diff options
Diffstat (limited to 'docs/docbook/devdoc/architecture.xml')
| -rw-r--r-- | docs/docbook/devdoc/architecture.xml | 184 |
1 files changed, 184 insertions, 0 deletions
diff --git a/docs/docbook/devdoc/architecture.xml b/docs/docbook/devdoc/architecture.xml new file mode 100644 index 0000000000..312a63af97 --- /dev/null +++ b/docs/docbook/devdoc/architecture.xml @@ -0,0 +1,184 @@ +<chapter id="architecture"> +<chapterinfo> + <author> + <firstname>Dan</firstname><surname>Shearer</surname> + </author> + <pubdate> November 1997</pubdate> +</chapterinfo> + +<title>Samba Architecture</title> + +<sect1> +<title>Introduction</title> + +<para> +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 messy SMB and CIFS +protocol. +</para> + +<para> +It also tries to answer some of the frequently asked questions such as: +</para> + +<orderedlist> +<listitem><para> + Is Samba secure when running on Unix? The xyz platform? + What about the root priveliges issue? +</para></listitem> + +<listitem><para>Pros and cons of multithreading in various parts of Samba</para></listitem> + +<listitem><para>Why not have a separate process for name resolution, WINS, and browsing?</para></listitem> + +</orderedlist> + +</sect1> + +<sect1> +<title>Multithreading and Samba</title> + +<para> +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. +</para> + +<para> +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.) +</para> + +<para> +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. +</para> + +</sect1> + +<sect1> +<title>Threading smbd</title> + +<para> +A few problems that would arise from a threaded smbd are: +</para> + +<orderedlist> +<listitem><para> + 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). +</para></listitem> + +<listitem><para> + if one thread dies (eg. a seg fault) then all threads die. We can + immediately throw robustness out the window. +</para></listitem> + +<listitem><para> + 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. +</para></listitem> + +<listitem><para> + 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. +</para></listitem> + +<listitem><para> + the per process file descriptor limit would mean that we could only + support a limited number of clients. +</para></listitem> + +<listitem><para> + we couldn't use the system locking calls as the locking context of + fcntl() is a process, not a thread. +</para></listitem> + +</orderedlist> + +</sect1> + +<sect1> +<title>Threading nmbd</title> + +<para> +This would be ideal, but gets sunk by portability requirements. +</para> + +<para> +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) +</para> + +<para> +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?) +</para> + +<para> +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. +</para> + +</sect1> + +<sect1> +<title>nbmd Design</title> + +<para> +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. The +first version used a single structure which was used by all the +pending states. As the initialisation of this structure was +done by adding arguments, as the functionality developed, it got +pretty messy. So, it was replaced with a higher-order function +and a pointer to a user-defined memory block. This suddenly +made things much simpler: large numbers of functions could be +made static, and modularised. This is the same principle as used +in NT's kernel, and achieves the same effect as threads, but in +a single process. +</para> + +<para> +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. +</para> + +</sect1> +</chapter> |