The lowdown on replication appliances


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Replication appliances can work across heterogeneous storage devices, but not all appliances are appropriate for every environment.

With growing heterogeneous server and storage environments, some enterprises are finding that host- and storage array-based replication approaches no longer satisfy their requirements. A replication appliance may be the solution. Operating system- and storage array-agnostic, replication appliances give companies increased flexibility to address one-time and continuous data replication needs while allowing applications to continue processing without data loss or to recover with minimal downtime.

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Replication appliances
Click here for a comprehensive list of replication appliances (PDF).
Replication appliances allow administrators to:
  • Deploy the appliance in their existing networked storage infrastructure
  • Choose from a variety of replication options that best suit particular applications
  • Replicate data between different brands and tiers of storage
  • Replicate data locally or remotely to provide different recovery options
  • Create consistent database snapshots for near real-time application recovery
But all replication appliances don't necessarily offer all of these features or deliver them in the same way. FalconStor Software's IPStor, McData Corp.'s UltraNet Replication Appliance (obtained with its acquisition of Computer Network Technology Corp.) and Sun Microsystems Inc.'s MirrorStore (part of its StorageTek acquisition) appliances provide an in-band Fibre Channel (FC) architecture that resides in the data path between the server and storage array and mirrors data to different storage arrays in a pass-through configuration. Kashya Inc.'s KBX5000, Topio Inc.'s Topio Data Protection Suite (TDPS) and Xiotech Corp.'s TimeScale use both in-band and out-of-band techniques. The appliances use FC connections to present disk targets to hosts, but also introduce out-of-band, host-based agents that copy write I/Os to the logical unit numbers (LUNs) that their appliances present to the host.

Replication appliances bring along their own set of administrative headaches, however. In-band FC disk targets take time to set up because administrators may be unfamiliar with how to configure the LUNs the appliances present or uncertain as to how an appliance that intercepts write I/Os will impact their applications. In addition, host-based agents require administrative time to install, possible server reboots and the introduction of drivers that may be incompatible with hosts or applications. There may also be a performance hit during an application's most intensive write I/O periods.

Replication appliance vendors are trying to address these concerns by combining in-band and out-of-band techniques to give administrators more ways to implement and manage the data replication process. But even with new options that attempt to address changing storage environments, replication appliances may still fall short in providing enterprise-wide solutions.

This was first published in November 2005

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