Data migration: Proceed with caution


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At that point, Veritas Volume Replicator's asynchronous feature can be used to keep the data in a near-real-time state, or its synchronous feature can keep the data in a real-time state between the two servers until the final switchover is complete. Veritas Volume Replicator can also be reconfigured so that the data is then synchronously or asynchronously replicated from the target server to the source server in case there's a need to fall back to the original configuration.

Both Topio's TDPS and Softek's TDMF-IP include an option to create the initial copy of data at the remote site. TDPS' offline synchronization feature takes a baseline of the blocks to be moved, starts copying them to tape and then tracks any changes to the blocks going forward. Unlike Symantec's Veritas Volume Replicator, after the backup is restored on the target server, TDPS transmits only the final state of the changed blocks on the source server after all updates are applied.

Softek TDMF-IP's Courier Transport option allows users to create a copy of the primary data on some type of portable media. This media is then physically transported to the target server and restored there. Issuing a smart resync command then identifies all the differences between the blocks on the target and source servers, and sends only the changes to the target server.

Capturing only the deltas on the blocks between the source and target servers results in a configuration

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where all changes since the initial copy of the data were made aren't restored, a deficiency that neither Softek nor Topio views as an issue. Topio and Softek consider their products' objective to be data migration, not replication. As a result, a recoverable image of the data on the target server needs to exist just prior to the moment of switching over from the source to the target server.

For users with large bandwidth connections and large amounts of data, Softek TDMF-IP offers the option to multithread the streams of data. Using this option, users can identify and create multiple logical groups of volumes and then assign each one its own time and network connection for migrating the data. The "gotcha" with multithreading multiple volumes is that it consumes a lot of network bandwidth, so users need to be cautious about when they run these types of migration jobs. The risk can be mitigated within TDMF-IP by setting network consumption limits for each network stream that can be changed dynamically to interoperate with various production schedules.

This was first published in August 2006

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