Dell Unity: Adding or removing a LUN from a Consistency Group (CG) with snapshots fails with the error code: 0X6000c21 (User Correctable)

Error Code: 0X6000c21 is encountered when attempting to add or remove a Logical Unit Number (LUN) from a Consistency Group which has snapshots.

An important consistency groups characteristic is that snapshots must cover all consistency group LUNs to ensure that the entire group can always be restored to a single crash consistent point-in-time. As a result, some operations are not permitted for consistency groups with existing snapshots. In particular:

• A LUN with one or more existing snapshots cannot be added to a consistency group. Any snapshots must be deleted before the LUN can be added to ensure that the snapshot state of all LUNs in the consistency group is consistent.
• No LUNs can be added to a consistency group that has one or more snapshots. Such an addition would create a consistency group that could not be fully restored because the newly added LUN would not be protected at the same point-in-time as the rest of the consistency group. Therefore, consistency group snapshots must be deleted before adding new LUNs to the group.
• Similarly, no LUNs can be removed from a consistency group with one or more snapshots. This removal would make it impossible to restore the full consistency group back to the snapped point-in-time. This is because the LUN would no longer be a part of the group. All consistency group snapshots must be deleted prior to removing any LUNs from the LUN group.

Reference document: Dell EMC Unity: Snapshots and Thin Clones

This is working as designed.

If there are any CG snapshots, they must be removed first, or the ability to add or remove a LUN fails. 

Reference Document:

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