Hp LeftHand P4000 SAN Solutions User Manual download pdf (Page 148)

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 162).

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 148) 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 107).

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.

148 Provisioning storage

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Hp LeftHand P4000 SAN Solutions User Manual Page 148