Third-party VSS causes NetWorker snapshot creation failures
With 2003 and earlier versions of Windows you were able to choose whether to use VSS to backup or not. Now with 2008 this is the only option so VSS problems have to be fixed for backups to complete successfully.
One issue we have recently encountered is the installation of 3rd party VSS providers that cause NetWorker back-ups to fail.
If the volume shadow copy service uses the 3rd party provider instead of the default Microsoft software provider we can see errors being generated at stages 7 and 8 of snapshot creation procedure below. Also we see other issues when the 3rd party provider has left a mount point or snapshot behind.
We have found that these errors will generally be cleared once NetWorker retries its backup so a retry of this backup will likely be successful.
The Shadow Copy Process
The shadow copy process is listed below:
- NetWorker asks the Volume Shadow Copy Service to enumerate the writers, gather the writer metadata, and prepare for shadow copy creation
- The writer creates an XML description of the backup components to the Volume Shadow Copy Service, and defines the restore method.
- The Volume Shadow Copy Service notifies the application-specific writer to prepare its data for making a shadow copy
- The writer prepares the data
- The Volume Shadow Copy Service initiates the “commit” shadow copy phase
- The Volume Shadow Copy Service tells the writers to quiesce their data
- The Volume Shadow Copy Service tells the provider to create the shadow copy
- The Volume Shadow Copy Service thaws the file system
- The Volume Shadow Copy Service queries the writers to confirm that write I/Os were successfully held during shadow copy creation
- If the writes were not successfully held (meaning that the shadow copy data is potentially inconsistent), the shadow copy is deleted and the requester is notified
- NetWorker can retry the process depending on the client retries set on the group
If the copy is successful, the Volume Shadow Copy Service gives the location information for the shadow copy back to the NetWorker.