Hp LeftHand P4000 SAN Solutions User Manual download pdf (Page 162)

13 Using snapshots

Snapshots are a copy of a volume for use with backup and other applications.

Types of snapshots

Snapshots are one of the following types:

• Regular or point-in-time —Snapshot that is taken at a specific point in time. However, an

application writing to that volume may not be quiesced. Thus, data may be in flight or cached

and the actual data on the volume may not be consistent with the application's view of the

data.

• Application-managed —Snapshot of a volume that is taken while the application that is

serving that volume is quiesced. Because the application is quiesced, the data in the snapshot

is consistent with the application's view of the data. That is, no data was in flight or cached

waiting to be written. This type requires the use of the HP StoreVirtual Application Aware

Snapshot Manager. For more information, see “Prerequisites for application-managed

snapshots” (page 163).

When an application uses two or more volumes, those associated volumes are called a volume

set. For example, you may set up Exchange to use two volumes to support a StorageGroup:

one for mailbox data and one for logs. Those two volumes make a volume set.

Snapshots versus backups

Backups are typically stored on different physical devices, such as tapes. Snapshots are stored in

the same cluster as the volume. Therefore, snapshots protect against data deletion, but not device

or storage media failure. Use snapshots along with backups to improve your overall data backup

strategy.

Uses and best practices for snapshots

You create snapshots from a volume on the cluster. At any time you can roll a volume back to a

specific snapshot, create a SmartClone volume, or use Remote Copy from a snapshot. You can

mount a snapshot to a different server and recover data from the snapshot to that server. When

planning to use snapshots, ensure that there is space available on the cluster to create the snapshot.

For information about snapshots and cluster space, see “The effect of snapshots on cluster space”

(page 147)

Snapshots can be used for these cases:

• Source for creating backups

Best Practice—Plan to use a single snapshot and delete it when you are finished.

• Data or file system preservation before upgrading software

Best Practice—Plan to use a single snapshot and delete it when you are finished.

• Protection against data deletion

Best Practice—Plan to use a series of snapshots, deleting the oldest on a scheduled basis.

When planning this schedule, consider the following questions:

◦ Is space available on the cluster to create the snapshots?

◦ What is the optimum schedule and retention policy for this schedule to snapshot a volume?

See “Planning snapshots” (page 163) for the average daily change rates for some common

applications.

For example, if you are using these backups as part of a disaster recovery plan, you

might schedule a daily snapshot of the volume and retain 7 copies. A second schedule

162 Using snapshots

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