Veritas Volume Manager 5.0.1 Administrator's Guide, HP-UX 11i v3, First Edition, November 2009

■ When cache usage reaches the high watermark value, highwatermark (default
value is 90 percent), 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).
The new required cache size cannot exceed the value of maxautogrow (default
value is twice 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 becomes exhausted, 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 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, you can increase the size of the cache manually.
See “Growing and shrinking a cache” on page 395.
Alternatively, you can use the vxcache set command to reduce the value of
highwatermark as shown in this example:
# vxcache -g mydg set highwatermark=60 cobjmydg
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.
If necessary, you can use the vxcache set command to change other autogrow
attribute values for a cache.
See the vxcache(1M) manual page.
Administering volume snapshots
Creating instant snapshots
394