Explanation of Dirty Region Logging (DRL), Disk Change Object (DCO), and Data Change Map (DCM)?

Explanation of Dirty Region Logging (DRL), Disk Change Object (DCO), and Data Change Map (DCM)?

Article: 100008298
Last Published: 2013-08-21
Ratings: 3 0
Product(s): InfoScale & Storage Foundation


What is the difference between Dirty Region Logging, Disk Change Object, and Data Change Map?


Veritas Storage Foundation for Windows (SFW) use various mechanisms to track inconsistencies between fault tolerant or replicated volumes, and to quickly synchronize or resynchronize inconsistent volumes. This document describes the mechanisms used by SFW (Dirty Region Logging and Disk Change Object) and Volume Replicator (Data Change Map), and highlights their similarities and differences.

Dirty Region Logging (DRL)
SFW can be configured to use DRL on mirrored volumes to quickly resynchronize all copies of a mirror when a system is restarted following a system crash. If DRL is not used, all copies of the mirror must be synchronized by copying the full contents of the volume to each plex, which can be an intensive and lengthy operation. A DRL log can be added during creation of the mirrored volume, or it can be added later. Multiple logs can be associated with a single mirrored volume for fault tolerance; however, a large number of logs can have an impact on performance. Figure 1 shows a mirrored volume with one DRL log configured.

Figure 1 - DRL Log

Similarly, logs can also be added to RAID-5 volumes in order to quickly reactivate the volume when a system is restarted following a crash. A RAID-5 log has the same characteristics as a DRL.

Disk Change Object (DCO)
DCO volumes are used by Volume Manager FastResync (FR). FR is used to quickly resynchronize mirrored volumes that have been temporarily split and rejoined. FR works by copying only changes to the newly reattached volume using FR logging. This process reduces the time required to rejoin a split mirror, and requires less processing power than full mirror resynchronization without logging. FR can be used with a standard mirrored volume, or it can be used with Veritas FlashSnap. FlashSnap enables the creation of independently addressable multi-purpose volumes (FlashSnap volumes), which are mirrors of volumes on a server. These can be detached from the local server and moved to another server for backup or other activities, and can then be reattached to the original volume on the local server and quickly resynchronized using FR.

A DCO volume is created when FR is enabled, or when a snapshot operation is started. The DCO volume keeps track of the changes made to a volume while a mirror is detached. Figure 2 shows a DCO associated with a FlashSnap volume, and Figure 3 shows a standard mirrored volume that has both a DCO and a DRL associated with it.

Figure 2 - DCO Volume

Figure 3 - DRL and DCO


Note: DCO and DRL keep track of regions on a volume where the mirrors are not synchronized; however, they perform different functions. DRL is responsible for determining whether a write to a mirrored volume has been completed on all mirrors and is used to resynchronize mirrors following a system crash, whereas a DCO retains a record of updates that have been missed by a detached mirror.

Data Change Map (DCM)
Volume Replicator uses a DCM to track the changes between primary and secondary volumes. The DCM is a bitmap that represents the data difference between the primary and secondary. It can used to:
Synchronize the Secondary data volumes incrementally
Synchronize the data volumes using auto-synchronize
Enable Replicator Log overflow protection when the log protection is set to DCM

Each data volume in the Replicated Volume Group (RVG) must have a valid DCM log associated with it before the DCM can be used. A DCM is used by default, and can be configured during the creation of the Replicated Data Set (RDS). Figure 4 shows the DCM in Volume Manager Disk View.

Figure 4 - DCM


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