Dirty region logging (DRL), if enabled, speeds recovery of mirrored volumes after a system crash. DRL tracks the regions that have changed due to I/O writes to a mirrored volume. DRL uses this information to recover only those portions of the volume.
If DRL is not used and a system failure occurs, all mirrors of the volumes must be restored to a consistent state. Restoration is done by copying the full contents of the volume between its mirrors. This process can be lengthy and I/O intensive.
If a version 20 DCO volume is associated with a volume, a portion of the DCO volume can be used to store the DRL log. There is no need to create a separate DRL log for a volume which has a version 20 DCO volume.