Since it has been around for a while there are numerous good articles around about that topic (see references). I will not just reproduce them here, but summarize the most important key facts you need to know if you come into touch with it for the first time.
1. How does CBT work and what is it good for?
If CBT is enabled for a virtual disk the VMkernel will create an additional file (named ...-ctk.vmdk) in the same directory where it stores a map of all the virtual disk's blocks. Once a block is changed it will be recorded in this map file. This way the VMkernel can easily tell a backup application what blocks of a file have changed since a certain point in time. The application can then perform an incremental backup by saving only these changed blocks.
CBT is also used by Storage VMotion that is able to move a virtual machine's disk files from one datastore to another while it is running.
2. How do you enable CBT?
CBT is to be enabled per virtual disk, and VMware's KB1031873 describes how to do this via editing a VM's advanced configuration parameters through the VI client. Unfortunately this requires the VM to be powered off. However, you can also change the setting while the VM is running by using an appropriate script like the one published here. To make the change effective then you need to perform a so called stun/unstun-cycle on the VM (i.e. power on/off, suspend/resume, create/delete snapshot).
It is important to know that CBT is not enabled by default, because it introduces a small overhead in virtual disk processing.
3. How do CBT and snapshots play together?
When you create a snapshot of a VM's virtual disk an additional ctk-file is created for the delta-disk file of the snapshot. Once this snapshot is deleted again the delta-ctk will be merged with the base-ctk, just like the delta-disk is merged with the base-disk.
4. Important notes and references
- KB1020128: Changed Block Tracking (CBT) on virtual machines
- KB1031873: Enabling Changed Block Tracking (CBT) on virtual machines
- While an application backs up a VM using CBT the VM cannot be vMotioned: KB2001004
- Inconsistency resolved in vSphere 4.0 U3 and vSphere 4.1: KB1021607
- KB1031106: Virtual machine freezes temporarily during snapshot removal on an NFS datastore in a ESX/ESXi 4.1 host
- Eric Siebert on CBT: A detailed introduction
- Additions by Duncan Epping: Even more details...