Enabling VMware Changed Block Tracking (CBT) helps reduce transferred data size and time for backup because CBT backs up only the blocks that are changed since the previous backup. This article will guide you through the process of enabling CBT in VMware vSphere.

To enable CBT in a virtual machine:

1. Power off the virtual machine. You will not be able to successfully complete the setting without powering off the virtual machine.
2. Right-click on the virtual machine and click Edit Settings.
3. Click the VM Options tab.
4. Click Advanced, and then click Edit Configuration next to Configuration Parameters. It will open the Configuration Parameters dialog.
5. Click Add parameters, add the ctkEnabled parameter, and set its value to true.
6. Click Add parameters, add scsi0:0.ctkEnabled, and set its value to true.^1,2
7. Power on the virtual machine.
8. In the home directory of the virtual machine, verify that each drive having CBT enabled has also a vmname-ctk.vmdk file.

The instructions above are referring to this website: VMware KB 1031873. The content might be subject to change in different versions of VMware vSphere. You may find more up-to-date details regarding enabling CBT in VMware VMs on VMware KB 1031873.

After CBT is enabled, the first backup will still be a full backup, and the subsequent backups will be incremental backup.