Veritas Storage Foundation Intelligent Storage Provisioning 5.0.1 Administrators Guide, HP-UX 11i v3, First Edition, November 2009

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Tuning the autogrow attributes
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.
The following behaviors occur when cache usage reaches the high watermark
value:
With 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).
If 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, the following behavior occurs:
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.
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, or use the vxcache
set command to reduce the value of highwatermark for the cache object, as shown
in the following example-
# vxcache -g mydg set highwatermark=60 cobjmydg
See Growing and shrinking a cache on page 106.
105Administering instant snapshots
Creating a shared cache volume and preparing space-optimized snapshots