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The VSS connection
Exchange agent support of Windows Server 2003's Volume Shadow Copy Service (VSS) framework is another feature that lets firms make point-in-time backups of an Exchange database using their backup software.

There are three components to the Microsoft VSS framework: a requestor, a writer and a provider. Backup software acts as the requestor and controls the overall process of creating a copy of the Exchange database. It initiates the copy by first calling the Microsoft Exchange 2003 writer, which prepares the Exchange database for the copy by handling the acquiesce and dismount of the Exchange database while maintaining its integrity and consistency.

After the Exchange database is in this suspended state, the backup software calls on the VSS provider to execute the copy function. Either the Windows 2003 OS or the external storage system acts as the provider of the copy utility, which performs the actual copy of the Exchange database. Once the copy completes, the backup software instructs Exchange to remount the database and resume normal processing of email. This leaves a copy of the Exchange database that the backup software can use to perform restores or as a source to make a secondary copy of the data to disk or tape.

The level of recovery one can perform depends on the VSS provider called by the backup software. The VSS provider offered by the Microsoft Windows Server 2003 OS uses a shadow copy or copy-on-write approach

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to create copies of the Exchange database, which is supported by most backup software products. Symantec's Backup Exec supports only Symantec's VSS provider, which is offered as part of Symantec's Veritas Storage Foundation for Windows.

The Microsoft VSS provider delivers rapid snapshots of the Exchange database, but it requires the volume containing the original Exchange database to remain available to back up or restore data. In this scenario, if the volume with the main production Exchange database becomes inaccessible, so do any snapshots created from it. Another potential problem with this type of snapshot is that it's read-only.

This was first published in September 2007

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