Enabling CBT for a VM

Product version: 9.0

Last modified: 30 September 2019

Question

How do I enable CBT manually?

Answer

Changed Block Tracking speeds up incremental backup and replication. Due to technical limitations, CBT cannot be enabled in Free ESXi automatically.

ESXi 5.1 and earlier

CBT can be enabled manually by editing the VM settings. To enable CBT in a virtual machine on Free ESXi 5.1 and earlier, follow the steps below:

  1. Power off the virtual machine. 

  2. Right-click the virtual machine and click Edit Settings.

  3. Click the Options tab.

  4. Click General under the Advanced section and then click Configuration Parameters. The Configuration Parameters dialog box opens.

  5. Click Add Row.

  6. Add the ctkEnabled parameter and set its value to “true”.

  7. Click Add Row, add scsi0:0.ctkEnabled, and set its value to “true”.

    Note
    The scsi0:0 parameter in the scsi0:0.ctkEnabled value indicates the SCSI device assigned to the hard disk that is added to the virtual machine. Every hard disk added to the virtual machine is given a SCSI device that appears similar to scsi0:0, scsi0:1, or scsi1:1. 

  8. Power on the virtual machine.

ESXi 5.5

CBT cannot be enabled without removing the VM from Inventory. To enable CBT in a virtual machine on Free ESXi 5.5, follow the steps below:

  1. Power off the virtual machine.

  2. Right-click the virtual machine and click Remove from Inventory.

  3. Open Datastore Browser and browse the datastore where the virtual machine configuration file <VM name>.vmx is located.

  4. Open the virtual machine folder on the datastore and download the <VM name>.vmx file.

  5. Open the downloaded file using a text editor.

  6. Manually add the ctkEnabled = "true" attribute.

  7. Manually add the scsi0:0.ctkEnabled = “true” attribute.

    Note
    The scsi0:0 parameter in the scsi0:0.ctkEnabled value indicates the SCSI device assigned to the hard disk that is added to the virtual machine. Every hard disk added to the virtual machine is given a SCSI device that appears similar to scsi0:0, scsi0:1, or scsi1:1. 

  8. Save the modified file.

  9. Upload the modified <VM name>.vmx file back to the datastore.

  10. Right-click the uploaded file and click Add to Inventory. The wizard for adding the virtual machine to the inventory opens.

  11. Follow the wizard and complete adding the virtual machine to inventory.

  12. Power on the virtual machine.

You can find more information on Enabling CBT in VMware VMs in VMware KB 1031873.

ESXi 6.0/6.5 Including Free ESXi

To enable CBT in a virtual machine, follow the steps below:

  1. Power off the virtual machine.

  2. Right-click the virtual machine in the list and choose Edit Settings.

  3. In the dialog box that opens, go to the VM Options tab.

  4. Expand the Advanced section and click Edit Configuration to open the Configuration Parameters dialog.

  5. In the dialog box, add a new parameter to the configuration as follows:

    1. Click Add parameter.

    2. A new parameter with the Click to edit key key and Click to edit value value appears in the list. Click the parameter key to open it for editing.

    3. In the parameter key editing box, enter ctkEnabled.

    4. Click the parameter value to open it for editing and enter TRUE.

  6. Similarly, add another parameter with the scsi0:0.ctkEnabled key and the TRUE value to the configuration.

  7. Click OK to close the Configuration Parameters dialog box.

  8. Power on the virtual machine.

  9. In the home directory of the virtual machine, verify that each disk that has CBT enabled also has a vmname-ctk.vmdk file.

ESXi 6.7

To enable CBT in a virtual machine, follow the steps below:

  1. Power off the virtual machine.

  2. Right-click the virtual machine in the list and choose Edit Settings.

  3. In the dialog box that opens, go to the VM Options tab.

  4. Expand the Advanced section and click Edit Configuration to open the Configuration Parameters dialog box.

  5. In the dialog box, add a new parameter to the configuration as follows:

    1. Click Add Row.

    2. Add the ctkEnabled parameter and then set its value to TRUE.

  6. Similarly, add another parameter with the scsi0:0.ctkEnabled key and the TRUE value to the configuration.

  7. Click OK to close the Configuration Parameters dialog box.

  8. Power on the virtual machine.

  9. In the home directory of the virtual machine, verify that each disk that has CBT enabled also has a vmname-ctk.vmdk file.

Note

  • The scsi0:0 string in the scsi0:0.ctkEnabled key indicates the SCSI device assigned to the hard disk that is added to the virtual machine. Every hard disk added to the virtual machine is given an SCSI device that appears similar to scsi0:0, scsi0:1, or scsi 1:1.

  • If your virtual machine has IDE disks, you will have to use other parameter keys. Refer to disk names on the Virtual Hardware tab of the Edit Settings dialog box.