What is Change Block Tracking (CBT)?

What is Change Block Tracking (CBT)?

Changed Block Tracking (CBT) is a VMware feature that helps performing the  incremental backups. 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. VMware Products like vSphere Replication and vSphere Data Protection makes use of this technology for the replication and backup.


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.


How do you identify whether the Virtual Disk has got the CBT enabled?

To check if a virtual disk has CBT enabled:


  • Open the vSphere Client and select a powered-off virtual machine.
  • Click Edit…Settings > Options > Advanced/General > Configuration Parameters.
  • The virtual machine’s configuration (.vmx) file contains the entry:

ctkEnabled = “TRUE” 


Note: Set the value to False to disable CBT.
  • For each virtual disk, the .vmx file contains the entry:

scsix:x.ctkEnabled = “TRUE”
  • For each virtual disk and snapshot disk there is a .ctkfile. For example:

vmname.vmdk
vmname-flat.vmdk
vmname-ctk.vmdk
vmname-000001.vmdk
vmname-000001-delta.vmdk
vmname-000001-ctk.vmdk

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