Like snapshot differentials or delta differencing, CBT backs up only the blocks that have changed, rather than backing up every block of every VM in the infrastructure. CBT requires ESX/ESXi hosts at version 4.0 or newer, VMs at virtual hardware version 7 or newer, and that I/O operations go through the ESX/ESXi storage stack.
CBT is disabled by default; administrators can enable CBT and some backup tools enable it automatically. If any blocks were changed since the last backup, Changed Block Tracking tags them and stores the information in a CTK file. CBT tells the vSphere or third-party backup tool to copy these changed blocks, avoiding copies of the entire VM. This reduces the amount of data undergoing backup.
VMware acknowledges that CBT could lose track of incremental changes in the event of a power failure or hard shutdown.
See also: vSphere Replication