Administration Reference

Vn

Purpose

Houses a chunk of AFS data in the disk cache

Description

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:

100
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, or if the argument is not included, from the third field of the /usr/vice/etc/cacheinfo file. 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. The afsd reference page 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.

The standard disk cache directory name is /usr/vice/cache, but it is acceptable to use a directory on a partition with more available space. To designate a different directory, change the value in the second field of the /usr/vice/etc/cacheinfo file before issuing the afsd command, or include the -cachedir argument to the afsd command.

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.

Cautions

Editing or removing a Vn file can cause a kernel panic. 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. If a Vn file is accidentally modified or deleted, rebooting the machine usually restores normal performance.

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