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 V1.68.1"><link rel="start" 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#id2522836">Introduction</a></span></dt><dt><span class="sect1"><a href="architecture.html#id2522880">Multithreading and Samba</a></span></dt><dt><span class="sect1"><a href="architecture.html#id2522912">Threading smbd</a></span></dt><dt><span class="sect1"><a href="architecture.html#id2522974">Threading nmbd</a></span></dt><dt><span class="sect1"><a href="architecture.html#id2523031">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="id2522836"></a>Introduction</h2></div></div></div><p>
2 This document gives a general overview of how Samba works
3 internally. The Samba Team has tried to come up with a model which is
4 the best possible compromise between elegance, portability, security
5 and the constraints imposed by the very messy SMB and CIFS
8 It also tries to answer some of the frequently asked questions such as:
9 </p><div class="orderedlist"><ol type="1"><li><p>
10 Is Samba secure when running on Unix? The xyz platform?
11 What about the root priveliges issue?
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="id2522880"></a>Multithreading and Samba</h2></div></div></div><p>
13 People sometimes tout threads as a uniformly good thing. They are very
14 nice in their place but are quite inappropriate for smbd. nmbd is
15 another matter, and multi-threading it would be very nice.
17 The short version is that smbd is not multithreaded, and alternative
18 servers that take this approach under Unix (such as Syntax, at the
19 time of writing) suffer tremendous performance penalties and are less
20 robust. nmbd is not threaded either, but this is because it is not
21 possible to do it while keeping code consistent and portable across 35
22 or more platforms. (This drawback also applies to threading smbd.)
24 The longer versions is that there are very good reasons for not making
25 smbd multi-threaded. Multi-threading would actually make Samba much
26 slower, less scalable, less portable and much less robust. The fact
27 that we use a separate process for each connection is one of Samba's
29 </p></div><div class="sect1" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2522912"></a>Threading smbd</h2></div></div></div><p>
30 A few problems that would arise from a threaded smbd are:
31 </p><div class="orderedlist"><ol type="1"><li><p>
32 It's not only to create threads instead of processes, but you
33 must care about all variables if they have to be thread specific
34 (currently they would be global).
36 if one thread dies (eg. a seg fault) then all threads die. We can
37 immediately throw robustness out the window.
39 many of the system calls we make are blocking. Non-blocking
40 equivalents of many calls are either not available or are awkward (and
41 slow) to use. So while we block in one thread all clients are
42 waiting. Imagine if one share is a slow NFS filesystem and the others
43 are fast, we will end up slowing all clients to the speed of NFS.
45 you can't run as a different uid in different threads. This means
46 we would have to switch uid/gid on _every_ SMB packet. It would be
49 the per process file descriptor limit would mean that we could only
50 support a limited number of clients.
52 we couldn't use the system locking calls as the locking context of
53 fcntl() is a process, not a thread.
54 </p></li></ol></div></div><div class="sect1" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2522974"></a>Threading nmbd</h2></div></div></div><p>
55 This would be ideal, but gets sunk by portability requirements.
57 Andrew tried to write a test threads library for nmbd that used only
58 ansi-C constructs (using setjmp and longjmp). Unfortunately some OSes
59 defeat this by restricting longjmp to calling addresses that are
60 shallower than the current address on the stack (apparently AIX does
61 this). This makes a truly portable threads library impossible. So to
62 support all our current platforms we would have to code nmbd both with
63 and without threads, and as the real aim of threads is to make the
64 code clearer we would not have gained anything. (it is a myth that
65 threads make things faster. threading is like recursion, it can make
66 things clear but the same thing can always be done faster by some
69 Chris tried to spec out a general design that would abstract threading
70 vs separate processes (vs other methods?) and make them accessible
71 through some general API. This doesn't work because of the data
72 sharing requirements of the protocol (packets in the future depending
73 on packets now, etc.) At least, the code would work but would be very
74 clumsy, and besides the fork() type model would never work on Unix. (Is there an OS that it would work on, for nmbd?)
76 A fork() is cheap, but not nearly cheap enough to do on every UDP
77 packet that arrives. Having a pool of processes is possible but is
78 nasty to program cleanly due to the enormous amount of shared data (in
79 complex structures) between the processes. We can't rely on each
80 platform having a shared memory system.
81 </p></div><div class="sect1" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2523031"></a>nbmd Design</h2></div></div></div><p>
82 Originally Andrew used recursion to simulate a multi-threaded
83 environment, which use the stack enormously and made for really
84 confusing debugging sessions. Luke Leighton rewrote it to use a
85 queuing system that keeps state information on each packet. The
86 first version used a single structure which was used by all the
87 pending states. As the initialisation of this structure was
88 done by adding arguments, as the functionality developed, it got
89 pretty messy. So, it was replaced with a higher-order function
90 and a pointer to a user-defined memory block. This suddenly
91 made things much simpler: large numbers of functions could be
92 made static, and modularised. This is the same principle as used
93 in NT's kernel, and achieves the same effect as threads, but in
96 Then Jeremy rewrote nmbd. The packet data in nmbd isn't what's on the
97 wire. It's a nice format that is very amenable to processing but still
98 keeps the idea of a distinct packet. See "struct packet_struct" in
99 nameserv.h. It has all the detail but none of the on-the-wire
100 mess. This makes it ideal for using in disk or memory-based databases
101 for browsing and WINS support.
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>
projects / samba / blob