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Getting started with remote data replication

I have two storage area networks (SANs) and I want to perform remote data replication between them. Currently, the primary SAN has 21 TB of data, and it is not feasible to replicate this amount over the WAN. Is it possible to make a full tape backup of the primary SAN, copy the data onto the secondary SAN, and then set up data replication from the primary SAN to the secondary SAN for data changed after the initial copy? Also, once I have this in place I would like to make tape backups of the secondary SAN. Is this a good strategy?

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This is a common issue with initial replication involving significant amounts of data. Data replication between storage arrays is typically done at the block level. A volume (or LUN) of equal size to the source is created on the target array and data is replicated at the block level until both copies are synchronized. After that, only changed blocks are replicated. Software installed on both arrays is responsible for keeping track of which blocks have been replicated and which blocks are changing and therefore in need of replication.

On the other hand, tape backup is usually a host-based process that copies data at the file level using software installed on the host; the backup server takes care of tracking what has been backed up via catalog or database. Restoring a full backup from tape to a remote array must be done using a host that has a file system and directory structure on that array to restore at the file level. While this will get the data on the remote array, it does not produce a "block map" that the storage array can understand and use for future replication.

There are limited options to get this done and they include:

 

  1. Install the target (remote) SAN in the same location as the source SAN to get the initial baseline replica and then move the target SAN to the remote location to resume the replication process for changed blocks only.
  2.  

  3. Work out an agreement with your carrier to temporarily have access to additional network bandwidth between both sites to complete the initial replica. Note that this may still take a long time depending on the bandwidth available. For example, 21 TB of data could take around 20 days over a 100 Mbps link. You can also take advantage of WAN optimization technology to help reduce the amount of redundant data being sent across the WAN link.

This was first published in November 2010

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