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=== How to enable CBT manually for a virtual machine ===

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.

Resolution: 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 <fc #ff0000>scsi0:0.ctkEnabled</fc>, and set its value to <fc #ff0000>true</fc>.
  - 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.

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

Notes:
<code>
1. Whenever a hard drive is added to the virtual machine, a SCSI device will be
 assigned to the hard drive. scsi0:0 in scsi0:0.ctkEnabled represents the SCSI device,
 while the others display similar to scsi0:1, or scsi 1:1.
2. You can enable or disable CBT individually on each hard drive.
</code>