| 1 | <html><head><meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"><title>Chapter 3. Samba Architecture</title><link rel="stylesheet" href="../samba.css" type="text/css"><meta name="generator" content="DocBook XSL Stylesheets V1.74.0"><link rel="home" href="index.html" title="SAMBA Developers Guide"><link rel="up" href="pt02.html" title="Part II. Samba Basics"><link rel="prev" href="pt02.html" title="Part II. Samba Basics"><link rel="next" href="debug.html" title="Chapter 4. The samba DEBUG system"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Chapter 3. Samba Architecture</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="pt02.html">Prev</a> </td><th width="60%" align="center">Part II. Samba Basics</th><td width="20%" align="right"> <a accesskey="n" href="debug.html">Next</a></td></tr></table><hr></div><div class="chapter" lang="en"><div class="titlepage"><div><div><h2 class="title"><a name="architecture"></a>Chapter 3. Samba Architecture</h2></div><div><div class="author"><h3 class="author"><span class="firstname">Dan</span> <span class="surname">Shearer</span></h3></div></div><div><p class="pubdate"> November 1997</p></div></div></div><div class="toc"><p><b>Table of Contents</b></p><dl><dt><span class="sect1"><a href="architecture.html#id2556692">Introduction</a></span></dt><dt><span class="sect1"><a href="architecture.html#id2556735">Multithreading and Samba</a></span></dt><dt><span class="sect1"><a href="architecture.html#id2556767">Threading smbd</a></span></dt><dt><span class="sect1"><a href="architecture.html#id2556828">Threading nmbd</a></span></dt><dt><span class="sect1"><a href="architecture.html#id2556873">nbmd Design</a></span></dt></dl></div><div class="sect1" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2556692"></a>Introduction</h2></div></div></div><p>
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| 2 | This document gives a general overview of how Samba works
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| 3 | internally. The Samba Team has tried to come up with a model which is
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| 4 | the best possible compromise between elegance, portability, security
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| 5 | and the constraints imposed by the very messy SMB and CIFS
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| 6 | protocol.
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| 7 | </p><p>
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| 8 | It also tries to answer some of the frequently asked questions such as:
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| 9 | </p><div class="orderedlist"><ol type="1"><li><p>
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| 10 | Is Samba secure when running on Unix? The xyz platform?
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| 11 | What about the root priveliges issue?
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| 12 | </p></li><li><p>Pros and cons of multithreading in various parts of Samba</p></li><li><p>Why not have a separate process for name resolution, WINS, and browsing?</p></li></ol></div></div><div class="sect1" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2556735"></a>Multithreading and Samba</h2></div></div></div><p>
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| 13 | People sometimes tout threads as a uniformly good thing. They are very
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| 14 | nice in their place but are quite inappropriate for smbd. nmbd is
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| 15 | another matter, and multi-threading it would be very nice.
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| 16 | </p><p>
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| 17 | The short version is that smbd is not multithreaded, and alternative
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| 18 | servers that take this approach under Unix (such as Syntax, at the
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| 19 | time of writing) suffer tremendous performance penalties and are less
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| 20 | robust. nmbd is not threaded either, but this is because it is not
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| 21 | possible to do it while keeping code consistent and portable across 35
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| 22 | or more platforms. (This drawback also applies to threading smbd.)
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| 23 | </p><p>
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| 24 | The longer versions is that there are very good reasons for not making
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| 25 | smbd multi-threaded. Multi-threading would actually make Samba much
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| 26 | slower, less scalable, less portable and much less robust. The fact
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| 27 | that we use a separate process for each connection is one of Samba's
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| 28 | biggest advantages.
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| 29 | </p></div><div class="sect1" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2556767"></a>Threading smbd</h2></div></div></div><p>
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| 30 | A few problems that would arise from a threaded smbd are:
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| 31 | </p><div class="orderedlist"><ol type="1"><li><p>
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| 32 | It's not only to create threads instead of processes, but you
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| 33 | must care about all variables if they have to be thread specific
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| 34 | (currently they would be global).
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| 35 | </p></li><li><p>
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| 36 | if one thread dies (eg. a seg fault) then all threads die. We can
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| 37 | immediately throw robustness out the window.
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| 38 | </p></li><li><p>
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| 39 | many of the system calls we make are blocking. Non-blocking
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| 40 | equivalents of many calls are either not available or are awkward (and
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| 41 | slow) to use. So while we block in one thread all clients are
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| 42 | waiting. Imagine if one share is a slow NFS filesystem and the others
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| 43 | are fast, we will end up slowing all clients to the speed of NFS.
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| 44 | </p></li><li><p>
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| 45 | you can't run as a different uid in different threads. This means
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| 46 | we would have to switch uid/gid on _every_ SMB packet. It would be
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| 47 | horrendously slow.
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| 48 | </p></li><li><p>
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| 49 | the per process file descriptor limit would mean that we could only
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| 50 | support a limited number of clients.
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| 51 | </p></li><li><p>
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| 52 | we couldn't use the system locking calls as the locking context of
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| 53 | fcntl() is a process, not a thread.
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| 54 | </p></li></ol></div></div><div class="sect1" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2556828"></a>Threading nmbd</h2></div></div></div><p>
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| 55 | This would be ideal, but gets sunk by portability requirements.
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| 56 | </p><p>
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| 57 | Andrew tried to write a test threads library for nmbd that used only
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| 58 | ansi-C constructs (using setjmp and longjmp). Unfortunately some OSes
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| 59 | defeat this by restricting longjmp to calling addresses that are
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| 60 | shallower than the current address on the stack (apparently AIX does
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| 61 | this). This makes a truly portable threads library impossible. So to
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| 62 | support all our current platforms we would have to code nmbd both with
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| 63 | and without threads, and as the real aim of threads is to make the
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| 64 | code clearer we would not have gained anything. (it is a myth that
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| 65 | threads make things faster. threading is like recursion, it can make
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| 66 | things clear but the same thing can always be done faster by some
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| 67 | other method)
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| 68 | </p><p>
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| 69 | Chris tried to spec out a general design that would abstract threading
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| 70 | vs separate processes (vs other methods?) and make them accessible
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| 71 | through some general API. This doesn't work because of the data
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| 72 | sharing requirements of the protocol (packets in the future depending
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| 73 | on packets now, etc.) At least, the code would work but would be very
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| 74 | clumsy, and besides the fork() type model would never work on Unix. (Is there an OS that it would work on, for nmbd?)
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| 75 | </p><p>
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| 76 | A fork() is cheap, but not nearly cheap enough to do on every UDP
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| 77 | packet that arrives. Having a pool of processes is possible but is
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| 78 | nasty to program cleanly due to the enormous amount of shared data (in
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| 79 | complex structures) between the processes. We can't rely on each
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| 80 | platform having a shared memory system.
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| 81 | </p></div><div class="sect1" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2556873"></a>nbmd Design</h2></div></div></div><p>
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| 82 | Originally Andrew used recursion to simulate a multi-threaded
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| 83 | environment, which use the stack enormously and made for really
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| 84 | confusing debugging sessions. Luke Leighton rewrote it to use a
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| 85 | queuing system that keeps state information on each packet. The
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| 86 | first version used a single structure which was used by all the
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| 87 | pending states. As the initialisation of this structure was
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| 88 | done by adding arguments, as the functionality developed, it got
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| 89 | pretty messy. So, it was replaced with a higher-order function
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| 90 | and a pointer to a user-defined memory block. This suddenly
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| 91 | made things much simpler: large numbers of functions could be
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| 92 | made static, and modularised. This is the same principle as used
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| 93 | in NT's kernel, and achieves the same effect as threads, but in
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| 94 | a single process.
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| 95 | </p><p>
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| 96 | Then Jeremy rewrote nmbd. The packet data in nmbd isn't what's on the
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| 97 | wire. It's a nice format that is very amenable to processing but still
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| 98 | keeps the idea of a distinct packet. See "struct packet_struct" in
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| 99 | nameserv.h. It has all the detail but none of the on-the-wire
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| 100 | mess. This makes it ideal for using in disk or memory-based databases
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| 101 | for browsing and WINS support.
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| 102 | </p></div></div><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="pt02.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="pt02.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="debug.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Part II. Samba Basics </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> Chapter 4. The samba DEBUG system</td></tr></table></div></body></html>
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