Introduction to Snapshots

A snapshot is a point-in-time copy of volume data that can protect against mistakes, viruses, or database corruption. Snapshot creation does not disrupt access to the volume.

Like volumes, snapshots appear on the network as iSCSI targets and can be set online and accessed by hosts with iSCSI initiators. You can recover volume data by restoring a volume from a snapshot or by cloning a snapshot, which creates a new volume.

To create a snapshot, you must first reserve snapshot space for the volume. Snapshot reserve is consumed from the same pool in which the volume resides, and is based on a percentage of the current volume reserve.

There are several ways to create snapshots:

A snapshot name is generated automatically, based on the volume name and the date and time when the snapshot was created (for example, staff1-2006-05-08-11:29:29.1). However, where the GUI displays snapshots under a volume name (for example, in the far left panel), the snapshots are identified only by the timestamp.

An iSCSI target name for a snapshot is also automatically generated. The iSCSI target name will consist of the iSCSI prefix (iqn.2001-05.com.equallogic), followed by a generated string, followed by the snapshot name.

As with volumes, host access to snapshots is controlled by access control records. A volume and its snapshots share a list of access control records. You can specify that a record apply only to the volume, only to the volume’s snapshots, or to both the volume and its snapshots. See Managing Access Controls for Volumes and Snapshots for more information.

PS Series group snapshot technology uses a copy-on-write technique. When users make changes to the base volume (or to the snapshot), the group tracks the changes in the snapshot reserve.

A warning is issued when the amount of free snapshot reserve for the volume falls below the user-defined warning threshold. You can increase the reserve, as needed, if pool space is available.

Note: A snapshot depends on its base volume. If a volume goes offline because of a problem (for example, because a member is offline due to network problems), all its snapshots will be set offline. If you delete a volume, all its snapshots are also deleted.

There are several mechanisms that control how many volume snapshots can exist at one time:

In some cases, you may want to preserve the data in a snapshot. To do this, you can clone the snapshot, as described in Cloning a Snapshot. For example, if a snapshot will be automatically deleted because of lack of snapshot reserve, you can clone the snapshot to preserve the data.