VMware CTK file

A VMware CTK file contains a list of all the changes made to a VMware virtual machine (VM) since it was last backed up. 

CTK change tracking files exist on all VMs where VMware's Changed Block Tracking (CBT) technology is enabled. CBT relies on the CTK file's information to back up only the VM information blocks that have changed, instead of backing up the entire VM.

There is only one CTK file in the virtual machine disk files, and it has the file extension .ctk. The CTK file is always much smaller than VMware Virtual Machine Disk File (VMDK) block it describes.  

Note: VMware snapshots also contain a .ctk file. Orphaned CTK files, left over erroneously from snapshots, can cause errors during backup.