The Effect Of Snapshots On Cluster Space; Managing Capacity Using Volume Size And Snapshots; Volume Size And Snapshots; Schedules To Snapshot A Volume And Capacity - HP StorageWorks P4000 User Manual

At any time you can roll back to a specific snapshot. When you do roll back, you must delete all the
snapshots created after that snapshot. Also, using an iSCSI initiator, you can mount a snapshot to a
different server and recover data from the snapshot to that server.

The effect of snapshots on cluster space

Snapshots take up space on the cluster. Because snapshots are a thin provisioned space, they save
space compared to a full provisioned space.
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

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
capacity management
Chapter 13 on page 213.
for detailed information about reviewing capacity.
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