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
Directive
The 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
Directive
The 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