~zulcss/samba/server-dailies-3.4

<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>

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.

</p><p>

It also tries to answer some of the frequently asked questions such as:

</p><div class="orderedlist"><ol type="1"><li><p>

Is Samba secure when running on Unix? The xyz platform?

What about the root priveliges issue?

</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>

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.

</p><p>

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.)

</p><p>

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.

</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>

A few problems that would arise from a threaded smbd are:

</p><div class="orderedlist"><ol type="1"><li><p>

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).

</p></li><li><p>

if one thread dies (eg. a seg fault) then all threads die. We can

immediately throw robustness out the window.

</p></li><li><p>

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.

</p></li><li><p>

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.

</p></li><li><p>

the per process file descriptor limit would mean that we could only

support a limited number of clients.

</p></li><li><p>

we couldn't use the system locking calls as the locking context of

fcntl() is a process, not a thread.

</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>

This would be ideal, but gets sunk by portability requirements.

</p><p>

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)

</p><p>

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?)

</p><p>

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.

</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>

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.

</p><p>

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

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>