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

Changed Block Tracking, a feature of ESX(i) 4.0 and later, tracks changes to the contents of virtual disks.

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.

Use of CBT is configured in the virtual machine settings. The CBT configuration parameters are set up independently for a virtual machine (ctkEnabled) and for each of its disks (scsi#:#.ctkEnabled). Acronis Backup backs up a disk by using CBT only when both parameters are set to true. There is also a third parameter that prohibits using CBT, regardless of the above two settings (ctkDisallowed). To use CBT, this parameter must be missing or set to false. For a new virtual machine, the CBT parameters are usually missing.

For information about setting up the CBT configuration parameters, refer to VMware Knowledge Base: http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1031873.

Acronis Backup enables you to start using CBT on multiple machines without having to manually set up multiple configuration parameters.

Available settings

The preset is: Enable and use CBT

• Enable and use CBT

  Acronis Backup automatically enables CBT for each virtual machine that is included in the backup plan, and for all disks of the virtual machine. The ctkDisallowed parameter always remains unchanged.

  The resulting CBT settings determine CBT usage during the backup and are kept after the backup is completed.

• Use, if it is enabled on VM

  If CBT is enabled for a virtual machine, Acronis Backup uses CBT according to the CBT settings of individual disks. Otherwise, CBT is not used.

• Do not use CBT

  Acronis Backup does not use CBT, regardless of the CBT settings for the virtual machines.

Restrictions

Because of the CBT support limitations in VMware vSphere, use of CBT is not possible in these cases:

• The virtual machine version is earlier than 7. You can view the virtual machine version in the virtual machine properties.
• The virtual machine has a Raw Device Mapping (RDM) disk attached in the physical compatibility mode.
• The virtual machine has a disk attached to a shared virtual SCSI bus. Disks attached to such buses can be used by multiple virtual machines at a time. To view whether a SCSI bus is shared, open the virtual machine properties, select the corresponding SCSI controller in the list of hardware, and examine the SCSI Bus Sharing setting. Use of CBT for the disk is possible only when the setting is None.

The presence of snapshots, although it is a CBT support limitation in VMware vSphere, is not a restriction for using CBT in Acronis Backup.

For details about CBT support in VMware vSphere, refer to VMware Knowledge Base: http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1020128.