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Understanding how storage products implement snapshots will allow you to achieve a better balance between application requirements and costs.
Snapshots provide many benefits across all types of applications and storage requirements. They can greatly enhance and simplify the data recovery process by reducing recovery time and providing more recovery point objectives (RPOs).
They can enhance QA by reducing testing cycle times, and provide a basis for efficient updates of disaster recovery (DR) sites. But implementing snapshots requires planning and application analysis to determine what type of snapshot to use and how many to take.
A snapshot is a point-in-time image of a collection of data. In general, snapshots can be broken into two types: full copy and differential copy. While most full-copy snapshots use similar techniques, there are several ways to implement differential snapshots, each with its advantages and disadvantages.
With a full-copy snapshot, the entire contents of a data set are copied to a different set of spindles. Copying the data may be done on a continuous basis, so the creation of the actual snapshot can happen quickly. Examples include EMC Corp.'s TimeFinder/Mirror, Hitachi Data Systems' (HDS) ShadowImage and Network Appliance (NetApp) Inc.'s SnapMirror. The primary advantage of this approach is that the entire data set is maintained on a different set of spindles, providing a high level of protection if primary data is destroyed. It also allows the data in the snapshot to be accessed without placing any additional I/O load on the primary data spindles, a distinct advantage when the snapshot is used as the basis of a tape backup.
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