afs_cache - Format of data stored in an AFS client disk cache
The disk cache on a client machine is composed of multiple Vn files that contain the data, a CacheItems file that records index information for all of the Vn files, and a VolumeItems file that records the mapping between volume name and mount point for volumes.
When it initializes, the Cache Manager creates the cache files in the configured cache location. The standard directory name is /var/cache/yfs, but it is acceptable to use a directory on a partition with more available space. To designate a different directory, include the -cachedir argument to the afsd command.
The CacheItems file records information about each file in the disk cache on a client machine (each Vn file). The information includes the file ID number and associated volume version number of the AFS file currently stored in the Vn file, which enables the Cache Manager to determine which Vn file contains the AFS data it needs to present to an application.
As it initializes, the Cache Manager creates the binary-format CacheItems file in the same local disk cache directory as the Vn files that the CacheItems file describes, and it must always remain there.
The VolumeItems file records the mapping between volume name and mount point for each volume that the Cache Manager has accessed since it initialized on a client machine using a disk cache. The Cache Manager uses the mappings to respond correctly to queries about the current working directory, which can come from the operating system or commands such as the UNIX pwd command.
As it initializes, the Cache Manager creates the binary-format VolumeItems file in the local disk cache directory, and it must always remain there.
A Vn file can store a chunk of cached AFS data on a client machine that is using a disk cache. As the Cache Manager initializes, it verifies that the local disk cache directory houses a number of Vn files equal to the largest of the following:
One and a half times the result of dividing the cache size by the chunk size (cachesize/chunksize * 1.5).
The result of dividing the cache size by 10 MB (10,240).
The Cache Manager determines the cache size from the -blocks argument to the afsd command. The default chunk size is 64 KB; use the -chunksize argument to the afsd command to override it. To override the default number of chunks resulting from the calculation, include the -files argument to the afsd command. afsd(8) describes the restrictions on acceptable values for each of the arguments.
If the disk cache directory houses fewer Vn files than necessary, the Cache Manager creates new ones, assigning each a unique integer n that distinguishes it from the other files; the integers start with 1 and increment by one for each Vn file created. The Cache Manager removes files if there are more than necessary. The Cache Manager also adds and removes Vn files in response to the fs setcachesize command, which can be used to alter the cache size between reboots.
Vn files expand and contract to accommodate the size of the AFS directory listing or file they temporarily house. As mentioned, by default each Vn file holds up to 64 KB (65,536 bytes) of a cached AFS element. AFS elements larger than 64 KB are divided among multiple Vn files. If an element is smaller than 64 KB, the Vn file expands only to the required size. A Vn file accommodates only a single element, so if there many small cached elements, it is possible to exhaust the available Vn files without reaching the maximum cache size.
Editing or removing the CacheItems or VolumeItems files or a Vn file can cause a kernel panic. If the contents of Vn files seem out of date, clear the files by using the fs flush or fs flushvolume command. If any of the cache files are accidentally modified or deleted, rebooting the machine usually restores normal performance.
To alter cache size (and thus the number of Vn files) between reboots, use the fs setcachesize command. Alternatively, alter the value of the -blocks, -files or -chunksize arguments to the afsd command invoked in the machine's AFS initialization file, and reboot. To refresh the contents of one or more Vn files, use the fs flush or fs flushvolume command.
afsd(8), fs_checkvolumes(1), fs_flush(1), fs_flushvolume(1), fs_setcachesize(1)
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