VMware Data Recovery uses Microsoft Windows Volume Shadow Copy Service (VSS) quiescing, which provides the backup infrastructure for certain Windows operating systems, as well as a mechanism for creating consistent point-in-time copies of data known as shadow copies.

VSS produces consistent shadow copies by coordinating with business applications, file-system services, backup applications, fast-recovery solutions, and storage hardware. VSS support is provided with VMware Tools, which runs in the guest operating system. VMware provides a VSS Requestor and a VSS Snapshot Provider (VSP). The requestor component is available inside a supported guest and responds to events from an external backup application. The requestor is instantiated by the VMware Tools service when a backup process is initialized. The VSP is registered as a Windows service and notifies the ESX/ESXi host when the applications are quiesced so it can take a snapshot of the virtual machine.

Data Recovery uses different quiescing mechanisms depending on the guest operating system that you run in your virtual machines.

Driver Type and Quiescing Mechanisms Used According to Guest Operating Systems

Guest Operating System

Driver Type Used

Quiescing Type Used

Windows XP 32-bit

Windows 2000 32-bit

Sync Driver

File-system consistent quiescing

Windows Vista 32-bit/64-bit

Windows 7 32-bit/64-bit

VMware VSS component

File-system consistent quiescing

Windows 2003 32-bit/64-bit

VMware VSS component

Application-consistent quiescing

On pre-ESX 4.1 Hosts:

Windows 2008 32-bit/64-bit Windows 2008 R2

VMware VSS component

File-system consistent quiescing

On ESX 4.1 and later Hosts:

Windows 2008 32-bit/64-bit

Windows 2008 R2

VMware VSS component

Application-consistent quiescing. For application-consistent quiescing to be available, three conditions must be met:

The UUID attribute must be enabled. This is enabled by default on virtual machines created on ESX 4.1 and later hosts. For virtual machines created on other hosts, complete the procedure Enable Windows 2008 Virtual Machine Application Consistent Quiescing.

The virtual machine must use only SCSI disks. For example, application-consistent quiescing is not supported for virtual machines with IDE disks. There must as many free SCSI slots in the virtual machine as the number of disks. For example, if there are 8 SCSI disks on SCSI adapter 1, there are not enough SCSI slots free to perform application quiescing.

The virtual machine must not use dynamic disks.

Other guest operating systems

Not applicable

Crash-consistent quiescing

Application consistent quiescing of Windows 2008 virtual machines is only available when those virtual machines are created in vSphere 4.1 and later. Virtual machines created in vSphere 4.0 can be updated to enable application consistent quiescing, as described in Enable Windows 2008 Virtual Machine Application Consistent Quiescing.

Because Data Recovery uses VSS, Data Recovery can create snapshots while ensuring application consistency. This means that applications write to disk any important data that is currently in memory, making sure that a later restore of that virtual machine can restore the application back into a consistent state.

Detailed information about VSS can be found at http://technet.microsoft.com/en-us/library/cc785914.aspx.

In most cases, the quiescing mechanisms provided with Data Recovery will properly quiesce applications. If your environment includes applications or operating systems that do not respond to included quiescing mechanisms as expected, Data Recovery supports the use of custom quiescing scripts. Deploy and run the custom quiescing scripts inside the protected virtual machine.

Locations of Custom Quiescing Scripts

Guest Operating System


Location of Script on Virtual Machine



C:\Program Files\VMware\VMware Tools\backupScripts.d

All scripts are invoked in ascending alphabetical order with freeze as the first argument.


C:\Program Files\VMware\VMware Tools\backupScripts.d

All scripts are invoked in descending alphabetical order with thaw or freezeFail as the first argument.






When running the scripts, you can also use the SYNC driver or VSS components on those virtual machines that support them.