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Backup in a snap: A guide to snapshot technologies

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Redirect-on-write (ROW) snapshot

Redirect-on-write is comparable to copy-on-write, but it eliminates the double write performance penalty. ROW also provides storage space-efficient snapshots like copy-on-write. What allows ROW to eliminate the write performance penalty is that the new writes to the original volume are redirected to the storage provisioned for snapshots. ROW redirection of new writes reduces the number of writes from two to one. So instead of writing one copy of the original data to the storage space plus a copy of the changed data required with COW, ROW writes only the changed data.

With redirect-on-write, the original copy contains the point-in-time snapshot data, and it's the changed data that ends up residing on the snapshot storage. There's some complexity when a snapshot is deleted. The deleted snapshot's data must be copied and made consistent back on the original volume. The complexity goes up exponentially as more snapshots are created, which complicates original data access, snapshot data and original volume data tracking, and snapshot deletion data reconciliation. Serious problems can occur when the original data set (upon which the snapshot is dependent) becomes fragmented.

Clone or split-mirror snapshot

A clone or split-mirror snapshot creates an identical copy of the data. The clone or split-mirror can be of a storage volume, file system or a logical unit number (LUN). The good thing about clones is

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that they're highly available. The bad thing is that because all of the data has to be copied, it can't be done instantaneously. A clone can be made instantaneously available by splitting a pre-existing synchronous volume mirror into two. However, when a split-mirror is used as a clone, the original volume has lost a synchronized mirror.

A very significant downside to this snapshot methodology is that each snapshot requires as much storage capacity as the original data. This can be expensive, especially if more than one snapshot clone is required to be kept live at any given time. One other downside is the impact to system performance because of the overhead of writing synchronously to the mirror copy.

Copy-on-write with background copy snapshot

Copy-on-write with background copy takes the COW instantaneous snapshot data and uses a background process to copy that data from its original location to the snapshot storage location. This creates a clone or mirror of the original data.

Copy-on-write with background copy attempts to take the best aspects of copy-on-write while minimizing its downsides. It's often described as a hybrid between COW and cloning.

Incremental snapshot

An incremental snapshot tracks changes made to the source data and snapshot data when the snapshot is generated. When an incremental snapshot is generated, the original snapshot data is updated or refreshed. There's a time stamp on the original snapshot data and on each subsequent incremental snapshot. The time stamp provides the capability to roll back to any point-in-time snapshot. Incremental snapshots allow you to get faster snapshots after the first one, and you use only nominally more storage space than the original data. This enables more frequent snapshots and longer retention of snapshots.

This was first published in October 2009

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