What’s the right ‘fit’ for CDP?

Posted on April 01, 2007


Some continuous data protection (CDP) products emphasize data protection and others emphasize business continuity, but it’s not an either/or purchasing decision.

By Dan Tanner

What will it take to make continuous data protection (CDP) catch on in the marketplace? That is a pressing question for vendors and users alike. Start-up vendors are weighing technology approaches and potential for market acceptance, while considering whether to hang on and ultimately go public, or get set to be acquired. Established vendors are considering CDP’s market value, as well as the price of an acquisition versus in-house development and/or time to revenue. Enterprise users are becoming better informed about CDP, although many are biding their time until they are more comfortable with the technology.

When CDP entered the marketplace, it was perceived as being additive to existing data-protection practices. Thus, it often wasn’t purchased unless it could be seen as (a) doing something extra, which could be extra protection or business continuance, to the most valuable data or applications, or (b) providing better protection/continuance for all data and applications. Essentially, (a) amounts to business continuity (BC), and (b) emphasizes data protection (DP). CDP does not necessarily ensure continuous data availability. DP is necessary but not sufficient for BC.

There are CDP products that provide value for both BC and DP, and there are also more-specialized offerings that emphasize one over the other. CDP products oriented toward BC are closely linked to critical applications, providing a high level of continuity. That is, they are oriented toward not only recovery to any point (meeting the finest-grained recovery point objective, or RPO, which all CDP products must do), but also offer the shortest recovery time (meeting the most-stringent recovery time objective, or RTO, demanded for business continuity).

These types of CDP products are highly specialized and are bonded to specific applications. As such, they become legacy when the application becomes legacy.

CDP products aimed primarily at RPO, but not emphasizing RTO, may cover more of the protection/ continuance landscape. These CDP products are more generalized and can even replace “mainstream” backup software. And since they’re not bonded to specific applications, they may have more longevity.

“Fit” and the “stack”

The fit of a CDP product may depend on its vertical capture point position in the IT stack (see figure). The further down the stack CDP is implemented, the more general and DP-oriented it is (but with less consistency for BC usage). This type of CDP is coming to be known as block-oriented; it captures changes to storage, but without file context. These products may be well-suited as replacements for traditional backup software.

The higher in the stack CDP is implemented, the more application-specific and BC-oriented it is, and the less general and DP-suited it is. This type of CDP captures changes to files, with context, but usually with the trade-off of application-specificity (i.e., it is not suitable for use as a replacement for general-backup software).

Click here to enlarge image

The horizontal capture points in the diagram relate to another implementation decision. CDP can be implemented on all hosts, or on selected hosts for selected applications, requiring code for each host environment (and sometimes being limited to certain operating environments). Alternatively, CDP can be implemented in the network as a packaged hardware/software appliance. Eventually, this approach could be implemented on an intelligent network switch-which has the advantage of “seeing” both of the end points. CDP could also be implemented directly on storage arrays, although no vendor is taking that approach today.

CDP for BC must be able to preserve true file changes with coherence among all of a file’s open applications. That is what fidelity and consistency are all about.

Is it necessary to choose whether you want CDP for BC on critical applications, or use CDP for DP and replace existing backup applications yet realize that your RTO for critical applications may not be optimal? No, it is not necessarily an either/or purchase decision: There are solutions available that deliver both BC and DP, though in varying degrees through different approaches.

Examples include offerings that use software to protect data (and virtualize storage) in-band at the block level, while also handling out-of-band, application-level change logging, using a copy-on-write technique. Another approach uses software to cover the full spectrum (at the block level) from scheduled replication to event-driven CDP, which with configuration tweaking could come very close to the RTO of application-specific BC-oriented CDP, while leaving file fidelity and consistency assurance responsibility to the user.

When deciding how to implement CDP, you should identify your priorities and look at where in the stack protection should occur. Regardless of the approach, the promise of CDP means faster data retrieval, enhanced data protection, and increased business continuity.

Dan Tanner is the founder of ProgresSmart (www.progressmart.com) and president of the ASNP New England Chapter.

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