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Backing up a LUN snapshot

Can you give me a detailed explanation on how you actually make a snapshot of a LUN on a SAN, and then back it up? Who or what hosts the snapshot, and how is it backed up?

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Although your question may seem simple to you, the answer is not. Like many SAN and storage questions, the answer is: It depends. It depends on the software, the storage hardware, the SAN and the implementation. It depends on where the snapshot software sits (in the server, in an appliance, in an intelligent switch, in a storage virtualization device or in a target storage device.) It depends on whether the snapshot is a full volume snapshot, incremental snapshot, read-only snapshot, read/write snapshot, clone snapshot or consistency group snapshot. It depends on whether you are backing up that snapshot to disk or tape and if the backup is local or remote. It depends on whether the backup software is backing the snapshot copy through the server, or if the target storage is replicating the snapshot as a backup to another target storage device. As you can see, there is no simple stock answer to your question without significant qualification.

So, let's answer your question for Microsoft Storage Server 2003 utilizing Volume Shadow Copy Services of VSS and specifically their Shadow Copy function.

Snapshots are generically created by defining a marker at a point in time and making sure that the data can be rolled back to that point in time. You can keep multiple snapshots, and snapshots typically require much less additional disk space than VSS Snapshot clones. The VSS shadow snapshots can be created in several different ways.

A common method is called "copy-on-write." The copy-on-write method defines a snapshot by LUN at a point in time, and then monitors the original dataset for changes. If a change is made, the change is recorded or tracked in a separate location. Over time, the size of a snapshot can continue to grow, especially when a snapshot is made of a quickly changing dataset. The snapshot manager presents different views of the dataset, typically as if they were different full backups of the data. The snapshot manager can also switch to any available view of the data on demand, thus, in a sense, restoring the data.

Microsoft's VSS shadow copy snapshot is not actually an independent copy of the data, whereas their clone snapshot is. If the original data is destroyed, the shadow snapshot data is useless because it contains only the recent changes to the data. This backup method gives you a rollback mechanism, but not an actual backup of the data. The advantage to this backup method is that you are only writing the changes, instead of all the data, to disk, so that the actual creation of the snapshot can occur very quickly. A disadvantage is that you do not have a recoverable backup if your original data is corrupted. Because a shadow copy snapshot backup does not provide a true backup, most solutions implement an additional step that streams the snapshot backup to tape.

Each and every snapshot product will have a different methodology to accomplish what you want. Check with your vendor for specifics.

This was first published in April 2006

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