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Using difference-based synchronization

You can synchronize the Secondary using difference-based synchronization when there is little difference between the Primary and Secondary data volumes in an RDS. Difference-based synchronization can be used to transfer data over the network when the application is active or inactive.

In difference-based synchronization, the syncrvg command generates MD5 checksums for the data blocks on the Primary data volume and the corresponding Secondary data volume and compares these checksums. The syncrvg command then transfers over the network only those blocks for which checksums do not match. These steps are repeated for the entire Primary data volume and Secondary data volume.

MD5 checksum is generated using the RSA Data Security, Inc. MD5 Message-Digest Algorithm. For more information on MD5 checksum, refer to the md5 manual page.

Difference-based synchronization is useful in situations such as:

• Storage Replicator Log (SRL) Overflow— To synchronize the Secondary when the SRL protection has not been set to use the Data Change Map (DCM).
• Failing Back to the Original Primary—To synchronize the original Primary data volumes with the new Primary data volumes.

The vradmin syncrvg command enables you to synchronize the Secondary RVG with the Primary RVG based on differences. You can issue the vradmin syncrvg command from any host in the RDS. The vradmin syncrvg command synchronizes the data volumes associated with the Secondary RVG in an RDS with the corresponding data volumes associated with its Primary RVG. The vradmin syncrvg command can be used to synchronize multiple Secondaries at the same time.

If a volume set is associated to an RVG, synchronizing the RVG only affects the component volumes of the volume set that are associated with the RVG. If the volume set includes component volumes that are not associated to the RVG, those volumes are not synchronized.

 To synchronize Secondary RVG with Primary RVG based on differences

Before issuing this command, verify that the RLINKs are detached. Use the
-c checkpoint option with the vradmin syncrvg command as follows:

# vradmin -g diskgroup -c checkpt_name syncrvg local_rvgname \

sec_hostname....

Note that you can use the -c option with the vradmin syncrvg command when performing difference-based synchronization, to automatically start a checkpoint with the specified name. After the data volumes are synchronized the checkpoint is ended. This checkpoint can then be used to start replication using the vradmin startrep command.

The argument local_rvgname is the name of the RVG on the local host and represents its RDS.

The argument sec_hostname is a space-separated list of the names of the Secondary hosts as displayed in the output of the vradmin printrvg command.

The argument checkpt_name specifies the name of the Primary checkpoint of your choice.

Example—synchronizing the Secondary based on differences

This example explains how to synchronize the Secondary RVG hr_rvg on the Secondary host london with the Primary RVG on host seattle.

 To synchronize the Secondary RVG, hr_rvg, on london with its Primary RVG on seattle based on differences

Before issuing this command, make sure that the RLINKs are detached.

# vradmin -g hrdg -c checkpt_presync syncrvg hr_rvg london

Note that you can use the -c option with the vradmin syncrvg command when performing difference-based synchronization to automatically start a checkpoint with the specified name. After the data volumes are synchronized the checkpoint is ended. This checkpoint can then be used to start replication using the vradmin startrep command.

The name checkpt_presync is the Primary checkpoint that you will create.