Enabling or Disabling Changed Block Tracking for Microsoft Hyper-V

You can optimize incremental backups by enabling Changed Block Tracking (CBT) for your backups. When CBT is enabled, incremental backups read only the allocated and modified portions of virtual disks. Depending on the version of Windows Server being used for the Hyper-V nodes, incremental backups can use Commvault CBT, native Microsoft CBT, or Cyclic Redundancy Check (CRC).

For more information, see Changed Block Tracking.


Commvault CBT tracks changed data blocks on VHDX-format disks with Hyper-V virtual machines running on Windows Server 2012 R2, and is available for all Windows Server 2012 R2 editions (Hyper-V Core, Standard, and Datacenter).

Commvault CBT can be used for backups of virtual machine disks on storage devices configured with 512 byte or 4K sector size.

Commvault CBT is not supported for the following backups:

  • Backups on remote SMB (Server Message Block Protocol) shares
  • Backups of shared VHDX disks

Before You Begin

To use Commvault CBT, you must meet the following requirements:

  •  Install the VSA software on all the nodes in the Hyper-V cluster.

    The first backup of a CBT enabled subclient activates the CVCBT driver on all the nodes. The driver communicates to the remote nodes for monitoring the virtual disks.

    Note: All nodes must have the VSA package installed, but you do not have to assign all nodes as proxies to be used for backup operations.

  • For all nodes, use Service Pack 2 or later of the Virtual Server Agent. If you have an earlier version of the Virtual Server Agent, install the latest service pack and restart the Hyper-V node.

    Ensure that all Hyper-V nodes use the same version of the CVCBT driver.

    Whenever you install a new service pack, you must restart the Hyper-V node to use the latest CVCBT driver version included with the service pack.


  • If a 2016 VSA Hyper-V proxy host is used to run a CBT-based backup copy job when a snap backup is run on a 2012 R2 proxy host with a CVCBT driver, the CBT validation will fail. In this case, even though CBT is enabled, CRC incremental backups are executed instead.

Enabling CBT

  1. From the CommCell Browser, navigate to Client Computers > virtualization_client > Virtual Server > Hyper-V > backup set.
  2. Right-click the subclient and select Properties.
  3. Click the Backup Options tab.
  4. On the Backup Options tab, select the Use Changed Block Tracking check box and click OK.


CBT is enabled for the subclient. After enabling CBT, the first backup run on the subclient activates the driver on all the nodes in the cluster.


  • If files were deleted from the virtual disks, the subsequent CRC or CBT enabled incremental backup will still consider the deleted files as modified extents and tracks them as changed blocks. Therefore, you may notice an increase in the backup size for that incremental backup.
  • CBT enabled backups of VHDX-format disks on a Cluster Shared Volume (CSV) might cause Hyper-V nodes to crash or restart intermittently. To resolve this issue, install the hotfix provided by Microsoft. For instructions, refer to Microsoft KB article 3130944.

Disabling CBT

When CBT is enabled, the CBT driver is automatically installed and enabled if it is not present on a Hyper-V node that performs a backup.

To disable CBT, perform the following actions:

  1. On the Backup Options tab of the subclient properties, disable CBT by clearing the Use Changed Block Tracking check box.
  2. For each Hyper-V node, at the command prompt using administrator privileges, run the following commands to disable or uninstall the CBT driver:
    • To disable the CVCBT driver, run the following command on the Hyper-V node:

      sc stop cvcbt

    • To uninstall the CVCBT driver, run the following command on the Hyper-V node:

      DriverInstaller.exe -cvcbt -op unInstall -inf "path"

      Where path is the location of the Commvault drivers folder. For example, you can use the following command:

      DriverInstaller.exe -cvcbt -op unInstall -inf "C:\Program Files\CommVault\ContentStore\Drivers"

To re-enable CBT, select the Use Changed Block Tracking option in the subclient properties. When CBT is enabled, the CBT driver is automatically installed and enabled when a Hyper-V node performs a backup.

What To Do Next

You can view the tracking status for running or completed backup jobs. For instructions, see Viewing Virtual Machine Backup Status.

Last modified: 8/4/2020 7:00:32 PM