Search the VMware Knowledge Base (KB)
View by Article ID

Changed Block Tracking (CBT) on virtual machines (1020128)

  • 90 Ratings


Changed Block Tracking (CBT) is a VMware feature that helps perform incremental backups. VMware Data Recovery uses this technology and so can developers of backup and recovery software.


Virtual machines running on ESX/ESXi hosts can track disk sectors that have changed. This feature is called Changed Block Tracking (CBT). On many file systems, CBT identifies the disk sectors altered between two change set IDs. On VMFS partitions, CBT can also identify all the disk sectors that are in use.

Virtual disk block changes are tracked from outside virtual machines, in the virtualization layer. When software performs a backup, it can request transmission of only the blocks that changed since the last backup, or the blocks in use. The CBT feature can be accessed by third-party applications as part of the vSphere APIs for Data Protection (VADP). Applications call VADP to request that the VMkernel return blocks of data that have changed on a virtual disk since the last backup snapshot.

For CBT to identify altered disk sectors since the last change ID, these items are required:
  • The host must be ESX/ESXi 4.0 or later.
  • The virtual machine owning the disks to be tracked must be hardware version 7 or later.
  • I/O operations must go through the ESX/ESXi storage stack. So NFS is supported, as is RDM in virtual compatibility mode, but not RDM in physical compatibility mode. VMFS is supported, whether backed by SAN, iSCSI, or local disk.
  • CBT must be enabled for the virtual machine (see below).
  • Virtual machine storage must not be (persistent or non-persistent) independent disk, meaning unaffected by snapshots.
For CBT to identify disk sectors in use with the special "*" change ID, these items are required:
  • The virtual disk must be located on a VMFS volume, backed by SAN, iSCSI, or local disk.
  • The virtual machine must have zero (0) snapshots when CBT is enabled, for a clean start.
In some cases, such as a power failure or hard shutdown while virtual machines are powered on, CBT might reset and lose track of incremental changes. In vSphere 4.1 and prior, Cold Migration (but not Storage vMotion) could reset but not disable CBT. In vSphere 5.x versions earlier to vSphere 5.5 Update 2, storage vMotion resets CBT. For more information, see Changed Block Tracking is reset after a storage vMotion operation in vSphere 5.x (2048201).

To check if a virtual disk has CBT enabled:
  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"  

    Note: Set the value to False to disable CBT. For more information, see Enabling Changed Block Tracking (CBT) on virtual machines (1031873).

  4. For each virtual disk, the .vmx file contains the entry:

    scsix:x.ctkEnabled = "TRUE"

  5. For each virtual disk and snapshot disk there is a .ctk file.

    For example:

Note: After a successful backup and full snapshot consolidation, there should be no snapshot related .ctk  files remaining in the datastore. For example: vmname-000001-ctk.vmdk.

Additional Information

For more information, see the section Changed Block Tracking on Virtual Disks in the Designing vSphere Backup Solutions section of the Virtual Disk API Programming Guide.

If you are using VMware Horizon View and linked clone virtual machines, you should not be using CBT. Always ensure that CBT is disabled for the parent virtual machine. For more information, see Deploying or recomposing View desktops fails when the parent virtual machine has CBT enabled (2032214).

For related Changed Block Tracking articles, see:


Change Block Tracking, CBT enabled virtual disks

See Also

Update History

06/23/2011 - Added reference to 1031873. 11/01/2011 - Add known caveats, review by VADP core team. 03/19/2015 - Added ESXi 5.5 and ESXi 6.0 to Products. Added note about VMware View Linked Clones to Additional Info section.

Request a Product Feature

To request a new product feature or to provide feedback on a VMware product, please visit the Request a Product Feature page.


  • 90 Ratings

Did this article help you?
This article resolved my issue.
This article did not resolve my issue.
This article helped but additional information was required to resolve my issue.

What can we do to improve this information? (4000 or fewer characters)

Please enter the Captcha code before clicking Submit.
  • 90 Ratings