+++ /dev/null
-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 messy 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. 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.
-
-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?
---------------------------------
-
git.samba.org / sfrench / samba-autobuild / .git / commitdiff