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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:

If the autogrow feature has been disabled:

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, you can increase the size of the cache manually.

See "Growing and shrinking a cache" on page 328.

Alternatively, you can 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.

Warning: Ensure that the cache is sufficiently large, and that the autogrow attributes are configured correctly for your needs.