Administrator Guide

About operating with a single controller
If you purchased a 2U controller enclosure with a single controller module, note that it does not offer redundant configuration
and, in the case of controller failure, leaves the system at risk for data unavailability. For more information, see About data
protection with a single controller.
NOTE: If you are operating a system with a single controller, some functionality described in the documentation may be
unavailable or not applicable to your system. For example, only one storage pool can exist and text about controller failover
and recovery is not applicable.
About snapshots
The system can create snapshots of virtual 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 cannot be expanded.
When you reach the maximum number of snapshots for your system, before you can create a new snapshot, you must delete an
existing snapshot. To view the maximum number of snapshots for your system, see the System configuration limits topic in
the PowerVault Manager help.
The system treats a snapshot like any other volume. The snapshot can be mapped to hosts with read-only access, read-write
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 the event log 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. For more information, see the Dell
EMC PowerVault ME4 Series Storage System CLI Guide.
Snapshot creation and levels
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. Since snapshots reference volumes, they take up no space unless they or the source volume or source
snapshot is modified.
Space does not have to be reserved for snapshots because all space in the pool is available for them. 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.
Because a snapshot can be the source of other snapshots, a single virtual volume can be the progenitor of many levels of
snapshots. Originating from an original base volume, the levels of snapshots create a snapshot tree that can include up to 254
snapshots, each of which can also be thought of as a leaf of the tree. When snapshots in the tree are the source of additional
snapshots, they create a new branch of the snapshot tree and are considered the parent snapshot of the child snapshots, which
are the leaves of the branch.
The tree can contain snapshots that are identical to the volume or have content that has been later modified. Once the
254-snapshot limit has been reached, you cannot create additional snapshots of any item in the tree until you manually delete
existing snapshots from the tree. You can only delete snapshots that do not have any child snapshots.
You cannot expand the base volume of a snapshot tree or any snapshots in the tree.
Rollback and reset snapshot features
With the rollback feature, if the contents of the selected snapshot have changed since it was created, the modified contents
will overwrite those of the source volume or snapshot during a rollback. Since virtual snapshots are copies of a point in time,
Getting started
27