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322 Administering volume snapshots
Creating instant snapshots
created, and it must also have the same region size. See “Creating a volume
for use as a full-sized instant or linked break-off snapshot” on page 323 for
details.
Creating a shared cache object
To create a shared cache object
1 Decide on the following characteristics that you want to allocate to the cache
volume that underlies the cache object:
■ The size of the cache volume should be sufficient to record changes to
the parent volumes during the interval between snapshot refreshes. A
suggested value is 10% of the total size of the parent volumes for a
refresh interval of 24 hours.
■ If redundancy is a desired characteristic of the cache volume, it should
be mirrored. This increases the space that is required for the cache
volume in proportion to the number of mirrors that it has.
■ If the cache volume is mirrored, space is required on at least as many
disks as it has mirrors. These disks should not be shared with the disks
used for the parent volumes. The disks should also be chosen to avoid
impacting I/O performance for critical volumes, or hindering disk
group split and join operations.
2 Having decided on its characteristics, use the
vxassist command to create
the volume that is to be used for the cache volume. The following example
creates a mirrored cache volume, cachevol, with size 1GB in the disk
group, mydg, on the disks mydg16 and mydg17:
# vxassist -g mydg make cachevol 1g layout=mirror \
init=active mydg16 mydg17
The attribute init=active is specified to make the cache volume
immediately available for use.
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.