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Why recursive VSS is good for backing up virtualized Windows Servers

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VSS primer

For many Windows Server-based applications, Microsoft provides some core “plumbing” for enabling backups called Volume Shadow Copy Service (VSS). Along with the VSS foundation within the Windows OS, there are three active components:

  1. VSS Requester includes components typically found in a traditional backup agent (often from a third party) that initiate the backup process.
  2. VSS Writer components in the application (e.g., SQL Server, Exchange or even the Windows file system) ensure the workload is ready to be backed up by performing tasks such as flushing memory-based transactions or other backup preparation.
  3. VSS Provider components in the storage layer (OS software based or hardware based) capture a snapshot of the data set to be protected.

Essentially, a VSS-based backup works in eight relatively straightforward steps:

  1. A backup agent’s VSS Requester queries VSS for workloads that are capable of being backed up.
  2. VSS enumerates any workloads that have registered their VSS Writer.
  3. The backup agent’s VSS Requester requests that the workload be prepared for backup.
  4. The workload’s VSS Writer does what’s specifically required for that workload to be backed up.
  5. After preparation,

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  1. the workload’s VSS Writer notifies VSS and its VSS Provider that its data is ready.
  2. The VSS Provider snaps the data set and notifies the VSS Requester that it has the data.
  3. The VSS Requester references the (usually temporary) snapshot within the VSS Provider and sends the appropriate data to the backup server.
  4. Upon completion of the backup and acknowledgement by the backup server, VSS can notify the VSS Provider that it’s free to release the snapped data and the VSS Writer that its data is secure. The workload can then do its post-backup maintenance tasks.

This was first published in June 2012

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