted 0 1284016821 -a1243 2 - -\change_inserted 0 1284016803 -d1245 2 -a1246 9 - if older code (which doesn't understand the feature) writes to the database. -\change_deleted 0 1284016101 - -\end_layout - -\begin_layout Subsection - -\change_inserted 0 1284015634 -Record Headers Are Not Expandible -a1249 2 - -\change_inserted 0 1284015634 -a1254 2 - -\change_inserted 0 1284015634 -a1258 2 - -\change_inserted 0 1284422552 -a1267 2 - -\change_inserted 0 1284422568 -a1271 2 - -\change_inserted 0 1284422646 -a1276 2 - -\change_inserted 0 1284422656 -a1280 2 - -\change_inserted 0 1284423065 -a1305 2 - -\change_inserted 0 1284423042 -a1310 2 -\change_unchanged - -a1457 2 - -\change_inserted 0 1283336713 -a1463 2 - -\change_unchanged -d1482 2 -d1485 1 -a1485 51 -\change_deleted 0 1283307675 -There are three details which become important: -\end_layout - -\begin_layout Enumerate - -\change_deleted 0 1283307675 -On encountering a full bucket, we use the next bucket. -\end_layout - -\begin_layout Enumerate - -\change_deleted 0 1283307675 -Extra hash bits are stored with the offset, to reduce comparisons. -\end_layout - -\begin_layout Enumerate - -\change_deleted 0 1283307675 -A marker entry is used on deleting an entry. -\end_layout - -\begin_layout Standard - -\change_deleted 0 1283307675 -The doubling of the table must be done under a transaction; we will not - reduce it on deletion, so it will be an unusual case. - It will either be placed at the head (other entries will be moved out the - way so we can expand). - We could have a pointer in the header to the current hashtable location, - but that pointer would have to be read frequently to check for hashtable - moves. -\end_layout - -\begin_layout Standard - -\change_deleted 0 1283307675 -The locking for this is slightly more complex than the chained case; we - currently have one lock per bucket, and that means we would need to expand - the lock if we overflow to the next bucket. - The frequency of such collisions will effect our locking heuristics: we - can always lock more buckets than we need. -\end_layout - -\begin_layout Standard - -\change_deleted 0 1283307675 -One possible optimization is to only re-check the hash size on an insert - or a lookup miss. - -\change_inserted 0 1283307770 -a1492 2 - -\change_inserted 0 1283336187 -a1500 2 - -\change_inserted 0 1283336586 -a1510 2 -\change_unchanged - -d1636 3 -a1638 8 -Proposed Solution -\change_deleted 0 1283336858 - -\end_layout - -\begin_layout Standard -The first step is to remove all the current heuristics, as they obviously - interact, then examine them once the lock contention is addressed. -a1647 2 -\change_inserted 0 1283336910 - -a1650 2 - -\change_inserted 0 1283337052 -a1655 2 -\change_unchanged - -a1776 2 -\change_inserted 0 1283309850 - -a1779 2 - -\change_inserted 0 1283337216 -a1813 2 - -\change_inserted 0 1284424151 -a1825 2 -\change_unchanged - -a1830 2 -\change_unchanged - -a2031 2 - -\change_inserted 0 1283336739 -a2040 2 -\change_unchanged - -a2117 2 -\change_inserted 0 1283337133 - -a2120 2 - -\change_inserted 0 1283337139 -a2121 2 -\change_unchanged - -a2136 2 - -\change_inserted 0 1283337235 -a2147 2 -\change_unchanged - -d2251 1 -a2251 7 -Proposed Solution -\change_deleted 0 1284423472 - -\end_layout - -\begin_layout Standard -None. -d2261 1 -a2261 1 -\change_inserted 0 1284423891 -d2263 1 -a2263 4 -\change_deleted 0 1284423891 -. - -\change_inserted 0 1284423901 -a2271 2 -\change_unchanged - -a2293 2 -\change_inserted 0 1284423495 - -a2312 2 - -\change_inserted 0 1284424201 -d2321 1 -a2321 3 - -\change_unchanged -We could solve a small part of the problem by providing read-only transactions. -a2505 2 -\change_inserted 0 1284423555 - -a2508 2 - -\change_inserted 0 1284423617 -a2512 2 - -\change_inserted 0 1284423719 -a2519 2 - -\change_inserted 0 1284423864 -a2530 2 - -\change_inserted 0 1284423850 -a2540 2 -\change_unchanged - -@ - - -1.9 -log -@Extension mechanism. -@ -text -@d56 2 -a57 2 -\change_inserted 0 1284016854 -9-September -d479 11 -d1303 1 -a1303 1 -\change_inserted 0 1284016847 -d1310 56 -d1945 1 -a1945 1 -\change_inserted 0 1283310945 -d1956 2 -d2402 2 -d2416 4 -d2421 12 -d2455 2 -d2476 12 -d2673 47 -@ - - -1.8 -log -@Remove bogus footnote -@ -text -@d56 2 -a57 2 -\change_inserted 0 1283307544 -1-September -d838 12 -d1198 103 -@ - - -1.7 -log -@Moving hash table does not work. -@ -text -@a1436 12 -\begin_inset Foot -status collapsed - -\begin_layout Plain Layout - -\change_inserted 0 1283336450 -If we make the hash offsets zone-relative, then this only restricts the - zone size, not the overall database size. -\end_layout - -\end_inset - -@ - - -1.6 -log -@Commit changes -@ -text -@d38 1 -a38 1 -\author "" -d53 7 -a59 1 -26-July-2010 -d1333 10 -d1361 3 -a1363 1 - There are three details which become important: -d1367 2 -d1373 2 -d1379 2 -d1385 2 -d1397 2 -d1407 2 -d1411 45 -d1582 2 -d1598 14 -d1733 62 -d1996 13 -d2086 10 -d2110 15 -a2124 1 -\begin_layout LyX-Code -@ - - -1.5 -log -@Soft transaction commit -@ -text -@d38 1 -a38 1 -\author "Rusty Russell,,," -a52 4 - -\change_deleted 0 1280141199 -10-May-2010 -\change_inserted 0 1280141202 -a53 2 -\change_unchanged - -a2028 2 - -\change_inserted 0 1280140902 -a2034 2 - -\change_unchanged -a2212 2 -\change_inserted 0 1280140661 - -a2215 2 - -\change_inserted 0 1280140703 -a2219 2 - -\change_inserted 0 1280708312 -a2226 2 - -\change_inserted 0 1280708400 -a2239 2 - -\change_inserted 0 1280140836 -a2243 2 - -\change_inserted 0 1280708255 -a2247 2 - -\change_inserted 0 1280708374 -a2252 2 - -\change_inserted 0 1280141181 -a2274 2 - -\change_inserted 0 1280141345 -@ - - -1.4 -log -@Merge changes -@ -text -@d38 1 -a38 1 -\author "" -d53 2 -d56 4 -d2035 10 -d2223 84 -@ - - -1.3 -log -@Transaction and freelist rethink. -@ -text -@d38 1 -a38 1 -\author "Rusty Russell,,," -d53 1 -a53 1 -27-April-2010 -d662 1 -a662 5 - behavior of disallowing -\change_inserted 0 1272940179 -nested -\change_unchanged -transactions should become the default. -a1210 2 -\change_inserted 0 1272944650 - -a1214 2 - -\change_inserted 0 1272944763 -a1218 2 -\change_unchanged - -a1223 2 -\change_unchanged - -a1301 2 - -\change_inserted 0 1273478114 -a1310 2 -\change_unchanged - -d1515 1 -a1515 11 -The free list -\change_deleted 0 1273469807 -should -\change_inserted 0 1273469810 -must -\change_unchanged - be split -\change_deleted 0 1273469815 -into multiple lists -\change_unchanged -to reduce contention. -a1520 2 -\change_inserted 0 1273470006 - -a1523 2 - -\change_inserted 0 1273492055 -a1539 2 - -\change_inserted 0 1273483888 -a1551 2 -\change_unchanged - -a1554 8 - -\change_deleted 0 1272942055 -There are various ways to organize these lisys, but because we want to be - able to quickly identify which free list an entry is in, and reduce the - number of locks required for merging, we will use zoning (eg. - each free list covers some fixed fraction of the file). - -\change_inserted 0 1273484187 -d1556 1 -a1556 7 - -\change_deleted 0 1273484194 -The algorithm for f -\change_inserted 0 1273484194 -F -\change_unchanged -reeing is simple: -d1560 1 -a1560 7 -Identify the correct -\change_deleted 0 1273482856 -free list -\change_inserted 0 1273482857 -zone -\change_unchanged -. -d1564 1 -a1564 7 -Lock the -\change_inserted 0 1273482895 -corresponding -\change_unchanged -list -\change_inserted 0 1273482863 -. -a1567 2 - -\change_inserted 0 1273482909 -d1573 1 -a1573 13 - -\change_deleted 0 1273482885 -, and p -\change_inserted 0 1273482888 -P -\change_unchanged -lace the freed entry -\change_deleted 0 1273492415 -at the head -\change_inserted 0 1273492415 -in the list for that zone -\change_unchanged -. -d1577 2 -a1578 7 -Allocation is a little more complicated, as we -\change_deleted 0 1273483240 -merge entries as we walk the list: -\change_inserted 0 1273484250 -perform delayed coalescing at this point: -\change_unchanged - -d1582 1 -a1582 19 -Pick a -\change_deleted 0 1273482955 -free list; -\change_inserted 0 1273482957 -zone -\change_unchanged - either the -\change_deleted 0 1273482962 -list -\change_inserted 0 1273482962 -zone -\change_unchanged - we last freed -\change_deleted 0 1273482966 -o -\change_inserted 0 1273482966 -i -\change_unchanged -nto, or based on a -d1594 1 -a1594 9 -Lock th -\change_inserted 0 1273482980 -e corresponding -\change_deleted 0 1273482973 -at -\change_unchanged - list. -\change_inserted 0 1273482982 - -a1597 2 - -\change_inserted 0 1273483084 -a1598 53 -\change_unchanged - -\end_layout - -\begin_layout Enumerate -If the top entry is -\change_deleted 0 1273492155 -well-sized, -\change_inserted 0 1273492159 --large enough, -\change_unchanged -remove it from the list and return it. -\end_layout - -\begin_layout Enumerate -Otherwise, -\change_inserted 0 1273492206 -coalesce entries in the list. -\change_deleted 0 1273492200 -examine the entry to the right of it in the file. - If it is free: -\end_layout - -\begin_deeper -\begin_layout Enumerate - -\change_deleted 0 1273492200 -If that entry is in a different list, lock that list too. -\end_layout - -\begin_layout Enumerate - -\change_deleted 0 1273492200 -If we had to place a new lock, re-check that the entry is free. -\end_layout - -\begin_layout Enumerate - -\change_deleted 0 1273492200 -Remove that entry from its free list and expand this entry to cover it. -\end_layout - -\begin_layout Enumerate - -\change_deleted 0 1273485554 -Goto step 3. -\end_layout - -\end_deeper -\begin_layout Enumerate - -\change_inserted 0 1273485311 -If there was no entry large enough, unlock the list and try the next zone. -d1602 1 -a1602 5 - -\change_deleted 0 1273483646 -Repeat step 3 with each entry in the list. -\change_unchanged - -d1606 2 -a1607 5 - -\change_deleted 0 1273483668 -Unlock the list and repeat step 2 with the next list. -\change_unchanged - -d1611 1 -a1611 7 -If no -\change_deleted 0 1273483671 -list -\change_inserted 0 1273483671 -zone -\change_unchanged - satisfies, expand the file. -d1615 2 -a1616 9 -This optimizes rapid insert/delete of free list entries -\change_inserted 0 1273485794 - by not coalescing them all the time. -\change_deleted 0 1273483685 -, and allows us to get rid of the tailer altogether -\change_unchanged -. - -\change_inserted 0 1273492299 -a1638 39 - -\change_deleted 0 1273476840 -The question of -\begin_inset Quotes eld -\end_inset - -well-sized -\begin_inset Quotes erd -\end_inset - - free entries is more difficult: the 25% overhead works in practice for - ldb because indexes tend to expand by one record at a time. - This can be resolved by having an -\begin_inset Quotes eld -\end_inset - -expanded -\begin_inset Quotes erd -\end_inset - - bit in the header to note entries that have previously expanded, and allocating - more space for them. - Whether the -\begin_inset Quotes eld -\end_inset - -increasing slack -\begin_inset Quotes erd -\end_inset - - algorithm should be implemented or first-fit used is still unknown: we - will determine this once these other ideas are implemented. -\change_inserted 0 1273483750 - -\end_layout - -\begin_layout Standard - -\change_inserted 0 1273492450 -a1644 2 - -\change_inserted 0 1273470441 -a1654 2 - -\change_inserted 0 1273476556 -a1659 2 - -\change_inserted 0 1273470423 -a1661 2 -\change_unchanged - -a1672 2 - -\change_inserted 0 1273476847 -a1676 2 - -\change_inserted 0 1273476886 -a1691 2 - -\change_inserted 0 1273477233 -a1699 2 - -\change_inserted 0 1273477534 -a1706 2 - -\change_inserted 0 1273482700 -a1712 2 - -\change_inserted 0 1273478079 -a1722 2 - -\change_inserted 0 1273477839 -a1726 2 - -\change_inserted 0 1273477925 -a1730 2 - -\change_inserted 0 1273477925 -a1734 2 - -\change_inserted 0 1273477925 -a1738 2 - -\change_inserted 0 1273477925 -a1742 2 - -\change_inserted 0 1273477925 -a1746 2 - -\change_inserted 0 1273477925 -a1750 2 - -\change_inserted 0 1273477925 -a1754 2 - -\change_inserted 0 1273477925 -a1758 2 - -\change_inserted 0 1273477925 -a1762 2 - -\change_inserted 0 1273477925 -a1766 2 - -\change_inserted 0 1273477925 -a1770 2 - -\change_inserted 0 1273477925 -a1774 2 - -\change_inserted 0 1273477925 -a1778 2 - -\change_inserted 0 1273477925 -a1782 2 - -\change_inserted 0 1273477925 -a1786 2 - -\change_inserted 0 1273477925 -a1790 2 - -\change_inserted 0 1273477925 -a1794 2 - -\change_inserted 0 1273477925 -a1798 2 - -\change_inserted 0 1273492522 -a1802 2 - -\change_inserted 0 1273492530 -a1806 2 - -\change_inserted 0 1273492546 -a1810 2 - -\change_inserted 0 1273478239 -a1814 2 - -\change_inserted 0 1273479960 -a1821 2 - -\change_inserted 0 1273480265 -a1830 2 - -\change_inserted 0 1273480354 -a1845 2 - -\change_inserted 0 1273478968 -a1851 2 - -\change_inserted 0 1273492604 -a1859 2 - -\change_inserted 0 1273479572 -a1862 2 -\change_unchanged - -a1870 2 - -\change_inserted 0 1273480282 -a1874 2 - -\change_inserted 0 1273478931 -a1878 2 - -\change_inserted 0 1273481549 -a1882 2 - -\change_inserted 0 1273481557 -a1886 2 - -\change_inserted 0 1273480307 -a1890 2 - -\change_inserted 0 1273480335 -a1894 2 - -\change_inserted 0 1273479897 -a1898 2 - -\change_inserted 0 1273479653 -a1902 2 - -\change_inserted 0 1273480371 -a1906 2 - -\change_inserted 0 1273480464 -a1910 2 - -\change_inserted 0 1273480399 -a1914 2 - -\change_inserted 0 1273480425 -a1918 2 - -\change_inserted 0 1273480453 -a1922 2 - -\change_inserted 0 1273480455 -a1926 2 - -\change_inserted 0 1273480450 -a1930 2 - -\change_inserted 0 1273480452 -a1935 2 -\change_inserted 0 1273478830 - -a1942 5 - -\change_deleted 0 1273481604 -In theory, we could get away with 2: one after we write the new data, and - one to somehow atomically change over to it. -\change_inserted 0 1273481632 -a1946 2 - -\change_inserted 0 1273481724 -a1950 2 - -\change_inserted 0 1273481713 -a1954 2 - -\change_inserted 0 1273481717 -a1958 2 - -\change_inserted 0 1273481730 -a1962 2 - -\change_inserted 0 1273481736 -a1966 2 - -\change_inserted 0 1273481744 -a1970 2 - -\change_inserted 0 1273481748 -a1974 2 - -\change_inserted 0 1273482185 -a1978 2 - -\change_inserted 0 1273482259 -a1989 50 - -\change_deleted 0 1273481848 -None. - Trying to rewrite the transaction code is a separate experiment, which - I encourage someone else to do. - At some point you say -\begin_inset Quotes eld -\end_inset - -use a real database -\begin_inset Quotes erd -\end_inset - -. -\end_layout - -\begin_layout Standard - -\change_deleted 0 1273481848 -But as a thought experiment: -\change_unchanged - -\end_layout - -\begin_layout Standard - -\change_deleted 0 1273481788 -Say there was a pointer in the header which said where the hash table and - free list tables were, and that no blocks were labeled with whether they - were free or not (it had to be derived from what list they were in). - We could create new hash table and free list in some free space, and populate - it as we want the post-committed state to look. - Then we sync, then we switch the offset in the header, then we sync again. -\end_layout - -\begin_layout Standard - -\change_deleted 0 1273481788 -This would not allow arbitrary changes to the database, such as tdb_repack - does, and would require more space (since we have to preserve the current - and future entries at once). - If we used hash trees rather than one big hash table, we might only have - to rewrite some sections of the hash, too. -\change_inserted 0 1273481854 - -\end_layout - -\begin_layout Standard - -\change_inserted 0 1273482102 -a1993 2 - -\change_inserted 0 1273482061 -a1998 2 - -\change_inserted 0 1273482063 -a2002 2 - -\change_inserted 0 1273482072 -a2006 2 - -\change_inserted 0 1273482139 -a2011 2 - -\change_inserted 0 1273482364 -a2015 2 - -\change_inserted 0 1273482163 -a2019 2 - -\change_inserted 0 1273482493 -a2037 2 - -\change_inserted 0 1273482536 -a2046 2 -\change_unchanged - -a2049 2 - -\change_inserted 0 1273482641 -a2058 2 - -\change_inserted 0 1273481827 -d2067 2 -a2068 11 -We could -\change_inserted 0 1273481829 -then -\change_unchanged -implement snapshots using a similar method -\change_deleted 0 1273481838 - to the above, only -\change_inserted 0 1273481840 -, -\change_unchanged - using multiple different hash tables/free tables. -@ - - -1.2 -log -@After first feedback (Ronnie & Volker) -@ -text -@d1314 13 -d1531 11 -a1541 1 -The free list should be split into multiple lists to reduce contention. -d1547 39 -d1596 7 -d1604 1 -a1604 1 -The algorithm for freeing is simple: -d1608 7 -a1614 1 -Identify the correct free list. -d1618 30 -a1647 1 -Lock the list, and place the freed entry at the head. -d1651 7 -a1657 2 -Allocation is a little more complicated, as we merge entries as we walk - the list: -d1661 19 -a1679 1 -Pick a free list; either the list we last freed onto, or based on a -d1691 17 -a1707 1 -Lock that list. -d1711 7 -a1717 1 -If the top entry is well-sized, remove it from the list and return it. -d1721 5 -a1725 1 -Otherwise, examine the entry to the right of it in the file. -d1731 2 -d1737 2 -d1743 2 -d1749 2 -d1756 8 -d1765 2 -d1770 2 -d1773 2 -d1778 7 -a1784 1 -If no list satisfies, expand the file. -d1788 28 -a1815 2 -This optimizes rapid insert/delete of free list entries, and allows us to - get rid of the tailer altogether. -d1819 2 -d1851 1 -a1851 1 -\change_inserted 0 1272941474 -d1857 303 -a2159 18 -\change_inserted 0 1272942759 -There are various ways to organize these lists, but because we want to be - able to quickly identify which free list an entry is in, and reduce the - number of locks required for merging, we will use zoning (eg. - each of the N free lists in a tdb file of size M covers a fixed fraction - M/N). - Note that this means we need to reshuffle the free lists when we expand - the file; this is probably acceptable when we double the hash table size, - since that is such an expensive operation already. - In the case of increasing the file size, there is an optimization we can - use: if we use M in the formula above as the file size rounded up to the - next power of 2, we only need reshuffle free lists when the file size crosses - a power of 2 boundary, -\emph on -and -\emph default -reshuffling the free lists is trivial: we simply merge every consecutive - pair of free lists. -d2164 107 -d2276 2 -d2280 59 -d2346 2 -d2363 2 -d2366 2 -d2371 2 -d2382 2 -d2389 57 -d2458 13 -d2474 32 -a2505 2 -We could implement snapshots using a similar method to the above, only using - multiple different hash tables/free tables. -@ - - -1.1 -log -@Initial revision -@ -text -@d1 1 -a1 1 -#LyX 1.6.4 created this file. For more info see http://www.lyx.org/ -d36 3 -a38 3 -\tracking_changes false -\output_changes false -\author "" -d662 5 -a666 1 - behavior of disallowing transactions should become the default. -d1215 21 -d1527 2 -d1533 3 -a1535 1 - The algorithm for freeing is simple: -d1642 26 -@ diff --git a/lib/ntdb/doc/design.pdf b/lib/ntdb/doc/design.pdf Binary files differindex 558dc1f8c2..838191469d 100644 --- a/lib/ntdb/doc/design.pdf +++ b/lib/ntdb/doc/design.pdf diff --git a/lib/ntdb/doc/design.txt b/lib/ntdb/doc/design.txt index bd2ffde4db..bd680f09ee 100644 --- a/lib/ntdb/doc/design.txt +++ b/lib/ntdb/doc/design.txt @@ -1,8 +1,8 @@ -TDB2: A Redesigning The Trivial DataBase +NTDB: Redesigning The Trivial DataBase Rusty Russell, IBM Corporation -1-December-2010 +19 June 2012 Abstract @@ -65,6 +65,11 @@ without significantly increasing complexity; all involved are far too aware of the dangers of second system syndrome in rewriting a successful project like this. +Note: the final decision was to make ntdb a separate library, +with a separarate 'ntdb' namespace so both can potentially be +linked together. This document still refers to “tdb” everywhere, +for simplicity. + 2 API Issues 2.1 tdb_open_ex Is Not Expandable @@ -182,7 +187,7 @@ This flag can also be changed at runtime. 2.3.1 Proposed Solution -Given the usage patterns, it seems that the “least-surprise” +Given the usage patterns, it seems that the“least-surprise” behavior of disallowing nested transactions should become the default. Additionally, it seems the outer transaction is the only code which knows whether inner transactions should be allowed, so @@ -193,7 +198,7 @@ expanded for this relatively-obscure case. 2.3.2 Status -Incomplete; nesting flag is still defined as per tdb1. +Complete; the nesting flag has been removed. 2.4 Incorrect Hash Function is Not Detected @@ -217,7 +222,7 @@ Complete. In response to scalability issues with the free list ([TDB-Freelist-Is] ) two API workarounds have been incorporated in TDB: tdb_set_max_dead() and the TDB_VOLATILE flag to tdb_open. The -latter actually calls the former with an argument of “5”. +latter actually calls the former with an argument of“5”. This code allows deleted records to accumulate without putting them in the free list. On delete we iterate through each chain @@ -235,8 +240,8 @@ will become a no-op. 2.5.2 Status -Incomplete. TDB_VOLATILE still defined, but implementation should -fail on unknown flags to be future-proof. +Complete. Unknown flags cause tdb_open() to fail as well, so they +can be detected at runtime. 2.6 <TDB-Files-Cannot>TDB Files Cannot Be Opened Multiple Times In The Same Process @@ -275,7 +280,7 @@ to allow other to create such an API. 2.6.2 Status -Incomplete. +Complete. 2.7 TDB API Is Not POSIX Thread-safe @@ -283,19 +288,19 @@ The TDB API uses an error code which can be queried after an operation to determine what went wrong. This programming model does not work with threads, unless specific additional guarantees are given by the implementation. In addition, even -otherwise-independent threads cannot open the same TDB (as in [TDB-Files-Cannot] +otherwise-independent threads cannot open the same TDB (as in[TDB-Files-Cannot] ). 2.7.1 Proposed Solution Reachitecting the API to include a tdb_errcode pointer would be a -great deal of churn; we are better to guarantee that the -tdb_errcode is per-thread so the current programming model can be -maintained. - -This requires dynamic per-thread allocations, which is awkward -with POSIX threads (pthread_key_create space is limited and we -cannot simply allocate a key for every TDB). +great deal of churn, but fortunately most functions return 0 on +success and -1 on error: we can change these to return 0 on +success and a negative error code on error, and the API remains +similar to previous. The tdb_fetch, tdb_firstkey and tdb_nextkey +functions need to take a TDB_DATA pointer and return an error +code. It is also simpler to have tdb_nextkey replace its key +argument in place, freeing up any old .dptr. Internal locking is required to make sure that fcntl locks do not overlap between threads, and also that the global list of tdbs is @@ -304,12 +309,13 @@ maintained. The aim is that building tdb with -DTDB_PTHREAD will result in a pthread-safe version of the library, and otherwise no overhead will exist. Alternatively, a hooking mechanism similar to that -proposed for [Proposed-Solution-locking-hook] could be used to +proposed for[Proposed-Solution-locking-hook] could be used to enable pthread locking at runtime. 2.7.2 Status -Incomplete. +Incomplete; API has been changed but thread safety has not been +implemented. 2.8 *_nonblock Functions And *_mark Functions Expose Implementation @@ -375,7 +381,7 @@ it is needed. 2.8.2 Status -Incomplete. +Complete. 2.9 tdb_chainlock Functions Expose Implementation @@ -427,7 +433,7 @@ otherwise EAGAIN. 2.10.2 Status -Incomplete. +Complete. 2.11 The API Uses Gratuitous Typedefs, Capitals @@ -477,7 +483,7 @@ Complete. 2.13 Various Callback Functions Are Not Typesafe -The callback functions in tdb_set_logging_function (after [tdb_log_func-Doesnt-Take] +The callback functions in tdb_set_logging_function (after[tdb_log_func-Doesnt-Take] is resolved), tdb_parse_record, tdb_traverse, tdb_traverse_read and tdb_check all take void * and must internally convert it to the argument type they were expecting. @@ -499,7 +505,7 @@ http://ccan.ozlabs.org/info/typesafe_cb.html 2.13.2 Status -Incomplete. +Complete. 2.14 TDB_CLEAR_IF_FIRST Must Be Specified On All Opens, tdb_reopen_all Problematic @@ -519,12 +525,12 @@ it alone has opened the TDB and will erase it. 2.14.1 Proposed Solution Remove TDB_CLEAR_IF_FIRST. Other workarounds are possible, but -see [TDB_CLEAR_IF_FIRST-Imposes-Performance]. +see[TDB_CLEAR_IF_FIRST-Imposes-Performance]. 2.14.2 Status -Incomplete, TDB_CLEAR_IF_FIRST still defined, but not -implemented. +Complete. An open hook is provided to replicate this +functionality if required. 2.15 Extending The Header Is Difficult @@ -537,7 +543,7 @@ not. 2.15.1 Proposed Solution -The header should contain a “format variant” value (64-bit). This +The header should contain a“format variant” value (64-bit). This is divided into two 32-bit parts: 1. The lower part reflects the format variant understood by code @@ -558,7 +564,7 @@ writes to the database. 2.15.2 Status -Incomplete. +Complete. 2.16 Record Headers Are Not Expandible @@ -576,7 +582,7 @@ would know the extension is not present on that record. 2.16.2 Status -Incomplete. +Complete. 2.17 TDB Does Not Use Talloc @@ -589,10 +595,10 @@ conveniently. The allocation within TDB is not complicated enough to justify the use of talloc, and I am reluctant to force another (excellent) library on TDB users. Nonetheless a compromise is -possible. An attribute (see [attributes]) can be added later to +possible. An attribute (see[attributes]) can be added later to tdb_open() to provide an alternate allocation mechanism, specifically for talloc but usable by any other allocator (which -would ignore the “context” argument). +would ignore the“context” argument). This would form a talloc heirarchy as expected, but the caller would still have to attach a destructor to the tdb context @@ -602,7 +608,7 @@ manage them (using talloc_free() or talloc_steal()). 2.17.2 Status -Deferred. +Complete, using the NTDB_ATTRIBUTE_ALLOCATOR attribute. 3 Performance And Scalability Issues @@ -635,11 +641,11 @@ can simply unlink the old tdb at that point. 3.1.2 Status -Incomplete; TDB_CLEAR_IF_FIRST still defined, but does nothing. +Complete. 3.2 TDB Files Have a 4G Limit -This seems to be becoming an issue (so much for “trivial”!), +This seems to be becoming an issue (so much for“trivial”!), particularly for ldb. 3.2.1 Proposed Solution @@ -679,7 +685,7 @@ Record sizes will be 64 bit, with an error returned on 32 bit platforms which try to access such records (the current implementation would return TDB_ERR_OOM in a similar case). It seems unlikely that 32 bit keys will be a limitation, so the -implementation may not support this (see [sub:Records-Incur-A]). +implementation may not support this (see[sub:Records-Incur-A]). 3.3.2 Status @@ -728,7 +734,11 @@ invalid. 3.4.2 Status -Complete. +Ignore. Scaling the hash automatically proved inefficient at +small hash sizes; we default to a 8192-element hash (changable +via NTDB_ATTRIBUTE_HASHSIZE), and when buckets clash we expand to +an array of hash entries. This scales slightly better than the +tdb chain (due to the 8 top bits containing extra hash). 3.5 <TDB-Freelist-Is>TDB Freelist Is Highly Contended @@ -783,7 +793,7 @@ Deleting a record occurs as follows: 7. Otherwise, prepend ourselves to the free list. -Disabling right-merging (step [right-merging]) causes +Disabling right-merging (step[right-merging]) causes fragmentation; the other heuristics proved insufficient to address this, so the final answer to this was that when we expand the TDB file inside a transaction commit, we repack the entire @@ -812,7 +822,7 @@ zone) which produces too many clashes for our hash table to handle well, and for coalescing we search by address. Thus an array of doubly-linked free lists seems preferable. -There are various benefits in using per-size free lists (see [sub:TDB-Becomes-Fragmented] +There are various benefits in using per-size free lists (see[sub:TDB-Becomes-Fragmented] ) but it's not clear this would reduce contention in the common case where all processes are allocating/freeing the same size. Thus we almost certainly need to divide in other ways: the most @@ -822,7 +832,7 @@ ordering. Unfortunately it is difficult to know what heuristics should be used to determine zone sizes, and our transaction code relies on -being able to create a “recovery area” by simply appending to the +being able to create a“recovery area” by simply appending to the file (difficult if it would need to create a new zone header). Thus we use a linked-list of free tables; currently we only ever create one, but if there is more than one we choose one at random @@ -862,9 +872,9 @@ coalescing at this point: This optimizes rapid insert/delete of free list entries by not coalescing them all the time.. First-fit address ordering ordering seems to be fairly good for keeping fragmentation low -(see [sub:TDB-Becomes-Fragmented]). Note that address ordering +(see[sub:TDB-Becomes-Fragmented]). Note that address ordering does not need a tailer to coalesce, though if we needed one we -could have one cheaply: see [sub:Records-Incur-A]. +could have one cheaply: see[sub:Records-Incur-A]. Each free entry has the free table number in the header: less than 255. It also contains a doubly-linked list for easy @@ -884,7 +894,7 @@ db when a transaction commit needs to enlarge the file. The 25% overhead on allocation works in practice for ldb because indexes tend to expand by one record at a time. This internal -fragmentation can be resolved by having an “expanded” bit in the +fragmentation can be resolved by having an“expanded” bit in the header to note entries that have previously expanded, and allocating more space for them. @@ -970,13 +980,13 @@ block: scale as fast as data, so I'm assuming a maximum key size of 32 bits. -4. 'full_hash' is used to avoid a memcmp on the “miss” case, but +4. 'full_hash' is used to avoid a memcmp on the“miss” case, but this is diminishing returns after a handful of bits (at 10 bits, it reduces 99.9% of false memcmp). As an aside, as the lower bits are already incorporated in the hash table resolution, the upper bits should be used here. Note that it's not clear that these bits will be a win, given the extra bits - in the hash table itself (see [sub:Hash-Size-Solution]). + in the hash table itself (see[sub:Hash-Size-Solution]). 5. 'magic' does not need to be enlarged: it currently reflects one of 5 values (used, free, dead, recovery, and @@ -1094,8 +1104,10 @@ Deferred. 3.9 <sub:TDB-Does-Not>TDB Does Not Have Snapshot Support -3.9.1 Proposed SolutionNone. At some point you say “use a real - database” (but see [replay-attribute]). +3.9.1 Proposed Solution + +None. At some point you say“use a real database” (but see[replay-attribute] +). But as a thought experiment, if we implemented transactions to only overwrite free entries (this is tricky: there must not be a @@ -1128,7 +1140,7 @@ failed. 3.10.1 Proposed Solution -None (but see [replay-attribute]). We could solve a small part of +None (but see[replay-attribute]). We could solve a small part of the problem by providing read-only transactions. These would allow one write transaction to begin, but it could not commit until all r/o transactions are done. This would require a new @@ -1175,7 +1187,7 @@ indefinitely. 3.12.1 Proposed Solution -Remove reliability guarantees; see [traverse-Proposed-Solution]. +Remove reliability guarantees; see[traverse-Proposed-Solution]. 3.12.2 Status @@ -1214,7 +1226,7 @@ normal (fast) usage, and occasionally empties the results into a transactional TDB. This kind of usage prioritizes performance over durability: as long as we are consistent, data can be lost. -This would be more neatly implemented inside tdb: a “soft” +This would be more neatly implemented inside tdb: a“soft” transaction commit (ie. syncless) which meant that data may be reverted on a crash. @@ -1226,12 +1238,12 @@ Unfortunately any transaction scheme which overwrites old data requires a sync before that overwrite to avoid the possibility of corruption. -It seems possible to use a scheme similar to that described in [sub:TDB-Does-Not] +It seems possible to use a scheme similar to that described in[sub:TDB-Does-Not] ,where transactions are committed without overwriting existing data, and an array of top-level pointers were available in the -header. If the transaction is “soft” then we would not need a -sync at all: existing processes would pick up the new hash table -and free list and work with that. +header. If the transaction is“soft” then we would not need a sync +at all: existing processes would pick up the new hash table and +free list and work with that. At some later point, a sync would allow recovery of the old data into the free lists (perhaps when the array of top-level pointers @@ -1249,7 +1261,7 @@ so it can coordinate cluster-wide transactions. 3.15.1 Proposed Solution<replay-attribute> Tridge points out that an attribute can be later added to -tdb_open (see [attributes]) to provide replay/trace hooks, which +tdb_open (see[attributes]) to provide replay/trace hooks, which could become the basis for this and future parallel transactions and snapshot support. |