Dell PowerVault ME4 Series Storage System Administrator’s Guide

Maintaining replication snapshot history from the Volumes topic

A replication set can be configured to maintain a replication snapshot history. As part of handling a replication, the replication set will automatically take a snapshot of the primary or secondary volumes, or both, thereby creating a history of data that has been replicated over time. This feature can be enabled for a secondary volume or for a primary volume and its secondary volume, but not for a volume group.

When this feature is enabled:

• For a primary volume, when a replication starts it will create a snapshot of the data image being replicated.
• For a secondary volume, when a replication successfully completes it will create a snapshot of the data image just transferred to the secondary volume. (This is in contrast to the primary volume snapshot, which is created before the sync.) If replication does not complete, a snapshot will not be created.
• You can set the number of snapshots to retain from 1 through 16, referred to as the snapshot retention count. This setting applies to management of snapshots for both the primary and secondary volume and can be changed at any time. Its value must be greater than the number of existing snapshots in the replication set, regardless of whether snapshot history is enabled. If you select a snapshot retention count value that is less than the current number of snapshots, an error message is displayed. Thus, you must manually delete the excess snapshots before reducing the snapshot count setting. When the snapshot count is exceeded, the oldest unmapped snapshot will be discarded automatically.
• The snapshots are named basename_nnnn where _nnnn starts at 0000 and increments for each subsequent snapshot. If primary volume snapshots are enabled, snapshots with the same name will exist on the primary and secondary systems. The snapshot number is incremented each time a replication is requested, whether or not the replication completes — for example, if the replication was queued and subsequently removed from the queue.
• If the replication set is deleted, any existing snapshots automatically created by snapshot history rules will not be deleted. You will be able to manage those snapshots like any other snapshots.
• Manually creating a snapshot will not increase the snapshot count associated with the snapshot history. Manually created snapshots are not managed by the snapshot history feature. The snapshot history feature generates a new name for the snapshot that it intends to create. If a volume of that name already exists, the snapshot history feature will not overwrite that existing volume. Snapshot numbering will continue to increment, so the next time the snapshot history feature runs, the new snapshot name will not conflict with that existing volume name.
• A snapshot created by this feature is counted against the system-wide maximum snapshots limit, with the following result:

  • If the snapshot count is reached before the system limit then the snapshot history is unchanged.

  • If the system limit is reached before the snapshot count then the snapshot history stops adding or updating snapshots.

• A mapped snapshot history snapshot will not be deleted until after it is unmapped.
• The snapshot basename and snapshot retention count settings only take effect when snapshot history is set to secondary or both, although these settings can be changed at any time.
• You can set the retention priority for snapshots to the following. In a snapshot tree, only leaf snapshots can be deleted automatically.

  Table 1. Snapshot retention prioritySnapshot retention priority

  Retention priority	Description
  never-delete	Snapshots will never be deleted automatically to make space. The oldest snapshot in the snapshot history will be deleted once the snapshot count has been exceeded. This is the default.
  high	Snapshots can be deleted after all eligible medium-priority snapshots have been deleted.
  medium	Snapshots can be deleted after all eligible low-priority snapshots have been deleted.
  low	Snapshots can be deleted. This parameter is unrelated to snapshot history, and because the default is never delete, snapshot history snapshots will normally not be affected in a low virtual memory situation.

When this option is disabled, snapshot history will not be kept. If this option is disabled after a replication set has been established, any existing snapshots will be kept, but not updated.