Changed Block Tracking (CBT)

This option applies only to Agent for VMware.

This option determines whether to use Changed Block Tracking (CBT) when performing an incremental or differential backup of an ESX(i) virtual machine.

To access this option, connect the console to the management server or to Agent for VMware. Then, select Options > Default backup options or Options > Default backup and recovery options > Default backup options, respectively. The default setting will be used unless you override it with a different setting in the backup plan.

How it works

The Changed Block Tracking mechanism tracks changes to the contents of virtual disks.

Changed Block Tracking was originally introduced in VMware ESX(i) 4.0. Starting with Acronis Backup 11.5 Update 5, there is an Acronis proprietary implementation of the CBT mechanism, which is unrelated to the VMware implementation. Acronis CBT does not use or change the virtual machine configuration settings related to VMware CBT (ctkEnabled, scsi#:#.ctkEnabled, and ctkDisallowed).

Without using CBT, Agent for VMware reads the virtual machine's file system to determine which blocks have changed. When using CBT, the agent obtains the list of changed blocks from the ESX(i) host. The agent only needs access to the file system to determine which of these blocks must be included in the backup. This leads to faster backups and reduces the load on the storage during a backup.

CBT is most effective for large amounts of data that changes little between backups. In particular, databases often consist of large files with relatively small daily changes.

Available settings

The preset is: Use CBT

• Use CBT

  Acronis Backup uses CBT for each virtual machine that is included in the backup plan.

• Do not use CBT

  Acronis Backup does not use CBT.