Best Practices: Will your data recovery strategy work for DR?


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Let's take the most common and often contentious example: database replication. I can't tell you how many times I've seen DBAs and storage folks at odds over selecting the best database replication tool. Storage vendors will tout their proprietary block-level replication solutions as superior to built-in database replication solutions such as Oracle's Data Guard. DBAs are more inclined to opt for the latter replication scenario. Why not implement a strategy that makes use of both?

Block-level solutions provide you with a "restartable" copy of your database, while database replication solutions provide a "recoverable" copy. When designing an enterprise-wide solution, I would lean toward an application-agnostic, block-level solution that's closer to the storage array than the app. For example, if you have an EMC environment, consider using either Symmetrix Remote Data Facility (SRDF) or MirrorView. You may want to include the database replicator because you shouldn't assume SRDF will provide an end-to-end solution. The combined approach will give you a restartable copy that can be made "current" to the last good transaction permitted by your recovery point objective (RPO). This isn't a black-and-white solution--the shades of grey are in how you implement the combination.

Interoperability is a key consideration when selecting technologies to help avoid finger-pointing among vendors when implementing a multivendor strategy. For example, the application clustering

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technology used with the data replication solution will determine if the clustering solution can effectively control the failover of the replication and start of applications at the remote site. Most clustering and application vendors now offer geographical clustering plug-ins to their cluster solutions that can be used to control data replication solutions from different vendors. Therefore, the choice of a clustering solution can't be made in a vacuum.

Cost matters
The bottom line is always price. Is the cost of implementing an elegant solution justified? Some business requirements may dictate large expenditures, but don't assume that every recovery plan has to be expensive. When you put all the options on the table, you'll be surprised how many there are. Block-level, array-to-array solutions tend to be the most expensive in terms of storage, licensing costs and the network bandwidth required to keep the two copies in sync. Application replication products are usually cheaper and have less-demanding bandwidth requirements, though they only work with the specific application.

Array-based replication solutions are application-agnostic and can be used to replicate any of the array's contents. But replicating everything on the array is an expensive proposition; if you need to trim costs, you may want to use a more selective approach.

This was first published in August 2007

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