VMware’s Change Block Tracking (CBT) Feature

As of ESX 4.0, VMware introduced a new feature called Change Block Tracking (CBT). For VM’s running version 7 virtual hardware, the VMkernel tracks the changed disk sectors on a per-VMDK basis. The tracking data is recorded in a separate file that has a CTK extension and works for all VMware disk types except Raw Device Mappings (RDMs) in physical mode. It is also supported on NFS and iSCSI based disks.

So why is CBT a big deal? CBT allows for backup software to track which sectors have changed since the last backup. This way the backup software only needs to worry about the updated sectors and not the entire VMDK as a whole, thus drastically reducing the amount of time needed to backup the VM.

To check if your virtual machine has CBT enabled:

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 .ctk file. For example:vmname.vmdk

VMware API Based Information from the VMware VCB Backup Guide

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