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<!-- $Id: mod_fsync.html,v 1.1 2004/11/04 20:19:35 tj Exp tj $ -->
<!-- $Source: /home/tj/proftpd/modules/doc/RCS/mod_fsync.html,v $ -->
<html>
<head>
<title>ProFTPD module mod_fsync</title>
</head>
<body bgcolor=white>
<hr>
<center>
<h2><b>ProFTPD module <code>mod_fsync</code></b></h2>
</center>
<hr><br>
<p>
On some kernels and/or filesystems, if there are files opened simultaneously
for reading and writing, the buffer cache algorithms may cause the
write I/O to swamp the read I/O, causing processes that are reading files
to slow down because of buffer cache misses. The Linux 2.4 kernel, for
example, suffers from this problem.
<p>
The <code>mod_fsync</code> module attempts to prevent such bottlenecks
by forcibly flushing to disk the buffers used for files open for writing
after a certain number of bytes have been written (for example, after
128 KB has been written to a file). This prevents the buffer cache from
being dominated by data from files being written, freeing up space for
data for files being read.
<p>
This module is contained in the <code>mod_fsync.c</code> file for ProFTPD
1.2.10rc1 and later, and is not compiled by default. Installation instructions
are discussed <a href="#Installation">here</a>.
<p>
The most current version of <code>mod_fsync</code> can be found at:
<pre>
<a href="http://www.castaglia.org/proftpd/">http://www.castaglia.org/proftpd/</a>
</pre>
<h2>Author</h2>
<p>
Please contact TJ Saunders <tj <i>at</i> castaglia.org> with any
questions, concerns, or suggestions regarding this module.
<h2>Directives</h2>
<ul>
<li><a href="#FsyncEngine">FsyncEngine</a>
<li><a href="#FsyncLog">FsyncLog</a>
<li><a href="#FsyncThreshold">FsyncThreshold</a>
</ul>
<hr>
<h2><a name="FsyncEngine">FsyncEngine</a></h2>
<strong>Syntax:</strong> FsyncEngine <em>on|off</em><br>
<strong>Default:</strong> FsyncEngine off<br>
<strong>Context:</strong> server config<br>
<strong>Module:</strong> mod_fsync<br>
<strong>Compatibility:</strong> 1.2.10rc1 and later
<p>
The <code>FsyncEngine</code> directive enables or disables the module's
runtime sync engine. If it is set to <em>off</em> this module does no
flushing of data outside of the normal I/O usage. Use this directive to
disable the module.
<p>
<hr>
<h2><a name="FsyncLog">FsyncLog</a></h2>
<strong>Syntax:</strong> FsyncLog <em>path</em><br>
<strong>Default:</strong> None<br>
<strong>Context:</strong> server config<br>
<strong>Module:</strong> mod_fsync<br>
<strong>Compatibility:</strong> 1.2.10c1 and later
<p>
The <code>FsyncLog</code> directive is used to a specify a log file for
<code>mod_fsync</code> reporting and debugging. The <em>path</em> parameter
must be the full path to the file to use for logging. Note that this path
must <b>not</b> be to a world-writeable directory and, unless
<code>AllowLogSymlinks</code> is explicitly set to <em>on</em>
(generally a bad idea), the path must <b>not</b> be a symbolic link.
<p>
<h2><a name="FsyncThreshold">FsyncThreshold</a></h2>
<strong>Syntax:</strong> FsyncThreshold <em>size</em><br>
<strong>Default:</strong> None<br>
<strong>Context:</strong> server config<br>
<strong>Module:</strong> mod_fsync<br>
<strong>Compatibility:</strong> 1.2.10rc1 and later
<p>
The <code>FsyncThreshold</code> directive configures a threshold value,
in bytes. When this number of bytes has been written,
<code>mod_fsync</code> will walk the list of files currently open for
writing, and will forcibly flush the data for those files to disk.
<p>
<b>Note</b>: This directive is required for <code>mod_fsync</code> to
operate.
<p>
<hr>
<h2><a name="Installation">Installation</a></h2>
To install <code>mod_fsync</code>, copy the <code>mod_fsync.c</code> file
into:
<pre>
<i>proftpd-dir</i>/contrib/
</pre>
after unpacking the latest proftpd-1.2.<i>x</i> source code. Then follow the
usual steps for using third-party modules in proftpd:
<pre>
./configure --with-modules=mod_fsync
make
make install
</pre>
<p>
<hr>
<h2><a name="Usage">Usage</a></h2>
<p>
<b>Example Configuration</b><br>
<pre>
<IfModule mod_fsync.c>
FsyncEngine on
FsyncLog /etc/ftpd/var/fsync.log
FsyncThreshold 131072 # 128KB
</IfModule>
</pre>
<p>
<hr><br>
Author: <i>$Author: tj $</i><br>
Last Updated: <i>$Date: 2004/11/04 20:19:35 $</i><br>
<br><hr>
<font size=2><b><i>
© Copyright 2004 TJ Saunders<br>
All Rights Reserved<br>
</i></b></font>
<hr><br>
</body>
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