diff options
| -rw-r--r-- | source3/architecture.doc | 135 | ||||
| -rw-r--r-- | source3/nmbd/nmbd.c | 2 | ||||
| -rw-r--r-- | source3/nmbd/nmbd_responserecordsdb.c | 2 |
3 files changed, 137 insertions, 2 deletions
diff --git a/source3/architecture.doc b/source3/architecture.doc new file mode 100644 index 0000000000..afcec6d50a --- /dev/null +++ b/source3/architecture.doc @@ -0,0 +1,135 @@ +Samba Architecture +------------------ + +First preliminary version Dan Shearer Nov 97 +Quickly scrabbled together from odd bits of mail and memory. Please update. + +This document gives a general overview of how Samba works +internally. The Samba Team has tried to come up with a model which is +the best possible compromise between elegance, portability, security +and the constraints imposed by the very message SMB and CIFS +protocol. + +It also tries to answer some of the frequently asked questions such as: + + * Is Samba secure when running on Unix? The xyz platform? + What about the root priveliges issue? + + * Pros and cons of multithreading in various parts of Samba + + * Why not have a separate process for name resolution, WINS, + and browsing? + + +Multithreading and Samba +------------------------ + +People sometimes tout threads as a uniformly good thing. They are very +nice in their place but are quite inappropriate for smbd. nmbd is +another matter, and multi-threading it would be very nice. + +The short version is that smbd is not multithreaded, and alternative +servers that take this approach under Unix (such as Syntax, at the +time of writing) suffer tremendous performance penalties and are less +robust. nmbd is not threaded either, but this is because it is not +possible to do it while keeping code consistent and portable across 35 +or more platforms. (This drawback also applies to threading smbd.) + +The longer versions is that there are very good reasons for not making +smbd multi-threaded. Multi-threading would actually make Samba much +slower, less scalable, less portable and much less robust. The fact +that we use a separate process for each connection is one of Samba's +biggest advantages. + +Threading smbd +-------------- + +A few problems that would arise from a threaded smbd are: + +0) It's not only to create threads instead of processes, but you + must care about all variables if they have to be thread specific + (currently they would be global). + +1) if one thread dies (eg. a seg fault) then all threads die. We can +immediately throw robustness out the window. + +2) many of the system calls we make are blocking. Non-blocking +equivalents of many calls are either not available or are awkward (and +slow) to use. So while we block in one thread all clients are +waiting. Imagine if one share is a slow NFS filesystem and the others +are fast, we will end up slowing all clients to the speed of NFS. + +3) you can't run as a different uid in different threads. This means +we would have to switch uid/gid on _every_ SMB packet. It would be +horrendously slow. + +4) the per process file descriptor limit would mean that we could only +support a limited number of clients. + +5) we couldn't use the system locking calls as the locking context of +fcntl() is a process, not a thread. + +Threading nmbd +-------------- + +This would be ideal, but gets sunk by portability requirements. + +Andrew tried to write a test threads library for nmbd that used only +ansi-C constructs (using setjmp and longjmp). Unfortunately some OSes +defeat this by restricting longjmp to calling addresses that are +shallower than the current address on the stack (apparently AIX does +this). This makes a truly portable threads library impossible. So to +support all our current platforms we would have to code nmbd both with +and without threads, and as the real aim of threads is to make the +code clearer we would not have gained anything. (it is a myth that +threads make things faster. threading is like recursion, it can make +things clear but the same thing can always be done faster by some +other method) + +Chris tried to spec out a general design that would abstract threading +vs separate processes (vs other methods?) and make them accessible +through some general API. This doesn't work because of the data +sharing requirements of the protocol (packets in the future depending +on packets now, etc.) At least, the code would work but would be very +clumsy, and besides the fork() type model would never work on Unix. (Is there an OS that it would work on, for nmbd?) + +A fork() is cheap, but not nearly cheap enough to do on every UDP +packet that arrives. Having a pool of processes is possible but is +nasty to program cleanly due to the enormous amount of shared data (in +complex structures) between the processes. We can't rely on each +platform having a shared memory system. + +nbmd Design +----------- + +Originally Andrew used recursion to simulate a multi-threaded +environment, which use the stack enormously and made for really +confusing debugging sessions. Luke Leighton rewrote it to use a +queuing system that keeps state information on each packet. During the +1.9.18 alpha series it was decided that this was too unwieldy to +manage. If the protocol was cleaner than it is then it would be OK +but with the way the protocol works you really need some data hiding. +The mistake we made was to transfer all the info from the packets to +more specialised structures. It bit us badly when we then found we +needed some detail of the original packet to handle some special +case. The specialised structures kept growing till they almost +contained all the info of the original packet! The code became +extremely hairy, which became particularly evident when Jeremy fixed +browsing on multiple subnets for 1.9.17. + +Then Jeremy rewrote nmbd. The packet data in nmbd isn't what's on the +wire. It's a nice format that is very amenable to processing but still +keeps the idea of a distinct packet. See "struct packet_struct" in +nameserv.h. It has all the detail but none of the on-the-wire +mess. This makes it ideal for using in disk or memory-based databases +for browsing and WINS support. + +nmbd now consists of a series of modules. It... + + +Samba Design and Security +------------------------- + +Why Isn't nmbd Multiple Daemons? +-------------------------------- + diff --git a/source3/nmbd/nmbd.c b/source3/nmbd/nmbd.c index 86f01d8e79..de83c0f67e 100644 --- a/source3/nmbd/nmbd.c +++ b/source3/nmbd/nmbd.c @@ -384,7 +384,7 @@ static void process(void) initiate_wins_processing(t); /* - * Go through the repsonse record queue and time out or re-transmit + * Go through the response record queue and time out or re-transmit * and expired entries. * (nmbd_packets.c) */ diff --git a/source3/nmbd/nmbd_responserecordsdb.c b/source3/nmbd/nmbd_responserecordsdb.c index bc0c0745f5..031853271b 100644 --- a/source3/nmbd/nmbd_responserecordsdb.c +++ b/source3/nmbd/nmbd_responserecordsdb.c @@ -229,7 +229,7 @@ never happen !\n", remote_broadcast_subnet->subnet_name)); return rrec; } - DEBUG(0,("find_response_record: repsonse packet id %hu received with no \ + DEBUG(0,("find_response_record: response packet id %hu received with no \ matching record.\n", id)); *ppsubrec = NULL; |