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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 expands 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 costs can escalate sharply if multiple snapshots must be maintained. An alternative is to store the full-copy snapshot on lower-cost media such as ATA disk. The viability of doing this depends on why the snapshot was created in the first place. If the snapshot is used as the basis for tape backup, then ATA could be a good fit. If the snapshot is to be used in the event of a disaster, ATA disks may not provide the performance required in a production environment.
Differential-copy snapshots
Differential-copy snapshots store only changes to a file system. As existing files are deleted or modified, the disk blocks associated with those changes are preserved. With this approach, significantly less disk space is required to maintain the snapshot. Examples of this approach are EMC's TimeFinder/Snap, HDS' Copy-on-Write Snapshot, Microsoft Corp.'s Volume Shadow Copy Service (VSS) and NetApp's Snapshot. Depending on the technique used, the creation of a differential snapshot can happen almost instantaneously.
The key advantage of this approach is that less disk space is required. The storage needed for maintaining the snapshot could be as little as 3% of the primary volume size. This could allow many snapshots to be stored. But the primary advantage of differential-copy snapshots is also its main disadvantage.
A differential snapshot requires access to the primary volume's data blocks to reconstruct the point-in-time image of the file system because all unchanged data exists only on the primary spindles. There are three considerations when choosing differential snapshots: performance, permanence and space management.
PERFORMANCE. Performance can be a concern when the snapshot is created and later when it's accessed. Depending on the underlying technique used, creating the snapshot can result in additional I/O load on the production, primary data spindles. This load could potentially impact users or systems accessing that data.
PERMANENCE. A differential snapshot requires access to the primary data set's blocks to reconstruct the point-in-time image of the file system. If the primary volumes are lost, then all associated snapshots are also lost.
SPACE MANAGEMENT. This is the trickiest of the three. A predetermined amount of capacity needs to be set aside to accommodate the changed data, which we'll call the "snap reserve." How a storage system reacts when the snap reserve isn't large enough depends on the manufacturer. Typically, one of two things happens: the oldest snapshot is deleted or free space in the primary file system is consumed. Neither is an appealing choice.
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