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Dell PowerVault ME4 Series Storage System Administrator’s Guide

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Setting up snapshot space management in the context of replication

The snapshot space management feature, accessible only through the CLI, enables users to monitor and control the amount of space that snapshots can consume in a pool. In addition to configuring a snapshot space limit, you can also specify a limit policy to enact when the snapshot space reaches the configured limit. The policy will 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 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 on setting snapshot retention priority, see Maintaining replication snapshot history from the Replications topic.

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's snapshot tree are ineligible for deletion. For more information on the create volume and create volume-set CLI commands, see the Dell PowerVault ME4 Series Storage System CLI Guide.

If you are using the replication feature and snapshot space management, there are specific 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 snapshot space to accommodate the maximum anticipated size of the two internal snapshots, which cannot be deleted, and any other snapshots that you would like to retain.
  • 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 Dell PowerVault ME4 Series Storage System CLI Guide.
  • You can later create more snapshot space by adding disks to the pool to increase its size.

If the internal snapshots are larger than anticipated and take up a lot of snapshot space, you can adjust the snapshot space thresholds or increase the snapshot space to prevent unintentional automatic deletion of snapshots that you want to retain. 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 Dell PowerVault ME4 Series Storage System CLI Guide.

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