Veritas Volume Manager 5.0 Administrator's Guide (September 2006)

335Administering volume snapshots
Creating instant snapshots
■ 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, either
increase the size of the cache manually as described in “Growing and shrinking a cache”
on page 336, or 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 for details.
Caution: Ensure that the cache is sufficiently large, and that the autogrow attributes are
configured correctly for your needs.