346Administering 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.