Dell PS4100 Technical Information download pdf (Page 38)

June 2012 Dell EqualLogic Configuration Guide v13.2 32

With replication, it does not matter if the volume is thin-provisioned or uses a traditional volume since

in either case only the data that has changed will be copied to the replica. On the secondary side,

volumes are always thin-provisioned to conserve available capacity used by the Replica Reserve for

that volume.

3.8.4 Fast failback

With fast failback enabled you can ensure that the volume data preserved by the failback snapshot on

the primary group always matches the volume data in the most recent replica stored on the secondary

group. If you have an event that causes failover to the secondary group and the workload

subsequently writes changes to the replica volume then failback snapshot supports a quicker fail back

to the primary group by replicating only the changes made to the replica volume during the time it

was active as a recovery volume on the secondary group. If the failback snapshot is not enabled, you

must replicate the entire volume contents back to the primary group to complete the failback

operation. Depending on the size of the volume, the failback scenario can take significantly longer to

complete if Fast Failback is not enabled.

3.8.5 Sizing Replica Reserve and Delegated Space

The volume replication process in EqualLogic PS Series storage consumes extra storage space on both

the primary and secondary group partners. In addition to the space consumed by the volume itself,

each volume will require additional space in the primary group for Replication Reserve and Local

Reserve, plus delegated space for storing replica sets in the secondary group. This is illustrated in

Figure 13 below. A single delegated space on the secondary group must be used for all volumes

received from a specific primary group.

Delegated space for a replication partnership must be assigned to a pool. Delegated space for

different replication partnerships can be assigned to different pools on the secondary group.

Note: Any type of application or operating system level process that causes block level data

changes will affect replication data size and time to complete. For example, file system

defragmentation of a large partition will significantly increase the amount of space consumed in

replica reserve, delegated space on the secondary group and time to complete the next scheduled

replication.

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Dell PS4100 Technical Information Page 38