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VSS Limitations

January 11, 2009
VSS is probably quite close to sliced bread on the great innovations list, however its not all a bed of roses, below are some considerations, thoughts to take away on limitations.
The NTBACKUP utility fully supports the use of VSS backups. However, when you include Exchange Server into the mix, things get a little complicated. NTBACKUP can back up an Exchange server, and it does support VSS backups — but it does not support VSS backups for Exchange. For that, you need a relatively high-end, third-party backup application.
Storage groups
You cannot perform Microsoft VSS backups of individual Exchange Server databases. You can only back up entire storage groups, because all the databases within a storage group share a common set of transaction log files. You can restore an individual database within a storage group, but to do so, you have to dismount the entire storage group. Another limitation has to do with Exchange servers that contain multiple storage groups. Microsoft VSS allows you to create backup jobs that back up multiple storage groups. However, you cannot simultaneously run multiple VSS backup jobs even if you create a separate job for each storage group.
Backup media 
A normal, non-VSS backup works by streaming data from the server to a tape drive or disk-based backup.

Microsoft VSS works by taking a snapshot of the Exchange information store and placing it onto a network volume, at the same time VSS tries to keep the database accessible to the users. To accomplish this, it places a lock on the databases and transaction logs within the information store that is being backed up. Once these resources are locked, VSS creates the snapshot as quickly as possible and unlocks the storage group. Users are never forced to disconnect from Exchange Server and usually don’t ever realize that anything has happened. Messages that are sent or received during this time are held in a queue until the storage group is unlocked.

The snapshot itself is a read-only image of the storage group. Because the image is read-only, the backup software doesn’t have to worry about changes being made to it while it is being backed up to tape.

The point is that you aren’t going to be able to perform a VSS-based Exchange backup directly to tape. You need a high capacity, high performance disk volume to which you can write the snapshot. Microsoft recommends using a Storage Area Network (SAN) for this purpose. From there, the information can be dumped to tape — but that part is optional.

Data integrity issues affect VSS Exchange backups

With Microsoft VSS backups of Exchange Server consists of a snapshot of the storage group. Since the backup software is simply making a snapshot, rather than streaming database pages to tape, the backup software never gets the chance to confirm the integrity of the data within the database. As a result, damaged database pages could be backed up. If the problem is left uncorrected, over time, all of your backups might contain some corrupt data.


Categories: Storage & Backup
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