Disaster Recovery Extra: Distance your data from disaster


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Evaluate your environment

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There are many ways to replicate data between geographically dispersed sites. The question is, which one is right for your environment? With so many options, the first step in developing a replication strategy is to evaluate your environment and weigh the different replication products against how they fit into your storage infrastructure. Obviously, each product and replication approach will have its pros and cons.

Many replication sales are accompanied by professional services engagements with tools to help you evaluate your needs. Steve Higgins, director of business continuity and security solutions at EMC Corp., says such tools measure reads, writes, simultaneous users, latency of the termination equipment and server load to help you size your replication environment properly. "It's like we've taken all the really smart people and put them into this tool," he says.

Symantec Corp.'s disaster recovery planning utility is free and can be downloaded from the Web. Called Veritas Volume Replicator Advisor, it runs on Windows and Solaris, and can collect data generated on systems running Symantec's Veritas Storage Foundation (Volume Manager), as well as process data collected by operating system utilities like AIX iostat and lvmstat, HP-UX and Linux sar, and perfmon under Windows. After collecting data for two weeks, it can help you determine your optimal bandwidth needs and, for future Veritas Volume Replicator customers, help size your Storage Replicator Log (SRL) files.

Array-based replication
Array-based replication is the most popular way to replicate data, at least for Storage readers. According to the survey mentioned earlier, 50% of respondents said they use array-based products to replicate data remotely. That's because, by and large, array-based replication is:

  • Non-disruptive to the application, as the processing is performed by the array
  • Application and operating system independent
  • Highly scalable
In other words, if you have a SAN and plan to replicate volumes from multiple servers, array-based replication is probably the way to go. The oldest and most venerable examples of array-based replication run on storage platforms initially developed for the mainframe. Among these are EMC Corp.'s Symmetrix Remote Data Facility (SRDF) for its Symmetrix family, Hitachi Data Systems' TrueCopy for Lightning and TagmaStore systems, and IBM Corp.'s Global Mirror and MetroMirror for the DS8000, DS6000 and Enterprise Storage Server (ESS) arrays.

Replication of this sort can do synchronous mirroring, usually good for no further than 200 miles, and asynchronous mirroring for longer distances. In addition, enterprise-oriented replication products have ways of bridging the gap between synchronous and asynchronous solutions by adding in a third data center. For example, EMC's SRDF/Star can be configured to write to two sites simultaneously: one synchronously and the other asynchronously. If the primary site fails, the company can quickly resynchronize the SRDF/S and SRDF/A sites by replicating only the differences between the sessions.

In the past couple of years, most midrange arrays have also started to include distance replication functions. For example, EMC Clariion can be outfitted with Mirror-View, Hewlett-Packard (HP) Co.'s Enterprise Virtual Array (EVA) models with Continuous Access, Network Appliance (NetApp) Inc.'s filers with SnapMirror, and Sun Microsystems Inc.'s StorEdge 6920 with Data Replicator, to name a few. In many ways, replication is becoming table stakes for any midrange array product.

This was first published in May 2006

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