Application consistent quiescing and VDR

August 18, 2010

Taking snapshots of your virtual machines seems an adequate way of protecting your virtual machine and it application data. However, protecting transactional systems like SQL, Active Directory, Exchange and other systems using snapshots requires a little more attention.

To create a working snapshot of your virtual machine, the virtual machine needs to be in a Quiesce state in order to guarantee a consistent and usable backup. This generally requires flushing any outstanding writes.

VMware Data Recovery (VDR) 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. Until VMware vSphere 4.1, VMware did not fully support the VSS-writers if you are running Windows Server 2008 virtual machines. A good friend of mine has an excellent article on this.

With VMware vSphere 4.1 this has changed and VMware now fully supports application level quiescing through VSS. VMware Data Recovery uses different quiescing mechanisms depending on the guest operating system that you run in your virtual machines. See the table below for the quiescing types.


For applicationconsistent 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 hosts.
  • 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.

Windows 2008 virtual machines created on ESX/ESXi 4.0 hosts can be enabled for application consistent quiescing on ESX/ESXi 4.1 hosts by enabling the disk UUID attribute. Use the following procedure to enable the disk UUID:

  1. Start the vSphere Client, and log in to a vCenter Server.
  2. Select Virtual Machines and Templates and click the Virtual Machines tab.
  3. Right-click the Windows 2008 virtual machine for which you are enabling the disk UUID attribute, and select Power > Power Off.
    The virtual machine powers off.
  4. Right-click the virtual machine, and click Edit Settings.
  5. Click the Options tab, and select the General entry in the settings column.
  6. Click Configuration Parameters
    The Configuration Paramters window appears.
  7. Click Add Row.
  8. In the Name column, enter disk.EnableUUID.
  9. In the Value column, enter TRUE.
  10. Click OK and click Save.
  11. Power on the virtual machine.

Application consistent quiescing is available for this virtual machine now that the UUID property has been enabled.