HP P4000 9.0 HP StorageWorks P4000 SAN Solution User Guide - Page 197

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 Chapter 13 on page 213. 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 for detailed information about reviewing capacity. P4000 SAN Solution user guide 197