Changed Block Tracking Feature on VMware Backup Plans

Changed Block Tracking Explained

Changed Block Tracking (CBT) is a feature that allows reading only those virtual machine disk blocks that have changed since a certain point in time.

Changed Block Tracking for VMware

The main advantage of Changed Block Tracking is significantly shorter incremental backups. The CBT mechanism records changes in a special Changed Block Tracking log file.

CBT can be used with any type of virtual disk, including those on VMFS (FC/iSCSI) and NAS/NFS volumes, except for Raw Device Mapping (RDM) disks in physical compatibility mode and disks attached to a shared virtual SCSI bus.

By default, the Changed Block Tracking feature is disabled for virtual machines, because CBT adds some load on the CPU.

System Requirements

The Changed Block Tracking feature has the following requirements:

• Virtual machine hardware version 7 or later
• VMFS, NFS, or iSCSI datastores
• Thick or thin provisioned disks
• No RDM disks in physical compatibility mode

How It Works

If the Changed Block Tracking feature is supported by the ESXi server and enabled in a VMware backup plan, Backup for Windows attempts to enable it on every backup plan run.

If existing snapshots are detected on the virtual machine, Changed Block Tracking is disabled, and the incremental backup reads the entire backup plan dataset.

The same happens if the Changed Block Tracking feature is not supported. In this case, Backup for Windows relies entirely on the ESXi server response and enables or disables CBT accordingly.

Enable Changed Block Tracking

To enable the Changed Block Tracking feature, create a new VMware backup plan or edit an existing one.

1. Follow the backup wizard steps to the Select Virtual Machines step.
2. Click Advanced Settings.
3. Select the Use Changed Block Tracking checkbox.

Check if a Virtual Disk has a CBT Enabled

To make sure that a virtual disk has CBT enabled, proceed as follows:

1. Open the vSphere Client and select a powered-off virtual machine.
2. Right-click the virtual machine and go to Edit Settings -> Options -> Advanced/General -> Configuration Parameters.
3. The configuration file (.vmx) of the virtual machine contains the entry: ctkEnabled = "TRUE".

Enable CBT on Virtual Machines

To enable or disable CBT on a virtual machine, perform the appropriate steps given for enabling or disabling:

Note: Ensure that the virtual machine is powered off before performing these steps.

To Enable CBT in a Virtual Machine

1. Power off the virtual machine.
2. Right-click the virtual machine, then click Edit Settings.
3. Switch to the Options tab.
4. Click General under the Advanced section, then click Configuration Parameters.
5. Click Add Row.
6. Add the ctkEnabled parameter, then set its value to true.
7. Click Add Row, add scsi0:0.ctkEnabled, then set its value to true.

8. Power on the virtual machine.