Snapshot Backup


Backing Up LUN Snapshots

The Snapshot Backup in SAN expert Marc Staimer offers a detailed description of what you need to consider when making a snapshot of the LUN and then making a backup. Could you give me a detailed explanation of how you actually made a LUN snapshot on a SAN, and then backed it up? Who or what saves a snapshot, and how is it backed up?

Even though your question may seem simple to you, the answer is no. Like many SAN questions and storage, the answer is: Depends. It depends on software, storage hardware, SAN and its implementation. That depends on where the snapshot software is (on the server, on the device, on the smart switch, on the storage virtualization device or on the target storage device.) 

That depends on whether the snapshot is a full volume snapshot, additional snapshots, read-only snapshots, read / write snapshots, clone snapshots or consistency group snapshots. That depends on whether you back up the snapshot to disk or tape and whether the backup is local or remote. 

That depends on whether the backup software supports copies of snapshots through the server, or if the target storage replicates the snapshot as a backup to another target storage device. As you can see, there are no simple answers to your questions without significant qualifications. 

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

Snapshots are generally made by defining markers at a point in time and ensuring that data can be rolled back to that point in time. You can save a lot of snapshots, and snapshots usually require far less disk space than the VSS Snapshot clone. VSS shadow snapshots can be made in several different ways.

A common method is called "copy-on-write." The copy-on-write method defines snapshots by LUN at a point in time, and then monitors the original dataset for changes. If changes are made, changes are recorded or tracked in a separate location. Over time, the size of photos can continue to grow, especially when photos are made from rapidly changing datasets. 

Snapshot managers

The Snapshot managers present different views of datasets, usually as if they are full data backups that are different. The snapshot manager can also switch to any data view on demand, so that, in a sense, recover data.

Snapshot copies of Microsoft VSS are actually not independent data copies, while clone snapshots are. If the original data is destroyed, the shadow snapshot data is useless because it only contains the latest changes to the data. This backup method gives you a rollback mechanism, but not actual data backup. 

The advantage of this backup method is that you only write changes, instead of all data, to disk, so making actual snapshots can happen very quickly. The disadvantage is that you don't have a backup that can be restored if your original data is damaged. 

Because backup copy of the shadow snapshot does not provide the correct backup, most solutions implement additional steps that drain snapshot backups to the record. Every snapshot product will have a different methodology to achieve what you want. Check with your vendor for specifications in Snapshot Backup.

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