Snapshots are fully supported with volume groups on a PowerStore system. A protection policy containing a snapshot rule can be assigned to the volume group to take snapshots at a defined interval. Snapshots can also be taken manually on the volume group or on individual volumes within the volume group at any time. This task can be done from the Snapshots tab within the Protection tab of the volume group or member volume.
Volumes can also be added or removed from a volume group without affecting data protection on the group. When a volume is removed from a volume group, no snapshots on the group are deleted or otherwise changed. If replication is configured, it continues and any changes to the group are propagated to the destination during the next sync. If the volume group has a protection policy that is assigned to it and a volume is removed, the policy is automatically assigned to the volume that is removed from the group to continue data protection. Replication on the volume that is being removed from the volume group will continue once a sync occurs on the volume group it was removed from.
When attempting the restore or refresh operations on a snapshot of a volume group, ensure that the number of volumes that were in the group when the snapshot was taken match the number of volumes in the volume group that is being restored or refreshed. For instance, if the snapshot was taken when the group had five members, it cannot be used for a restore if the group does not currently contain the five original members. To access this data, you can create a thin clone from the snapshot. To view the number of members of the group when the snapshot was taken, reference the Volume Members column on the snapshot tab.
The write-order consistency setting is a property of the volume group. This setting is enabled by default but can be changed at the creation of the volume group or later. The write-order consistency setting controls whether a snapshot is created at a consistent time across all members of the group. If enabled, the system takes a snapshot at the exact same time across all objects to keep the point-in-time image consistent for the entire group. If disabled, there is a chance that the snapshots on individual volumes within volume group are taken at slightly different times with possibly newly written data. When the snapshot is taken, the write-order consistency setting is marked as a property of the snapshot and affects what operations can be done on the snapshot. A column in the snapshot list for volume groups exists to view the write-order consistent property for each snapshot.
When write-order consistency is Yes for a snapshot, the restore and refresh operations have different capabilities than when it is No. When enabled on the snapshot, the restore and refresh operations affect the entire volume group, regardless of the current setting on the volume group. For instance, if Restore is used, all members of the group are restored from the snapshot image. This behavior is the same for the refresh operation. If write-order consistency is No for the snapshot, the restore and refresh operations can be issued to individual volumes within a volume group.
The write-order consistency setting also affects the ability to assign a protection policy to a volume group and its members. When write-order consistency is enabled on the group, users can only assign a protection policy to the volume group itself. Assigning a protection policy to an individual member is not supported. When write-order consistency is disabled on the volume group, users can choose to assign a policy to the group, or its individual members, but not both. This action provides flexibility for protecting the various members of the group with different protection policies.
When a volume group is deleted, the user can delete the volume group and retain its members or delete the group along with its members. When only the volume group is deleted, all snapshots taken of the group are also deleted. Any snapshots that are taken of the individual volumes remain. In either case, any thin clones that are created of the volume group or from a snapshot of the volume group also remain unaffected.