Dell EMC Unity: FAQ for Unity Replications

1. How to prevent a file system from being replicated with NAS Server?
Technically, when a NAS Server is replicated, all the file systems associated with it will also be replicated as separate replication sessions automatically. If a file system does not need to be replicated, its replication session can be deleted afterwards.

If a file system is created after the NAS Server replication, it will not be replicated automatically.

2. How to delete NAS Server replication?
The associated file system replications should be deleted prior to deleting any NAS Server replication.

3. How to recreate a file system replication?
Delete the original file system replication, and re-create it with correct settings. There is no need to touch the NAS Server replication in such situation.

4. How to switch over a replication?
The "Switch Over" operation on VNX is called "Failover with Sync" on Unity. Refer to KB 502754 for the detailed steps.

5. How to restore a replication to the original state after Failover?
On VNX, the switched/failed over replications can be started with "-overwrite_destination" option, so that the changes on DR site can be discarded. Some customers used this option for test purpose. However there is no such option on Unity 4.x, both Failback and Resume operations sync the changes on DR site to Prod site.

Note: Starting with Dell EMC Unity OE version 5.0, when running a failback operation, the following options are available:
  Keep local data changes by updating the remote resource - Preserves the changes that were made on the destination site by replicating them back to the original source
  Keep remote data by discarding all the local changes - Discards any changes made on the destination site after the failover by replicating in the original direction and overwriting any changed data

6. How to perform a Reverse operation on Dell EMC Unity?
The one step Reverse operation on VNX is no longer supported on Unity. As a workaround, perform a "Failover with sync" and then "Resume" to reverse a replication.

Refer to KB 502754 for the detailed steps.

7. What is the difference between Failback and Resume on Unity?
Both operations are designed for the situation when Failover has been done, and users are accessing the data on DR site.
For Unity 4.x, Failback removes access to DR site, and then sync the changes from DR to Prod. Once the Sync completes, allow access to Prod site. If there has been significant change accumulated on DR, it may take a long time to sync and cause long DU. Unity 5.x introduces a new option "Keep remote data by discarding all the local changes", which will discards any changes made on the destination site and allow the failback to finish quickly.

Resume does not remove access to DR site. It just sync the changes from DR to Prod and reverses the direction of the replication, so there is no DU for a Resume operation. However, if it is necessary to restore the replication to the original direction, "Failover with sync" and Resume need to be done at a scheduled time, because a short outage window is required for "Failover with sync".