Data Change Map (DCM) log fails to completely drain.

Article:TECH55550  |  Created: 2007-01-15  |  Updated: 2014-05-02  |  Article URL http://www.symantec.com/docs/TECH55550
Article Type
Technical Solution

Product(s)

Environment

Issue



In most configurations, a DCM (Data Change Map) is used when a RL (Replicator Log) overflows. Unlike the RL; which is a transaction log, the DCM log is a simple bitmap. Each "bit" in this bitmap corresponds with a region of blocks on the replicated volume. When a write occurs within a particular region of the volume, the corresponding bit is marked as "changed" (Figure 1).

Figure 1
 

Even if the amount of data that is written to the volume is very small, the entire region will be marked as "changed" and will need to be replicated.


Environment



Storage Foundation for Windows Volume Replicator option


Cause



In some cases, a process that is running on the primary may periodically write data to the same region of a volume. If the next write occurs before the DCM has finished draining, the region may be marked as "changed" again. This may result in a situation where the same region must be replicated repeatedly and the DCM is unable to completely drain.

Note: During an initial synchronization, some or all regions are marked as "changed." The initial resynchronization is similar to cases where an SRL overflow has occurred, except that all regions containing non-zero filesystem data are marked as "changed".


Solution



In most cases, the DCM will drain. In some cases, where there is repeated heavy activity to the same region(s), it may be necessary to stop the applications or processes that are writing to the replicating volume. In severe cases, manually disabling access to the volume will completely prevent writes from occurring on the primary. This allows the DCM to drain.

WARNING: Before disabling access to a volume, verify that no open handles exist. Forcibly ending open handles will have unpredictable results, including possible data corruption.

Disabling access to a Volume (non-clustered):
1. From VEA (Veritas Enterprise Administrator), expand Replication Network.
2. Expand the RDS (Replicated Data Set).
3. Expand the primary RVG.
4. Right-click on the replicated volume(s). Select Disable Data Access.

Disabling access to a Volume (clustered):
1. Use the cluster mechanism to offline the resources, leaving only the Disk Group and the IP resource used for Replication network traffic online.

keywords: stuck in DCM, DCM mode


Legacy ID



294170


Article URL http://www.symantec.com/docs/TECH55550


Terms of use for this information are found in Legal Notices