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Although continuous data protection (CDP) is the logical extension of snapshots, there are two problems with comparing the technologies. One is that the meaning of CDP is still shaking out. The other is that CDP and snapshots tend to blend into each other. CDP provides a constant, running record of the state of system storage -- in effect a constantly updated snapshot. Instead of copying and storing the data at set intervals like a data snapshot, CDP captures and stores the data constantly, saving every change as it is made.
There are a number of vendors offering CDP products, but they don't offer the same features, and they certainly don't agree on what constitutes CDP. As a result, storage managers must study the offerings carefully to understand the features and uses of each vendor's product.
There are two definitions of CDP, each with its own following. One camp, composed primarily of newer, smaller, CDP vendors such as Mendocino Software and Revivio Inc., insists that the ability to restore from any point in time is critical to the definition of CDP. The other, which includes Veritas and EMC, sees CDP as snapshots plus data replication.
Revivio is typical of the small companies' approach to CDP. Its product, which is called CPS (for continuous protection system), copies block-level data as it is changed and time stamps it to keep track of when it was changed. In the event of a restoration, the storage manager picks the specific time to restore from. The Revivio system combines blocks that have not been overwritten since the restore time with its copies of the blocks that have changed and presents the combined image to the system. Once the administrator has verified that the combined image is correct, it is used to continue running the application while the Revivio system restores the lost data in the background. Once the restoration is complete, the system goes back to monitoring and storing changed blocks.
One might consider CDP a more sophisticated kind of snapshot with additional capabilities, such as the ability to restore from any point in time. This makes the CDP concept attractive to many storage managers. However, CDP has some significant drawbacks as well. For example, unlike snapshots, the various CDP systems require additional storage and may require separate CDP appliances.
Perhaps the biggest drawback is simply that CDP is so new and still somewhat unsettled. Over the next year or two, we can expect the CDP market to offer a more standardized set of products and features, and data snapshot products to acquire more CDP features.
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About the author: Rick Cook has been writing about mass storage since the days when the term meant an 80 K floppy disk. The computers he learned on used ferrite cores and magnetic drums. For the last twenty years, he has been a freelance writer specializing in storage and other computer issues.