The advantages of asynchronous replication

Before the delta set is applied on the target volumes, the target system first takes a snapshot of the target volumes. This ensures the target storage system has a recoverable image of data should the replication not complete before the delta set is transmitted. Once the delta set is sent and applied to the target volumes, that becomes the new primary recovery copy with previous snapshots kept or discarded based on retention policies set by users. This replication process then repeats based upon the user-defined replication interval.

Bill Snow, IT director for the construction firm Moss & Associates in Fort Lauderdale, FL, uses Compellent's Storage Center storage systems to replicate data. Compellent Storage Center employs a method similar to EMC's Clariion, but differs in that it creates a snapshot on the primary storage system and then replicates the snapshot. Snow retains these snapshots for various lengths of time and uses them in lieu of backups. He keeps hourly snapshots for 48 hours, daily snapshots for two weeks and a weekly snapshot for a month. Moss & Associates still uses IBM Tivoli Storage Manager (TSM) to create monthly backups to tape for long-term offsite storage but, says Snow, "I rely on snapshots created during the replication process to act as my front line for backup and recovery."

Other uses
While business continuity is usually the initial driver for the adoption of asynchronous replication, other IT functions such as content

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distribution, data migrations, load balancing, and testing and development usually aren't far behind.

Eric Midkiff, network administrator at Thomas Memorial Hospital in South Charleston, WV, stores his Picture Archiving and Communication System (PACS) data on two EqualLogic Inc. PS Series storage systems in separate data centers--one in the hospital and the other approximately a mile away--and replicates data between them. Initially done for data protection and rapid recoveries of PACS images, production servers are now used at both sites to access and store PACS data, replicating data between them every 15 minutes. "This allows us to distribute content and load balance between the two sites providing higher performance for our users in both sites," says Midkiff.

EMC's SAN Copy is the only storage system-based replication tool that allows users to migrate data from volumes on another vendor's storage arrays directly to a Clariion without first virtualizing the other vendor's storage array. SAN Copy avoids this virtualization step by allowing administrators to present LUNs on other vendors' storage systems directly to the Clariion. The caveats with this technique are that the Clariion storage system must be able to access the other storage system's LUN through the SAN, the LUN on the Clariion must be the same size or larger than the source LUN, and the source and destination LUNs must be offline during the data migration.

This was first published in September 2007

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