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The lowdown on replication appliances

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Replication options
As storage networks grow, the types of applications they host necessitate different types of recovery scenarios. Because any application, server, network, storage array or site can fail, replication appliances need to offer different replication options. Although replication appliances are deployed to recover from large disasters like site failures, they also include options that permit local replication to recover from local system failures.

Topio's TDPS out-of-band appliance typifies the approach enterprises would take for disaster recovery or to centralize all backups to one site. Topio's architecture calls for a replication appliance at an offsite location to function as both a management workstation and a central target to which all of the agents send their data over IP in asynchronous mode. The snapshot option can also be configured so that the server agent holds the copied writes on the server until a consistent, recoverable image is constructed on the central management server. This option allows the regular production write I/Os to continue without impacting the application.

Replication appliance pros and cons

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In-band approaches such as FalconStor's IPStor or McData's UltraNet Replication Appliance allow synchronous replication between two different storage arrays. This option lets users keep a real-time copy of the data on two arrays in the same location; if one array fails or needs to go offline for maintenance, the server can continue its processing using data on the other array. Alternatively, these appliances include snapshot functionality that breaks off copies of the data periodically--in the event of data corruption, the application can recover from a specified point in time.

But there are limitations to in-band and out-of-band approaches when doing replication. Appliances from most vendors are limited to the number of appliances they can replicate across and have only limited ways of synchronizing the data if multiple replication appliances exist. Conversely, products like TDPS, which synchronize the data from multiple incoming replication data streams, fail to adequately address local replication needs and are not well suited for high write I/O applications. Even the less-intrusive, fabric-based techniques are currently limited to Cisco SAN director deployments, and there's little practical knowledge at this point about the impact this has on write I/Os or even how the SANTap feature fully works.

Replication appliances continue to gain new features and functions as vendors adapt to myriad user environments. Yet most replication appliances remain limited to handling either a small number of high-performing apps or a large number of lightly performing apps, while fabric-based replication features continue to evolve. As a result, enterprises should resist the urge to deploy any replication appliance on an enterprise-wide basis, and install them only as application demands dictate.

This was first published in November 2005

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