Application consistent quiescing and VDR

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.

7 Responses to Application consistent quiescing and VDR

  1. […] Comments Application consistent quiescing and VDR « vKnowledge on Is your VMware snapshot application consistent ??Jim on VMware vCenter Converter […]

  2. I have confirmed that 4.1 now supports application consistent snapshots. What I _haven’t_ confirmed is whether or not they are still using VSS_BT_COPY instead of VSS_BT_FULL. The former does not tell the app they’ve been backed up, and the latter does (so it can do things like truncate transaction logs). MAN I wish I was going to VMworld…

  3. patters says:

    My Exchange and SQL DBs are on their own SAN LUNs which I use my storage vendors’ VSS driver to snapshot with Backup Exec (and the MS software iSCSI initiator). Would your method work if these LUNs are not on VMFS and are RDMs instead?

  4. @W. Curtis Preston: Please let us know when you confirm…I am sure we would all like to know. This has been something I have been waiting for. If I have time this week I will see if I can confirm either way. On the note of going to VMworld…I am with you on that…didn’t get to go this year!

  5. Andrew Fidel says:

    Is there a way to prestage the disk.EnableUUID changes? It would be easy to go through and make the change en-mass and have them take effect during the next server reboot but taking the time to modify each machine during an outage window just wouldn’t be possible for most larger environments.

  6. ScotH says:

    Anyone know if VSS_BT_COPY or BSS_BT_FULL is used? I have been avoiding snapshots on our SQL server because of the lack of VSS support.

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