The steps are described in VMware KB 2139574 ("Resetting Changed Block Tracking for VMware vSphere virtual machines"):
To disable CBT:
Warning: Prior to applying this procedure, enusre the virtual machine is not running on snapshots. For more information, see:
- Consolidating snapshots in "vSphere 5.x/6.0" (VMware KB 2003638)
- Determerning if there are leftover delta files or snapshots that VMware vSphere or Infrastructure Client cannot detect (VMware KB 1005049)
- Shut down the virtual machine.
- Remove any existing virtual machine snapshots.
- Right click the VM, click "Edit settings", find the "Options" tab and click "Configuration Parameters"
- Set the "ctkEnabled" value to false
- Set the "scsi0:x.ctkEnabled" value to false for each disk of the VM in question
- Remove or rename any files ending with the *-ctk.vmdk file extension in the virtual machine directory.
- Power on the virtual machine.
- Power off the VM again.
Then enable back CBT:
- Right click the VM, click "Edit settings", find the "Options" tab and click "Configuration Parameters"
- Set the "ctkEnabled" value to true
- Set the "scsi0:x.ctkEnabled" value to true for each disk of the VM in question
- In the home directory of the virtual machine, verify that each hard disk contains a vmname-ctk.vmdk file.
- Power on the VM
- Run the backup