Veritas FlashSnap Point-In-Time Copy Solutions 5.0 AdministratorÆs Guide, HP-UX 11i v3, First Edition, May 2008

29Setting up volumes for instant snapshots
Creating a shared cache object
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:
■ 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.