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224 lines
9.3 KiB
HTML
224 lines
9.3 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
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"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml">
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<head>
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<meta name="generator" content="HTML Tidy, see www.w3.org" />
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<title>Apache module mod_file_cache</title>
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</head>
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<!-- Background white, links blue (unvisited), navy (visited), red (active) -->
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<body bgcolor="#FFFFFF" text="#000000" link="#0000FF"
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vlink="#000080" alink="#FF0000">
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<!--#include virtual="header.html" -->
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<h1 align="CENTER">Module mod_file_cache</h1>
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<p><strong>This module should be used with care. You can easily
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create a broken site using mod_file_cache, so read this
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document carefully.</strong></p>
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<p><em>Caching</em> frequently requested files that change very
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infrequently is a technique for reducing server load.
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mod_file_cache provides two techniques for caching frequently
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requested <em>static</em> files. Through configuration
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directives, you can direct mod_file_cache to either open then
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mmap()a file, or to pre-open a file and save the file's open
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<em>file handle</em>. Both techniques reduce server load when
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processing requests for these files by doing part of the work
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(specifically, the file I/O) for serving the file when the
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server is started rather than during each request.</p>
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<p><code>mod_file_cache</code> is not compiled into the server
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by default. To use <code>mod_file_cache</code> you have to
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enable the following line in the server build
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<code>Configuration</code> file:</p>
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<pre>
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AddModule modules/experimental/mod_file_cache.o
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</pre>
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<br />
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<br />
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<p>Notice: You cannot use this for speeding up CGI programs or
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other files which are served by special content handlers. It
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can only be used for regular files which are usually served by
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the Apache core content handler.</p>
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<p>This module is an extension of and borrows heavily from the
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mod_mmap_static module in Apache 1.3.</p>
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<h2>Summary</h2>
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<p><code>mod_file_cache</code> caches a list of statically
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configured files via <code>MMapFile</code> or
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<code>CacheFile</code> directives in the main server
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configuration.</p>
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<p>Not all platforms support both directives. For example,
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Apache on Windows does not currently support the MMapStatic
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directive, while other platforms, like AIX, support both. You
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will receive an error message in the server error log if you
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attempt to use an unsupported directive. If given an
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unsupported directive, the server will start but the file will
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not be cached. On platforms that support both directives, you
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should experiment with both to see which works best for
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you.</p>
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<h3><code>MmapFile</code> Directive</h3>
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<p>The <code>MmapFile</code> directive of
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<code>mod_file_cache</code> maps a list of statically
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configured files into memory through the system call
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<code>mmap()</code>. This system call is available on most
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modern Unix derivates, but not on all. There are sometimes
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system-specific limits on the size and number of files that can
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be mmap()d, experimentation is probably the easiest way to find
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out.</p>
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<p>This mmap()ing is done once at server start or restart,
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only. So whenever one of the mapped files changes on the
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filesystem you <em>have</em> to restart the server (see the <a
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href="../stopping.html">Stopping and Restarting</a>
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documentation). To reiterate that point: if the files are
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modified <em>in place</em> without restarting the server you
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may end up serving requests that are completely bogus. You
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should update files by unlinking the old copy and putting a new
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copy in place. Most tools such as <code>rdist</code> and
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<code>mv</code> do this. The reason why this modules doesn't
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take care of changes to the files is that this check would need
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an extra <code>stat()</code> every time which is a waste and
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against the intent of I/O reduction.</p>
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<h3><code>CacheFile</code> Directive</h3>
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<p>The <code>CacheFile</code> directive of
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<code>mod_file_cache</code> opens an active <em>handle</em> or
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<em>file descriptor</em> to the file (or files) listed in the
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configuration directive and places these open file handles in
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the cache. When the file is requested, the server retrieves the
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handle from the cache and passes it to the sendfile() (or
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TransmitFile() on Windows), socket API.</p>
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<p>Insert more details about sendfile API...</p>
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<p>This file handle caching is done once at server start or
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restart, only. So whenever one of the cached files changes on
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the filesystem you <em>have</em> to restart the server (see the
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<a href="../stopping.html">Stopping and Restarting</a>
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documentation). To reiterate that point: if the files are
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modified <em>in place</em> without restarting the server you
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may end up serving requests that are completely bogus. You
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should update files by unlinking the old copy and putting a new
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copy in place. Most tools such as <code>rdist</code> and
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<code>mv</code> do this.</p>
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<h2>Directives</h2>
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<ul>
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<li><a href="#mmapfile">MMapFile</a></li>
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<li><a href="#cachefile">CacheFile</a></li>
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</ul>
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<hr />
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<h2><a id="mmapfile" name="mmapfile">MMapFile</a></h2>
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<p><a href="directive-dict.html#Syntax"
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rel="Help"><strong>Syntax:</strong></a> MMapFile
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<em>filename</em> [<em>filename</em>] ...<br />
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<a href="directive-dict.html#Default"
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rel="Help"><strong>Default:</strong></a> <em>None</em><br />
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<a href="directive-dict.html#Context"
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rel="Help"><strong>Context:</strong></a> server-config<br />
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<a href="directive-dict.html#Override"
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rel="Help"><strong>Override:</strong></a> <em>Not
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applicable</em><br />
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<a href="directive-dict.html#Status"
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rel="Help"><strong>Status:</strong></a> Experimental<br />
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<a href="directive-dict.html#Module"
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rel="Help"><strong>Module:</strong></a> mod_file_cache<br />
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<a href="directive-dict.html#Compatibility"
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rel="Help"><strong>Compatibility:</strong></a> Only in Apache
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1.3 (via mod_mmap_statis) or later.</p>
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<p>The <code>MMapFile</code> directive maps one or more files
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(given as whitespace separated arguments) into memory at server
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startup time. They are automatically unmapped on a server
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shutdown. When the files have changed on the filesystem at
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least a HUP or USR1 signal should be send to the server to
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re-mmap them.</p>
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<p>Be careful with the <em>filename</em> arguments: They have
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to literally match the filesystem path Apache's URL-to-filename
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translation handlers create. We cannot compare inodes or other
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stuff to match paths through symbolic links <em>etc.</em>
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because that again would cost extra <code>stat()</code> system
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calls which is not acceptable. This module may or may not work
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with filenames rewritten by <code>mod_alias</code> or
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<code>mod_rewrite</code>.</p>
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Example:
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<pre>
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MMapFile /usr/local/apache/htdocs/index.html
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</pre>
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<hr />
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<h2><a id="cachefile" name="cachefile">CacheFile</a></h2>
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<p><a href="directive-dict.html#Syntax"
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rel="Help"><strong>Syntax:</strong></a> CacheFile
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<em>filename</em> [<em>filename</em>] ...<br />
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<a href="directive-dict.html#Default"
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rel="Help"><strong>Default:</strong></a> <em>None</em><br />
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<a href="directive-dict.html#Context"
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rel="Help"><strong>Context:</strong></a> server-config<br />
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<a href="directive-dict.html#Override"
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rel="Help"><strong>Override:</strong></a> <em>Not
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applicable</em><br />
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<a href="directive-dict.html#Status"
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rel="Help"><strong>Status:</strong></a> Experimental<br />
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<a href="directive-dict.html#Module"
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rel="Help"><strong>Module:</strong></a> mod_file_cache<br />
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<a href="directive-dict.html#Compatibility"
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rel="Help"><strong>Compatibility:</strong></a> Only available
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in Apache 2.0 or later.</p>
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<p>The <code>CacheFile</code> directive opens handles to one or
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more files (given as whitespace separated arguments) and places
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these handles into the cache at server startup time. Handles to
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cached files are automatically closed on a server shutdown.
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When the files have changed on the filesystem, the server
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should be restarted to to re-cache them.</p>
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<p>Be careful with the <em>filename</em> arguments: They have
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to literally match the filesystem path Apache's URL-to-filename
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translation handlers create. We cannot compare inodes or other
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stuff to match paths through symbolic links <em>etc.</em>
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because that again would cost extra <code>stat()</code> system
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calls which is not acceptable. This module may or may not work
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with filenames rewritten by <code>mod_alias</code> or
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<code>mod_rewrite</code>.</p>
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Example:
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<pre>
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CacheFile /usr/local/apache/htdocs/index.html
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</pre>
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<p><strong>Note</strong>: don't bother asking for a for a
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directive which recursively caches all the files in a
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directory. Try this instead... See the <a
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href="core.html#include">Include</a> directive, and consider
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this command:</p>
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<pre>
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find /www/htdocs -type f -print \
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| sed -e 's/.*/mmapfile &/' > /www/conf/mmap.conf
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</pre>
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<!--#include virtual="footer.html" -->
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</body>
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</html>
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