Volume Shadow Copy Service: Overview and Troubleshooting

Volume Shadow Copy Service: Overview and Troubleshooting

We are going to overview the basics of the Volume Shadow Copy Service concept so you can better understand underlying backup processing. You will also find a quick VSS troubleshooting guide and a few useful links that can help in most VSS-related issues.

What Is VSS and How It Works

Microsoft Volume Shadow Copy Service (VSS) is a system interface (or a framework) that allows third-party tools to perform centralized backup and restoration. Its main feature is an ability to perform a backup while applications being backed up are continuing writing and reading a data.

VSS consists of three components:

  1. VSS Requestor - typically the backup tool which asks to create a shadow copy.
  2. VSS Provider - a system interface used to actually create the shadow copy.
  3. VSS Writer - special service for a VSS-compatible application that ensures the data to be backed up is ready for shadow copy creation.

When the VSS-compatible application asks a provider to create a shadow copy, the provider instructs a writer to prepare the data for snapshot. Then the provider initiates the creation of a shadow copy snapshot, which actually re-targets system write requests to a special object - the real data stays ‘read-only’ and is available for normal backup.

After the VSS backup tool has finished its job, the snapshot is committed to the file system. Now the file system includes all operations made while the backup was running.

If you are familiar with virtual machines’ snapshotting, then you can imagine the VSS snapshotting process as a snapshot creation process on VMware or Hyper-V servers. The concept is the same.

Volume Shadow Copy Service framework is the same on server and desktop OS editions, but Windows server can have more providers for third-party apps (SQL, ISA, Exchange, etc).

Here the quick set of advantages you get when performing a VSS-enabled backup:

  • The backup performs faster since there is no need to pause system applications or duplicate data which needs to be backed up.
  • Snapshot process made it possible to create instant data backups and perform user-driven restoration.
  • You can backup or even copy those files users work on now.
  • Snapshotting mechanism allows you to have small instant copies (as Microsoft says, roughly 30MB for any 1GB of real data).

Volume Shadow Copy Service Troubleshooting

If VSS framework has some issues, you can not complete VSS-enabled backups - the symptoms are as follows:

  • Your backup application shows the backup job as failed, or completed with a VSS-related exception.
  • You will get similar errors in the Windows Event Log in case of any issues. Here are only a few examples:
    • “Error calling CreateFile on volume \\?\Volume…”.
    • “The shadow copy of volume \\?\Vol… was aborted during detection”.
    • “The Volume Shadow Copy integration service is either not enabled, not running, or not initialized”.

In any case the first and most important troubleshooting step is checking the Windows Event Log to find a root cause. As a quick and simple fix for temporary weird errors - reboot your system first before trying anything else.

If reboot did not help - try the following troubleshooting steps:

  • Before any backup, run a VSS framework that needs an app-related VSS writer in a “stable” state. Moreover, all writers should be operational and stable in a healthy system. Check writers status using this command (paste the command into CMD console:

Here is an AWS CLI script:

VSSadmin list writers

You will get the list of all writers registered - check “State” field - if there is an error as depicted below - the VSS writer is not operational:

Writer name: 'Microsoft Exchange Writer'
Writer Id: {GUID}
Writer Instance Id: {GUD}
State: [14] Failed
Last error: Retryable error
  • Restart corresponding app’s service and run VSSadmin list writers command again to verify if the problem has gone away. In our example we should restart the “Microsoft Exchange Information Store” service.
  • If you get no writers listed - you need to fix the registry.
  • Next you should check registered VSS providers using this command:
VSSadmin list providers

If you do not use third-party Volume Shadow Copy Service providers, then the output will show “Microsoft Software Shadow Copy provider”.

  • If the issue is still there - try checking the trace log. First of all, enable VSS tracing.
  • Try to run a backup again so your issue can rise up again. Then stop VSS services:
net stop VSS
net stop SWPRV

And stop VSS tracing using the command:

reg delete HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\VSS\Debug\
Tracing /f
  • Check the log file generated to find out the root cause of the issue.
  • Run the BETest Tool that can show additional details using complex VSS backup simulations.
  • After BETest is finished - check the “Output.txt” log for more details.

Use the Microsoft TechNet website to find details about any error messages you will find. In most cases you will get ready to use a solution, or even an automatic fix script.

Summary

Microsoft Volume Shadow Copy Service allows you to create fast and effective backups even on the “hot” data that is accessed and modified by users or apps during a backup run. But VSS errors can arise since this framework involves the cooperation of third-party tools.

You can perform fast VSS troubleshooting using our guide, but don’t hesitate to check Microsoft KB in more difficult cases. You can also try CloudBerry Backup as a VSS-compatible app for FREE and test whether your issue is app-specific or not.