Changed Block Tracking (CBT) is a feature of VMware vSphere that transmits only the data blocks that have been changed since the last backup, increasing the backup efficiency and rate. This topic describes how to enable CBT for virtual machines (VMs).
Procedure
Start the vSphere Web Client.
Use a browser to access your vSphere Web Client and log on to the vCenter Server.
Select a VM.
In the Hosts and Clusters view, select the VM for which you want to enable CBT.
Stop the VM.
To modify the VM settings, make sure that the VM is stopped. Right-click the VM and choose .
Edit the VM settings.
Right-click the VM and select Edit Settings.
Add CBT parameters.
On the VM Options tab, click Advanced. Then, click EDIT CONFIGURATION in the Configuration Parameters section.
Enter ctkEnabled and set the value to true.
Enable the CBT feature for each virtual disk.
The CBT feature must be enabled independently for each virtual disk (vmdk) of each VM. You can add the following parameter:
Enter scsi0:0.ctkEnabled and set the value to true. The parameter settings indicate that the disk is at position 0 of SCSI controller 0.
If a VM has multiple disks, you must add similar parameters to each disk, such as scsi0:1.ctkEnabled and scsi1:0.ctkEnabled.
Restart the VM.
Save the settings and close the settings window. Then, restart the VM for the new settings to take effect.
Reset CBT.
When you enable the CBT feature for the first time, you may need to reset CBT to make sure that all changes are accurately tracked. You can perform the following steps to reset CBT:
Stop the VM.
Delete all .ctk.vmdk files from the directory of the VM.
Restart the VM.