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346 Administering volume snapshots
Creating instant snapshots
Tuning the autogrow attributes of a cache
The highwatermark, autogrowby and maxautogrow attributes determine
how the VxVM cache daemon (
vxcached) maintains the cache if the autogrow
feature has been enabled and
vxcached is running:
■ 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 347, 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.