A data change map (DCM) is an object that contains a bitmap and can be optionally associated with a data volume on the primary replicated volume group (RVG).  It is almost identical to the dirty region log (DRL) in a VERITAS Volume Manager (VxVM) volume, but the bits represent ranges that are different between the primary and the secondary, rather than between mirrors.

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.  The DCM becomes active only when updates are unable to flow between the primary and the secondary, and the SRL is no longer large enough to hold accumulated updates, i.e. the SRL is full.  When the updates are able to flow from the primary to the secondary, the administrator initiates a resynchronization and causes VVR to incrementally synchronize the secondary with the primary by looking up the bitmap.  Note that the secondary is inconsistent between the time the DCM resynchronization starts and completes because the DCM resynchronization writes are not necessarily in the same order as application writes.  After the resynchronization is complete, the secondary RVG is consistent and replication resumes as usual.

The features automatic synchronization and SRL overflow protection with DCM use the DCM.  Each data volume in the RVG must have a valid DCM associated in order for these features to be used.