Article VMware and VSS: What your VMware backup vendor isn’t telling you VMware and VSS: Application Backup and Recovery Author: Anton Gostev Applies to: Date: August 25, 2008

More and more organizations are choosing VMware Infrastructure to virtualize their mission‐critical applications (Active Directory, Exchange, SharePoint, SQL Server) to create a flexible, easily administered virtual infrastructure.

Virtual machines (VMs) and any applications they contain must be protected against failure. Typically, in the virtual world, this is done by performing an image‐level backup of the whole machine (for instance, using VMware Consolidated Backup). This method results in what is known as a crash‐consistent image. Restoring a crash‐consistent image is essentially equivalent to rebooting a server after a hard reset. For operating systems, this has not been an issue, since they can easily handle this type of activity. For database applications as well as for applications featuring replication, however, such a restore will often result in lost data, data corruption, or application failure.

To facilitate a correct application backup, application vendors provide various means for creating a consistent backup of the application and database data. One example is Microsoft Volume Shadow Copy Service (VSS). Using VSS along with an image‐level backup of virtual machines (VM) running supported applications allows you to create a transactionally consistent backup image. With such a backup image, you can successfully recover both the VM, and any supported application installed on the VM.

Of course, the only purpose of a backup is recovery.  And in the case of VSS‐integrated backup and restore, it is critical to properly initialize the application, instructing it to perform the restore procedure from the shadow copy instead of performing a regular start up. Otherwise the application will not restore properly. In other words, an improperly restored transactionally consistent backup image is no more valuable than the restoration of a crash‐consistent image, with both scenarios potentially resulting in data loss or application failure.  

Here is an excellent blog post that shows you several examples of restoration of the virtual domain controller from the VSS-aware backup:
VSS and VMware ESX: What your VMware backup vendor isn’t telling you

If you find this information useful, consider taking a look at the original whitepaper:
VMware and VSS: Application Backup and Recovery

We strongly encourage all organizations to verify their backup approach using this simple test to ensure that the solution you have chosen is able to back up and correctly restore all your VMs hosting mission‐critical Windows applications.