Dell PowerVault ME5 Series Administrator's Guide

Snapshots

The system can create snapshots of volumes up to the maximum number supported by your system. Snapshots provide data protection by enabling you to create and save source volume data states at the point in time when the snapshot was created. Snapshots can be created manually or you can schedule snapshot creation. After a snapshot has been created, the source volume can be expanded, but the snapshot volume cannot be expanded.

To view the maximum number of snapshots for your system, see System configuration limits.

When you reach the maximum number of snapshots for your system, before you can create a new snapshot, you must delete an existing snapshot.

The system treats a snapshot like any other volume.

• Snapshots attach to hosts with read-write access by default. You can then change permissions to read-only access or no access, depending on the purpose of the snapshot.
• Snapshots use the rollback feature, which replaces the data of a source volume or snapshot with the data of a snapshot that was created from it.
• Snapshots also use the reset snapshot feature, which enables you to replace the data in a snapshot with the current data in the source volume. When you reset a snapshot, the snapshot name and mappings are not changed.

The set snapshot-space CLI command enables you to set the percent of the pool that can be used for snapshots (the snapshot space). Optionally, you can specify a limit policy to enact when the snapshot space reaches the percentage. You can set the policy to either notify you via events that the percentage has been reached (in which case the system continues to take snapshots, using the general pool space), or to notify you and trigger automatic deletion of snapshots. If automatic deletion is triggered, snapshots are deleted according to their configured retention priority.

Creating snapshots is a fast and efficient process that merely consists of pointing to the same data to which the source volume or snapshot points. (A snapshot takes up no space unless it is directly modified, or the source volume or source snapshot to which the snapshot refers to is modified.) Space does not have to be reserved for snapshots because all space in the pool is available for them, based upon the limit policy that is set. It is easy to take snapshots of snapshots and use them in the same way that you would use any volume. Since snapshots have the same structure as volumes, the system treats them the same way.