N-able Backup: Backup completes with error: "File size during backup differs from size during scan. Please check if VSS is enabled and operational on the system"

Last Modified

Thu Dec 31 13:12 GMT 2020

Description

  • N-able Backup completes with error: "File size during backup differs from size during scan. Please check if VSS is enabled and operational on the system"

Environment

  • N-able Backup

Solution

  • Error occurs on backups of Windows systems when we scan a file's size (during initial scan phase of backup) and then take a snapshot of the file during the backup phase, and the two sizes are different. This can be due to a few different reasons:
    • VSS is not enabled, so we cannot take a snapshot and have to rely on the file's space-on-disk size while the file is in use in memory.
    • Another program is forcefully disabling VSS and our backup cannot utilize it (such as antivirus).
    • The backup is occurring on a network drive or share that cannot be accessed by the System Writer.
  • Resolving the problem may require investigating all possible options.
  • To check VSS:
    1. Run these commands in an elevated (administrator) Command Prompt:
      • vssadmin list writers
        vssadmin list providers
        vssadmin list shadowstorage
    2. Check for:
      • vssadmin list writers
        • Check for missing System, ASR, WMI, BITS, NTDS (if a domain controller), Registry Writer, COM+ REGDB Writer, or Task Scheduler Writer
          • Depending on system configuration other writers may be present or missing. To determine what features your system requires, consult Microsoft documentation for the operating system and features you are using.
          • The most common cause of this issue is a missing or failed System Writer: Troubleshoot missing System Writer
        • Verify there are no writers in a Failed or Error state (Retryable or Non-retryable errors may both appear)
      • vssadmin list providers
        • Verify that there are no third-party VSS writers (such as StorageCraft's ShadowProtect VSS Writer) as these can cause conflicts.
          • One third-party VSS writer that generally should not conflict is the writer from VMware tools.
        • Verify the Microsoft Shadowcopy VSS Provider is present and does not display errors
      • vssadmin list shadowstorage
        • Ensure that shadowstorage has been assigned to each volume on the system that is being backed up
  • If VSS appears to be correct/good and issue is still occurring, consider adding the following exceptions to your antivirus solution, or temporarily uninstall or disable your antivirus to try running a backup:
    • Paths:
      • C:\Program Files\Backup Manager\
      • C:\ProgramData\MXB\ (if using standalone N-able Backup, or using integrated MSP Backup and Recovery through N-able N-central)
      • C:\ProgramData\Managed Online Backup\ (if using integrated Backup and Recovery through N-able RMM)
    • Processes:
      • C:\Program Files\Backup Manager\BackupFP.exe
      • C:\Program Files\Backup Manager\ClientTool.exe
      • C:\Program Files\Backup Manager\BackupUP.exe
      • C:\Program Files\Backup Manager\BackupIP.exe
      • C:\Program Files\Backup Manager\ProcessController.exe