Dell Unity: Best Practices to Manage Consistency Groups with Replication Configured

A CG is an addressable instance of LUN storage that can contain one or more LUNs (up to 50) and is associated with one or more attached hosts. Consistency groups help organize the storage that is allocated for a particular host or hosts. Snapshots that are taken of a consistency group apply to all LUNs associated with the group. 

Adding a LUN to a consistency group:

• You can create one or more new LUNs and add them to the consistency group.
• Or you can move existing LUNs that are not already assigned to a consistency group to a consistency group

Change consistency group or consistency group thin clone properties:

• If the LUN already has data reduction applied, you can view the capacity: What amount of space is allocated after Data Reduction was applied, what amount of space is not allocated, and the space saved for the entire system after data reduction.
• You can also choose to disable data reduction by removing the checkmark from the Data Reduction checkbox and clicking the Apply button. Existing data remains as is. Any newly-written data does not have data reduction applied.
• If a LUN is being moved, you can view the status and change the move session's priority setting. If the move is completed, you can view the move status for up to three days after the operation completed.
• A thin clone cannot have data reduction applied independently of its base LUN. However, if you apply data reduction to the base LUN, the thin clone also has data reduction applied.
• You cannot change the tiering policy for a thin clone independently of its base LUN in the consistency group. However, if you change the tiering policy of the base LUN, the thin clone inherits the changed tiering policy.
• When you replicate a thin clone, it becomes a standard LUN on the destination storage system, with no ties to the original base LUN.

Expand LUNs in a consistency group:

• You can expand any of the LUNs in a consistency group when the consistency group is the source in an asynchronous replication session.
• Expanding a LUN does not affect asynchronous replication functions, such as pause, resume, failover, and failback, for a consistency group.
• Expanding LUNs in consistency groups that have synchronous replication that is enabled is not supported.
• When you expand the LUN, the capacity of the consistency group also changes.

Delete a consistency group:

• Deleting a consistency group also deletes any of its snapshots, and all the LUNs in that group. If the consistency group contains LUNs with attached hosts, you cannot delete the consistency group.
• If the job fails, check the job details on the Jobs page for more information. The consistency group deletion may have been prevented if any LUNs within it have attached hosts.  

Preventing Orphaned CG Objects on Disaster Recovery (DR) Unity Sites:

When modifying CGs that are replicated to a DR Unity site, there are steps to take to minimize the likelihood of leaving behind "orphaned" CG objects. These objects can occupy space on the DR site without being part of an active replicating consistency group and may not always be visible. The following are guidelines to help reduce these types of issues:

1. Monitor Remote Pool Space Usage: When expanding a LUN on the source side, ensure that the remote pool on the DR site has sufficient space. If the pool is low on space, the remote CG replica object may fail to expand, leading to issues. Unisphere generates an alert if the usage exceeds the default threshold.
2. Avoid Modifying CGs with Active Replication: If you must add or remove a LUN from a replicating CG, you must first idle the replication. Pausing replication can cause issues, so wait until it is idle before making changes.
3. Remove Replication Before Modifying CG: To remove or add a LUN from a CG, delete the replication session from the Protection and Mobility/Replication/Sessions tab. Then, make the necessary changes to the source side CG.
4. Remove CG Snapshots: Before adding or removing a LUN, ensure that there are no CG snapshots. If snapshots exist, remove them first to avoid errors.
5. Clean Up DR Site CG Objects: After modifying the source side CG, locate the corresponding CG and LUNs on the DR site and delete them. This step is crucial to prevent issues with new replication configurations.
6. Reconfigure Replication: Once the source side CG has been modified, reconfigure replication by opening the CG, selecting the Replication tab, and choosing the configure replication button. This triggers a full sync operation for the newly modified CG.

Dell Unity Family Configuring and managing LUNs Version 5.5 (March 2025)