Administrator Guide

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, a modified snapshot cannot
be reverted. If you want a virtual snapshot to provide the capability to revert the contents of the source volume or snapshot to when the
snapshot was created, create a snapshot for this purpose and archive it so you do not change the contents.
For snapshots, the reset snapshot feature is supported for all snapshots in a tree hierarchy. However, a snapshot can only be reset to the
immediate parent volume or snapshot from which it was created.
About copying volumes
For virtual storage, this feature enables you to copy a virtual base volume or snapshot to a new virtual volume.
The volume copy feature enables you to copy a base volume and snapshot to a new volume. This feature creates a complete “physical”
copy of a base volume or virtual snapshot within a storage system. It is an exact copy of the source as it existed at the time the copy
operation was initiated, consumes the same amount of space as the source, and is independent from an I/O perspective. In contrast, the
snapshot feature creates a point-in-time logical copy of a volume, which remains dependent on the source volume.
The volume copy feature provides the following benefits:
• Additional data protection: An independent copy of a volume provides additional data protection against a complete source volume
failure. If the source volume fails, the copy can be used to restore the volume to the point in time when the copy was created.
• Non-disruptive use of production data: With an independent copy of the volume, resource contention and the potential performance
impact on production volumes is mitigated. Data blocks between the source and the copied volumes are independent, versus shared
with snapshots, so that I/O is to each set of blocks respectively. Application I/O transactions are not competing with each other when
accessing the same data blocks.
For more information about creating a copy of a virtual base volume or snapshot, see Copying a volume or snapshot.
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Getting started