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

Chapter 2, Setting up Volumes for Instant Snapshots

Creating a Shared Cache Object

3. Use the vxmake cache command to create a cache object on top of the cache volume that

you created in the previous step:

# vxmake [-g diskgroup] cache cache_object cachevolname=volume \

[regionsize=size] [autogrow=on] [highwatermark=hwmk] \

[autogrowby=agbvalue] [maxautogrow=maxagbvalue]]

If the region size, regionsize, is specified, it must be a power of 2, and be greater than or

equal to 16KB (16k). If not specified, the region size of the cache is set to 64KB.

Note All space-optimized snapshots that share the cache must have a region size that is equal

to or an integer multiple of the region size set on the cache. Snapshot creation also fails

if the original volume’s region size is smaller than the cache’s region size.

If the cache is to be allowed to grow in size as required, specify autogrow=on. By default,

the ability to automatically grow the cache is turned off.

In the following example, the cache object, cobjmydg, is created over the cache volume,

cachevol, the region size of the cache is set to 32KB, and the autogrow feature is enabled:

# vxmake -g mydg cache cobjmydg cachevolname=cachevol \

regionsize=32k autogrow=on

4. Having created the cache object, use the following command to enable it:

# vxcache [-g diskgroup] start cache_object

For example to start the cache object, cobjmydg:

# vxcache -g mydg start cobjmydg

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:

◆ When cache usage reaches the high watermark value, highwatermark (default value is 90

percent), and the new required cache size would not exceed the value of maxautogrow

(default value is twice the size of the cache volume in blocks), 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).

◆ 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: