Volume Replicator 7.4.1 Administrator's Guide - Windows

Understanding checkpoints

Volume Replicator checkpoints are user-defined markers in the Primary Replicator Log. The RVG and RLINK (Secondary) checkpoints are two types of checkpoints. The RVG checkpoint has a start (checkstart) and an end (checkend) and can be used for initial synchronization. The RLINK (Secondary) checkpoint is used to restore Secondary volumes in case of failure.

The Secondary data volumes must be synchronized with the Primary data volumes before replication can start, that is, after you add a Secondary to the RDS, after a Secondary data volume error, or after Replicator Log overflow. Volume Replicator enables you to synchronize the Secondary data volumes while the application is active on the Primary. If you use the Automatic Synchronization feature of Volume Replicator to synchronize the Secondary data volumes over the network, Volume Replicator ensures that the Secondary data volumes are consistent and up-to-date when the synchronization process completes.

If you use the backup and checkpoint method for synchronizing the Secondary and if the Primary application is active during the backup process, then, after you restore the backup on the Secondary, the Secondary data volumes are inconsistent and not up-to-date.

To make the Secondary consistent and up-to-date, Volume Replicator must transfer all the blocks that changed during the backup process, in the order that they changed. In a Volume Replicator environment, all writes to the Primary data volumes are logged to the Replicator Log; therefore, Volume Replicator can transfer the writes that occurred during the backup to the Secondary. To do this, Volume Replicator must know the start and end of the backup process. RVG checkpoints are used to indicate this start position (checkstart) and end position (checkend) in the Replicator Log.

Because the checkpoint information is stored in the Replicator Log, checkpoints become invalid when the Replicator Log wraps around. The same checkpoint and tape backups can be used to synchronize the data volumes on multiple Secondary hosts if the checkpoint remains valid.

Figure: Figure illustrates how Volume Replicator handles checkpoints

A backup utility may copy previous contents of the blocks corresponding to Write 3 (event 5) but copy updated contents of the blocks corresponding to Write 4 (event 7).

However, Volume Replicator logs all the writes to the Replicator Log (events 4 and 6). Note that a checkstart was performed (event 1) before the backup was started (event 2) and a checkend was performed (event 9) after the backup was completed (event 8). When you start replication with this checkpoint after the backup is restored on Secondary, Volume Replicator can transfer all the writes between checkstart and checkend and make the Secondary data volumes up-to-date and consistent.