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