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Data migration: Proceed with caution

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Server-to-remote server data migrations
Migrating data at a block level from the source server to a remote target server introduces a different set of issues:

  • Creating the initial copy of the data on the target server
  • Synching the data once the initial copy is in place
  • Going forward to the target server
  • Falling back to the source server
  • Shutting down the application to switch between servers

How the initial copy of the data is created on the target server depends on three variables: distance, bandwidth and the amount of data. When the target server is close, there's a minimal amount of data to move and ample network bandwidth between the source and target servers; creating and synchronizing the data between the source and target servers is no big deal. However, it becomes an issue when you need to migrate a large amount of data over a limited bandwidth network connection; each product offers a way to accomplish this objective.

Symantec's Veritas Volume Replicator uses checkpoint initialization, replicated volume group (RVG), storage replicator log (SRL) and replication link (RLINK) to move large amounts of data from the source to the target server by creating a full hot block-level backup to tape. The SRL is used to map changes during the backup; Veritas

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Volume Replicator places a check-start pointer in the SRL when the backup begins and a check-end pointer at the end of the backup. The tapes with the backup are then sent to the target server and the data is restored there.

Once restored, Veritas Volume Replicator on the source server communicates with the target server and sends over the two SRLs it owns: the one created during the backup and the one with all data since the completion of the backup. The target server's copy of Veritas Volume Replicator first replays the SRL created during the backup to create a consistent but out-of-date backup on the target server. It then replays all entries created in the SRL since the end of the backup to create an up-to-date copy of data at the remote site.

This was first published in August 2006

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