VERITAS FlashSnap Point-In-Time-Copy Solutions Administrator's Guide

Creating a Shared Cache Object
26 VERITAS FlashSnap Point-In-Time Copy Solutions Administrator’s Guide
◆ 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, 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.
Caution 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.