Requirements and guidelines for snapshots

Snapshot images and snapshot groups

• Each snapshot image is associated with exactly one snapshot group.
• A snapshot group is created the first time you create a scheduled or instant snapshot image for an associated object. This creates reserved capacity.

  You can view snapshot groups from the Pools & Volume Groups page.

• Scheduled snapshot images do not occur when the storage array is offline or powered off.
• If you delete a snapshot group that has a snapshot schedule, the snapshot schedule is also deleted.
• If you have a snapshot volume that you no longer need, you can reuse it, along with any associated reserved capacity, instead of deleting it. This creates a different snapshot volume of the same base volume. You can re-associate the snapshot volume or snapshot consistency group snapshot volume with the same snapshot image or a different snapshot image, as long as the snapshot image is in the same base volume.

Snapshot consistency group

• A snapshot consistency group contains one snapshot group for each volume that is a member of the snapshot consistency group.
• You can associate a snapshot consistency group with only one schedule.
• If you delete a snapshot consistency group that has a snapshot schedule, the snapshot schedule is also deleted.
• You cannot individually manage a snapshot group that is associated with a snapshot consistency group. Instead, you must perform the manage operations (create snapshot image, delete snapshot image or snapshot group, and rollback snapshot image) at the snapshot consistency group level.

Base volume

• A snapshot volume must have the same Data Assurance (DA) and security settings as the associated base volume.
• You cannot create a snapshot volume of a failed base volume.
• If the base volume resides on a volume group, the member volumes for any associated snapshot consistency group can reside on either a pool or volume group.
• If a base volume resides on a pool, all member volumes for any associated snapshot consistency group must reside on the same pool as the base volume.

Reserved capacity

• Reserved capacity is associated with only one base volume.
• Using a schedule can result in a large number of snapshot images. Make sure you have sufficient reserved capacity for scheduled snapshots.
• The reserved capacity volume for a snapshot consistency group must have the same Data Assurance (DA) and security settings as its associated base volume for the snapshot consistency group member volume.

Pending snapshot images

Snapshot image creation might remain in a Pending state in the following conditions:

• The base volume that contains this snapshot image is a member of an asynchronous mirror group.
• The base volume is currently in a synchronization operation. The snapshot image creation completes as soon as the synchronization operation is complete.

Maximum number of snapshot images

• If a volume is a member of a snapshot consistency group, System Manager creates a snapshot group for that member volume. This snapshot group counts towards the maximum allowable number of snapshot groups per base volume.
• If you attempt to create a snapshot image on a snapshot group or snapshot consistency group, but the associated group has reached its maximum number of snapshot images, you have two options:
  • Enable automatic deletion for the snapshot group or snapshot consistency group.
  • Manually delete one or more snapshot images from the snapshot group or snapshot consistency group and retry the operation.

Auto-deletion

If the snapshot group or snapshot consistency group is enabled for automatic deletion, System Manager deletes the oldest snapshot image when the system creates a new one for the group.

Rollback operation

• You cannot perform the following actions when a rollback operation is in progress:
  • Delete the snapshot image that is being used for the rollback.
  • Create a new snapshot image for a base volume that is participating in a rollback operation.
  • Change the associated snapshot group’s Repository-Full Policy.
• You cannot start a rollback operation when any of these operations are in progress:
  • Capacity expansion (adding capacity to a pool or volume group)
  • Volume expansion (increasing the capacity of a volume)
  • RAID level change for a volume group
  • Segment size change for a volume
• You cannot start a rollback operation if the base volume is participating in a volume copy.
• You cannot start a rollback operation if the base volume is a secondary volume in a remote mirror.
• A rollback operation fails if any of the used capacity in the associated snapshot repository volume has unreadable sectors.