Dell PowerVault ME5 Series Administrator's Guide

Setting up snapshot space management in the context of replication

Snapshot space management is the ability to control the number of snapshots and the amount of storage space they can consume in a pool.

When you create virtual volumes through the create volume and create volume-set CLI commands, you can set the retention priority for snapshots of the volume. If automatic deletion of snapshots is enabled, the system uses the retention priority of snapshots to determine which, if any, snapshots to delete. Snapshots are considered to be eligible for deletion if they have any retention priority other than never-delete. Snapshots are configured to be eligible for deletion by priority and age. The oldest, lowest priority snapshots are deleted first. Internal replication snapshots and snapshots that are mapped or are not leaves of a volume snapshot tree are ineligible for deletion. For more information on the create volume and create volume-set CLI commands, see the CLI Reference Guide

There are several factors to consider when managing snapshot space for the primary and secondary systems, especially when setting up the snapshot space and policies for the pool:

• Make sure that there is enough space in the pool for the base volume and replication-set needs. See guidelines provided in Creating a virtual pool for replication.
• To adjust the snapshot space of the pool, increase the value of the limit parameter of the set snapshot-space CLI command. For more information on the set snapshot-space CLI command, see the CLI Reference Guide.
• To create more snapshot space, increase the size of the pool, add disk groups to the pool, or expand existing ADAPT groups. It may be necessary to insert additional disks if the system doesn't have any available.

If the snapshots are taking up more space than anticipated, consider the following approaches to address the issue:

• Limit the number of snapshots created by the replication-set.
• Fine-tune the overall snapshot space allocation and policies for the pool.

Limiting the number of snapshots created is the most straightforward approach. You can set the number of snapshots taken by the snapshot history feature via the CLI. Use the set replication-set CLI command, specifying the snapshot- count parameter. Reducing the snapshot-count number will save space. For more information on the set replication-set CLI command, see the CLI Reference Guide.

Fine-tuning the overall snapshot space allocation and policies for the pool is the other approach. You can adjust the snapshot space limit and limit-policy parameters of the pool to reduce the number of retained snapshots, or you can relax the snapshot- retention-priority parameter used with the set replication-set CLI command. Relaxing the snapshot- retention-priority parameter allows the pool to be more aggressive in deleting older, less important snapshots when there is pressure on the pool to free-up snapshot space. For more information on the set replication-set CLI command, see the CLI Reference Guide.

To monitor the snapshot space for virtual pools, use the show snapshot-space CLI command. To monitor the size of the internal snapshots, use the show snapshots CLI command with its type parameter set to replication. For more information on the show snapshots CLI command, see the CLI Reference Guide.