mod_cache_disk
Disk based storage module for the HTTP caching filter.
Extension
mod_cache_disk.c
cache_disk_module

mod_cache_disk implements a disk based storage manager for mod_cache.

The headers and bodies of cached responses are stored separately on disk, in a directory structure derived from the md5 hash of the cached URL.

Multiple content negotiated responses can be stored concurrently, however the caching of partial content is not yet supported by this module.

Atomic cache updates to both header and body files are achieved without the need for locking by storing the device and inode numbers of the body file within the header file. This has the side effect that cache entries manually moved into the cache will be ignored.

The htcacheclean tool is provided to list cached URLs, remove cached URLs, or to maintain the size of the disk cache within size and/or inode limits. The tool can be run on demand, or can be daemonized to offer continuous monitoring of directory sizes.

# Note:

mod_cache_disk requires the services of mod_cache, which must be loaded before mod_cache_disk.

# Note:

mod_cache_disk uses the sendfile feature to serve files from the cache when supported by the platform, and when enabled with EnableSendfile. However, per-directory and .htaccess configuration of EnableSendfile are ignored by mod_cache_disk as the corresponding settings are not available to the module when a request is being served from the cache.

CacheRoot
The directory root under which cache files are stored
CacheRoot directory
server config
virtual host

The CacheRoot directive defines the name of the directory on the disk to contain cache files. If the mod_cache_disk module has been loaded or compiled in to the Apache server, this directive must be defined. Failing to provide a value for CacheRoot will result in a configuration file processing error. The CacheDirLevels and CacheDirLength directives define the structure of the directories under the specified root directory.

CacheRoot c:/cacheroot
CacheDirLevels
The number of levels of subdirectories in the cache.
CacheDirLevels levels
CacheDirLevels 2
server config
virtual host

The CacheDirLevels directive sets the number of subdirectory levels in the cache. Cached data will be saved this many directory levels below the CacheRoot directory.

A high value for CacheDirLevels combined with a low value for CacheDirLength will result in a relatively deep hierarchy, with a small number of subdirectories at each level.

The result of CacheDirLevels* CacheDirLength must not be higher than 20.

CacheDirLength
The number of characters in subdirectory names
CacheDirLength length
CacheDirLength 2
server config
virtual host

The CacheDirLength directive sets the number of characters for each subdirectory name in the cache hierarchy. It can be used in conjunction with CacheDirLevels to determine the approximate structure of your cache hierarchy.

A high value for CacheDirLength combined with a low value for CacheDirLevels will result in a relatively flat hierarchy, with a large number of subdirectories at each level.

The result of CacheDirLevels* CacheDirLength must not be higher than 20.

CacheMinFileSize
The minimum size (in bytes) of a document to be placed in the cache
CacheMinFileSize bytes
CacheMinFileSize 1
server config
virtual host
directory
.htaccess

The CacheMinFileSize directive sets the minimum size, in bytes, for a document to be considered for storage in the cache.

CacheMinFileSize 64
CacheMaxFileSize
The maximum size (in bytes) of a document to be placed in the cache
CacheMaxFileSize bytes
CacheMaxFileSize 1000000
server config
virtual host
directory
.htaccess

The CacheMaxFileSize directive sets the maximum size, in bytes, for a document to be considered for storage in the cache.

CacheMaxFileSize 64000
CacheReadSize
The minimum size (in bytes) of the document to read and be cached before sending the data downstream
CacheReadSize bytes
CacheReadSize 0
server config
virtual host
directory
.htaccess

The CacheReadSize directive sets the minimum amount of data, in bytes, to be read from the backend before the data is sent to the client. The default of zero causes all data read of any size to be passed downstream to the client immediately as it arrives. Setting this to a higher value causes the disk cache to buffer at least this amount before sending the result to the client. This can improve performance when caching content from a reverse proxy.

This directive only takes effect when the data is being saved to the cache, as opposed to data being served from the cache.

CacheReadSize 102400
CacheReadTime
The minimum time (in milliseconds) that should elapse while reading before data is sent downstream
CacheReadTime milliseconds
CacheReadTime 0
server config
virtual host
directory
.htaccess

The CacheReadTime directive sets the minimum amount of elapsed time that should pass before making an attempt to send data downstream to the client. During the time period, data will be buffered before sending the result to the client. This can improve performance when caching content from a reverse proxy.

The default of zero disables this option.

This directive only takes effect when the data is being saved to the cache, as opposed to data being served from the cache. It is recommended that this option be used alongside the CacheReadSize directive to ensure that the server does not buffer excessively should data arrive faster than expected.

CacheReadTime 1000