Veritas Storage Foundation 5.1 SP1 Advanced Features Administrator"s Guide (5900-1503, April 2011)

blocks), vxcached grows the size of the cache volume by the value of autogrowby
(default value is 20% of the size of the cache volume in blocks).
■ When cache usage reaches the high watermark value, and the new required
cache size would exceed the value of maxautogrow, vxcached deletes the oldest
snapshot in the cache. If there are several snapshots with the same age, the
largest of these is deleted.
If the autogrow feature has been disabled:
■ When cache usage reaches the high watermark value, vxcached deletes the
oldest snapshot in the cache. If there are several snapshots with the same age,
the largest of these is deleted. If there is only a single snapshot, this snapshot
is detached and marked as invalid.
Note: The vxcached daemon does not remove snapshots that are currently
open, and it does not remove the last or only snapshot in the cache.
If the cache space fills up, the snapshot is detached and marked as invalid. If this
happens, the snapshot is unrecoverable and must be removed. Enabling the
autogrow feature on the cache helps to avoid this situation from occurring.
However, for very small caches (of the order of a few megabytes), it is possible for
the cache to become exhausted before the system has time to respond and grow
the cache. In such cases, use the vxcache command to increase the size of the
cache, or to reduce the value of highwatermark.
If necessary, you can use the vxcache set command to change other autogrow
attribute values for a cache. For example, you can use the maxautogrow attribute
to limit the maximum size to which a cache can grow. To estimate this size,
consider how much the contents of each source volume are likely to change
between snapshot refreshes, and allow some additional space for contingency.
Warning: Ensure that the cache is sufficiently large, and that the autogrow
attributes are configured correctly for your needs.
See the vxcache(1M) manual page and the “Administering Volume Snapshots”
chapter in the Veritas Volume Manager Administrator’s Guide for more information
including how to grow, shrink and remove a storage cache.
Setting up volumes for instant snapshots
Creating a shared cache object
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