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Disaster Recovery Extra: Distance your data from disaster

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But some observers worry that some midrange arrays aren't sufficiently equipped to run processing-intensive replication. "Vendors reduce the price by using a lesser microprocessor and less cache," says Arun Taneja, president and founder of the Taneja Group, Hopkinton, MA.

Shortly after Sept. 11, New York City-based Folksamerica Reinsurance Co. implemented a new Fibre Channel (FC) SAN from NetApp. Included in the package were SnapMirror licenses that Folksamerica uses to replicate its 50 or so servers to a Qwest data center in Colorado, where an exact replica of its production environment is stored. The data is replicated asynchronously according to different policies, depending on the importance of the system, explains Erik Tomasi, vice president of infrastructure. That keeps bandwidth costs down and has prevented the filers from suffering from a noticeable performance hit, he says.

If there are any downsides to Folksamerica's DR setup, it has nothing to do with NetApp replication. "The biggest drawback of our current environment is the cost of keeping exact copies of our servers out in Colorado, and the challenge of keeping them synchronized," says Tomasi. "In general, replicating data to Colorado is a great idea," he notes, but in the future, he'll investigate ways to make better use of the equipment sitting in a DR site.

But despite its popularity, array-based replication has its critics. The most oft-cited criticism of array-based

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replication is that it limits the type of storage you can use. With array-based replication, the arrays at the primary and secondary sites must mimic one another. In other words, if there's an HDS Lightning at your primary site, you'll need an HDS Lightning at the replication site with the same amount of capacity. Not only does that limit your array choices, but it may also force you to buy a higher tier of storage for the DR site than is strictly necessary or affordable.

That was the thinking behind foregoing array-based replication at TradeCard Inc., which runs an outsourced supply-chain automation platform for a network of 1,500 business buyers and sellers. Last year, the company embarked on a plan to implement replication of its production and training databases to a DR site approximately 85 miles away from its main data center in New York City. As an EMC Clariion shop, the decision not to buy MirrorView—which offers both synchronous and asynchronous remote mirroring capabilities—came down to the freedom to buy non-EMC Clariion storage in the future.

"We didn't want to be stuck with that array technology. What if we wanted to add a NAS box down the road?" asks Anthony Ercolino, TradeCard's vice president of data center operations. Financially, says Ercolino, MirrorView was also the most expensive product the company evaluated.

That complaint hasn't exactly fallen on deaf ears among storage vendors, who are increasingly devising novel ways for users to replicate to a less-expensive tier of disk, albeit in the same class of array. For example, Hitachi TrueCopy users still have to replicate between Lightning arrays, but the FC volumes at the primary site can be replicated to serial ATA drives at the DR site. The caveat is that you need to architect your remote site to ensure that it can keep up with the volume of writes being sent from the primary array.

This was first published in May 2006

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