10.5 HP StoreVirtual Storage User Guide (AX696-96269, March 2013)

Plan how you intend to use snapshots, and the schedule and retention policy for schedules to
snapshot a volume. Snapshots record changes in data on the volume, so calculating the rate of
changed data in the client applications is important for planning schedules to snapshot a volume.
NOTE: Volume size, provisioning, and using snapshots should be planned together. If you intend
to use snapshots, review “Using snapshots” (page 165).
Managing capacity using volume size and snapshots
When you create a snapshot of a volume, the original volume is actually saved as the snapshot,
and a new volume (the “writable” volume) with the original name is created to record any changes
made to the volume’s data after the snapshot was created. Subsequent snapshots record only
changes made to the volume since the previous snapshot. Snapshots are always created as a thin
provisioned space, no matter whether its original volume is full or thin provisioned.
Volume size and snapshots
One implication of the relationship between volumes and snapshots is that the space used by the
writable volume can become very small when it records only the changes that have occurred since
the last snapshot was taken. This means that less space may be required for the writable volume.
Over time, you may find that space allocated for snapshots becomes larger, and the volume itself
becomes relatively small.
Schedules to snapshot a volume and capacity
When you have schedules to snapshot a volume, the recurrence or frequency, and the retention
policy for the schedules affect the amount of space used in the cluster. For example, it is possible
for a new snapshot and one snapshot scheduled for deletion to coexist in the cluster for some
period of time. If there is not sufficient room in the cluster for both snapshots, the scheduled snapshot
will not be created, and the schedule will not continue until an existing snapshot is deleted.
Therefore, if you want to retain (n) snapshots, the cluster should have space for (n+1).
Deleting snapshots
Another factor to note in planning capacity is the fact that when a snapshot is deleted, that
snapshot’s data is added to the snapshot or volume directly above it (the next newer snapshot).
The amount of space allocated for the volume or snapshot directly above the deleted snapshot
increases. See “Ongoing capacity management” (page 151) for detailed information about reviewing
capacity.
Ongoing capacity management
One of the critical functions of managing a SAN is monitoring usage and capacity. The CMC
provides detailed information about overall cluster capacity and usage, as well as detail about
provisioning and storage system capacity.
Number of volumes and snapshots
For information about the recommended maximum number of volumes and snapshots that can be
created in a management group, see “Configuration Summary overview” (page 110).
Reviewing SAN capacity and usage
You can review detailed information about the capacity of your cluster, the volumes it contains,
and the provisioning of the storage systems in the cluster. This information is presented in a series
of tab windows displayed at the cluster level.
Ongoing capacity management 151