The Data Change Map (DCM) is a component of VVR that is used to track writes when the SRL overflows and thus enables you to avoid complete resynchronization of the data on the Secondary. The DCM contains a bitmap and can be optionally associated with a data volume on the Primary RVG.
The DCM becomes active only when the SRL is no longer large enough to hold accumulated updates. While the DCM is active, each bit that has been set in the DCM represents a region whose contents are different between the Primary and the Secondary. At an appropriate time, the administrator initiates a resynchronization and causes VVR to incrementally synchronize the Secondary with the Primary by looking up the bitmap.
When the resynchronization of the DCM starts, the Secondary becomes inconsistent because the DCM resynchronization writes are not necessarily in the same order as the application writes. As a result, the Secondary cannot be used for disaster recovery while the DCM is resynchronizing. After the resynchronization of the DCM is complete, the Secondary RVG is consistent and replication resumes as usual.
The Automatic Synchronization, SRL Overflow Protection with DCM, and Fast Failback features use the DCM. Each data volume in the RVG must have a valid DCM associated with it before the DCM can be used.