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Differential snapshots: Copy-on-write

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Differential-copy snapshots
Differential-copy snapshots only store 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 primary advantage of this approach is that less disk space is required. Depending on the application, the storage overhead for maintaining the snapshot could be as little as 3% of the primary volume size. This potentially allows many snapshots to be stored. But, as with full-copy snapshots, the primary advantage of differential-copy snapshots is also its primary 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 the snapshot is accessed. Depending on the underlying technique used, the creation of 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 is not 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.

The size of the snap reserve is a function of how quickly the data changes in the primary file system and the number of snapshots to be retained. The rate of change is a function of what the primary file system is used for. For network-attached storage (NAS), Windows home directories and public shares have a data change rate that's typically 3% to 5% of the volume size per day. For storage area networks (SANs), the change rate is dependent on the application using the storage.

This was first published in December 2004

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