Problem Description
A SQL Server backup job using Volume Shadow Copy Service (VSS) technology fails during the initial phase. The job immediately reports a failure, and the log shows an error indicating that VSS initialization could not be completed successfully.
Traceback / Logs
The critical error in the logs is the failure to initialize the VSS backup components:
level=error ... message="Failed to initialize vss backup" error="VSS error: IVssBackupComponents.InitializeForBackup: VSS_E_UNEXPECTED - A volume shadow copy service (VSS) component encountered an unexpected error. [...]"
level=error ... method=InitBackup message="Error in initializing backup" Error="Failed to initialize backup: Failed to initialize backup"
level=error ... message="Ending Job with Status: 'Failed'"
Cause
The VSS_E_UNEXPECTED error during the IVssBackupComponents.InitializeForBackup call is a generic error indicating a failure within the Volume Shadow Copy Service itself before the snapshot creation process could even begin.
Common causes include:
Unstable VSS Writers: One or more VSS Writers (especially the SQL Server VSS Writer) are in a failed, corrupted, or unstable state.
VSS Component Corruption: Corruption in the registration or installation of core VSS components (DLLs/services).
Missing or Failed Dependencies: A dependent service required by VSS or the SQL Writer is stopped or failed.
Resolution
Follow these steps in order to resolve the VSS initialization failure:
Step 1: Check VSS Writer Status
Determine if all necessary VSS writers are in a stable state.
Open an elevated Command Prompt or PowerShell window.
Run the command: vssadmin list writers
Review the output and ensure the following conditions are met for all writers, especially the SqlServerWriter:
State: Must be [11] Stable
Last Error: Must be No error
Step 2: Restart VSS and SQL Writer Services
If the status check reveals any writer is not stable, restart the relevant services.
Open the Services snap-in (services.msc).
Locate the SQL Server VSS Writer service.
Restart the service.
Locate the Volume Shadow Copy service.
Restart the service.
Step 3: Check Event Viewer for Specific Errors
The VSS_E_UNEXPECTED error is often preceded by a more specific error in the Windows Event Logs.
Open Event Viewer (eventvwr.msc).
Navigate to Windows Logs $\rightarrow$ Application and System logs.
Filter or search for VSS or SQLWriter errors occurring immediately before the backup job timestamp. This may reveal the root cause (e.g., specific memory or disk I/O problems).
Step 4: Re-register VSS Components (⚠️ Administrative Action Required)
If the issue persists, the VSS components might be corrupted. Re-registering the core VSS DLLs may resolve this, but it is an advanced procedure.
⚠️ WARNING: Consult your Windows Server Administration team before running these commands. Re-registering system DLLs can impact other backup/replication processes and general operating system stability if performed incorrectly.
Open an elevated Command Prompt or PowerShell window.
Stop the VSS services:
net stop vss
net stop swprv
Run the following commands to re-register the required DLLs (only use this if previous steps fail):
regsvr32 /s %systemroot%\system32\ole32.dll
regsvr32 /s %systemroot%\system32\vss_ps.dll
regsvr32 /s %systemroot%\system32\vssapi.dll
regsvr32 /s %systemroot%\system32\swprv.dll
regsvr32 /s %systemroot%\system32\sqlwriter.dll
Start the VSS services:
net start vss
net start swprv
After performing the resolution steps, attempt the SQL backup job again.