The product uses the 'volume shadow copy' (VSS) technology built in to Windows to safely perform backups of open files. VSS provides the functionality to take a snapshot of a volume, allowing backups to backup data from the unchanging snapshot instead of the volume that is changing all the time. This technology is available on Windows XP, Vista, Server 2003 (or better) operating systems. On older operating systems it will still try to backup open files when possible, and then will check after a file has been backed up to make sure it was not modified while being backed up.
VSS has three parts that work together:
- VSS Providers: Providers implement the logic to actually take and maintain a snapshot of a volume. Windows comes with a default provider authored by Microsoft. It works with any volume so long as there is at least one volume in the system that is formatted with NTFS. Expensive NAS/DAS devices may provide their own hardware-based VSS provider.
- VSS Writers: Writers represent server applications that need to flush cached data to disk before a snapshot is taken, so that the snapshot represents good, consistent data. Both Exchange 2003 (or better) and SQL Server 2000 (or better) provide VSS writers. Many times problems with VSS are caused by the failure of some application-specific writer, outside the control of the backup application.
- VSS Requestor: Requestors represent applications that need to make, release, and use snapshots. For example, a backup application.
When open file backup fails, the problem is almost always caused by the failure of the Microsoft VSS provider or one of the application-specific VSS writers. The top of the backup log should contain some informational messages that give more information about which provider or writer failed, including an error message and number. The error number and the word VSS can often be used in a web search to find specific information about the possible causes of the error.
Additionally, both VSS providers and VSS writers oftentimes write events to the Windows event log. You should inspect the Windows application and system logs (Start, run, eventvwr) for any vss, volsnap, or other entries around the time when the backup started. You should consider events from applications whose VSS writer has failed. For example, if the backup log indicates that the Exchange writer failed, then you should also look for Exchange events in the Windows event log.
Another source of information is the vssadmin command line utility that comes with Windows. You should try the following commands at a command prompt to see the status of the providers and writers in your system, and whether any are in a failed state:
vssadmin list providers
vssadmin list writers
Note that in Windows Vista you will need to execute these commands from an Administrative command prompt (start task manager, show all tasks, File menu, New task, cmd).
Most VSS related errors are temporary and are fixed by rebooting the server. You should also try applying the following Microsoft hotfixes to ensure the issue is not related to a bug in VSS:
Beyond these steps how to resolve the problem is usually very specific and related to the specific VSS provider or writer failure. Many others on the web have usually encountered the same error. Searching for the VSS error code listed in the backup log (usually begins with 0x8.......) along with VSS and/or the name of the VSS writer will usually find relevant information. Our support team is also able to assist you in finding solutions to failures in your VSS providers and writers.
When you contact our support team please send them the backup log showing the VSS errors, any related errors and events from the windows event log, and the output of the vssadmin commands listed above. This will help our team find the resolution for your specific failure. The types of failures are very diverse and very specific, so the more information you provide the better our support team will be able to assist you.
Re-registering VSS System Files
CAUTION: The below scripts are only tested to work on Windows XP or below and Server 2003 or below. Do not run the below scripts on more modern operating systems without first getting confirmation from Microsoft.
Certain VSS errors may be resolved by re-registering the operating system's lower-level VSS components. This step should only be taken after other diagnostics have been failed to correct the failure. To re-register VSS components, execute the following within a command prompt:
cd /d %windir%\system32
net stop vss
net stop swprv
regsvr32 /i swprv.dll
regsvr32 /i eventcls.dll
net start swprv
net start vss
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