How to enable Changed Block Tracking (CBT) on VMware Virtual Machines


Resolve the error "Changed block tracking must be enabled on the VM for this backup type."



Changed Block Tracking (CBT) is a VMware feature that helps Unitrends agents perform incremental backups of virtual machines.

Applies To

Unitrends backup appliance software version 6.0 or later

VMware ESX/ESXi 4.0 or later



Virtual machines running on ESX/ESXi hosts can track disk sectors that have changed. This feature is called Changed Block Tracking (CBT). On many file systems, CBT identifies the disk sectors altered between two change set IDs. On VMFS partitions, CBT can also identify all the disk sectors that are in use.

Virtual disk block changes are tracked from outside virtual machines, in the virtualization layer. When software performs a backup, it can request transmission of only the blocks that changed since the last backup, or the blocks in use.
The CBT feature is accessed by the Unitrends agent as part of the vSphere APIs for Data Protection (VADP). The Unitrends agent calls VADP to request that the VMkernel return blocks of data that have changed on a virtual disk since the last backup snapshot.

Protected VMs which do not have CBT enabled will receive the following error when backup types requiring CBT are attempted.

Changed block tracking must be enabled on the VM for this backup type.

For CBT to identify altered disk sectors since the last change ID, the following items are required:

  • The host must be ESX/ESXi 4.0 or later.
  • The virtual machine owning the disks to be tracked must be hardware version 7 or later.
  • I/O operations must go through the ESX/ESXi storage stack.  So NFS is supported, as is RDM in virtual compatibility mode, but not RDM in physical compatibility mode. Of course VMFS is supported, whether backed by SAN, iSCSI, or local disk.
  • CBT must be enabled for the virtual machine (see below).
  • Obviously, virtual machine storage must not be (persistent or non-persistent) independent disk, meaning unaffected by snapshots.

For CBT to identify disk sectors in use with the special * change ID, the following items are required:

  • The virtual disk must be located on a VMFS volume, backed by SAN, iSCSI, or local disk. RDM is not VMFS.
  • The virtual machine must have zero (0) snapshots when CBT is enabled, for a clean start.

In some cases, such as a power failure or hard shutdown while virtual machines are powered on, CBT might reset and lose track of incremental changes. Likewise offline Storage vMotion, but not online Storage vMotion, could reset but not disable CBT.

To check if a virtual disk has CBT enabled, open the vSphere Client, select a powered-off virtual machine, and click Edit...Settings > Options > Advanced/General > Configuration Parameters.

  • The virtual machine's configuration (.vmx) file contains the entry:

    ctkEnabled = TRUE

    Note: Set the value False to disable CBT.
  • For each virtual disk, the .vmx file contains the entry:

    scsix:x.ctkEnabled = TRUE
  • For each virtual disk and snapshot disk there is a .ctk file. For example:

Note: After the above changes, a new full will need to be run on the affected VM.


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