Apache Module mod_file_cache
Summary
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.
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()ed, 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.
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.
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
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 file-path 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.
Example
CacheFile /usr/local/apache/htdocs/index.html
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 file-path 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.
Example
MMapFile /usr/local/apache/htdocs/index.html
