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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. The interesting thing about both techniques is that some of the advantages are also 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
Simplified capacity management is another advantage of a full-copy snapshot. If you have a 1TB source, then you need a 1TB destination. As long as the destination is expanded with the source, everything continues to work.
The disadvantage of using a completely separate set of spindles is the cost. Each snapshot requires 100% of the disk space associated with the source, and the cost can escalate sharply if there's a need to maintain multiple snapshots. A possible solution is to store the full-copy snapshot on lower-cost media such as ATA disk. The viability of doing this depends on the motivation for creating the snapshot in the first place. If the snapshot is used as the basis of a tape backup, then ATA could be a good solution. If the snapshot is to be used in the event of a disaster, the ATA disks may not be able to provide the performance required in a production environment.
This was first published in December 2004