Changed Block Tracking for VMware Backups

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 on VMware ESXi servers, because CBT adds some load on the server 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.

4. Follow the backup wizard steps to save the backup plan configuration.# Changed Block Tracking for VMware Backups

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