Administrator Guide

space used by the primary volume. At most, the two internal snapshots together for each volume may consume twice the
amount of disk space as the primary volume from which they are snapped.
Even though the internal snapshots are hidden from the user, they do consume snapshot space (and thus pool space) from the
virtual pool. If the volume is the base volume for a snapshot tree, the count of maximum snapshots in the snapshot tree may
include the internal snapshots for it even though they are not listed. Internal snapshots and internal volume groups count against
system limits, but do not display.
Creating a virtual pool for replication
When you create a virtual pool, specify that it has enough space for three times the anticipated size of the primary volume to
account for the primary volume plus the same amount of space for each of the two internal snapshots. This is the maximum
amount of space that you will need for replication. Also, for a pool on the primary system, allow additional space for other uses
of the pool.
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 EMC 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 EMC 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 EMC PowerVault ME4 Series Storage System CLI Guide.
Replication and empty allocated pages
Deleting data from a volume can result in deallocation of pages on that volume. Pages deallocated before the initial replication
will not be copied to the secondary volume. Pages deallocated since the last replication cause a page consisting of zeroes to
be written to the secondary volume during replication. This can result in a difference in the number of allocated pages between
the primary and secondary volumes. A virtual storage background task automatically reclaims pages consisting of all zeroes,
eventually freeing up the secondary volume snapshot space that these reclaimed pages consumed. Freeing up this space is not
immediate and happens over a period of time.
Working in the Replications topic
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