vSphere 4 was released with so many features that it seems we’re still trying to get all of them implemented. One of the most impactful features of is the changed block tracking (CBT) functionality. (CBT is supported with vSphere version 4 virtual machines and higher.)
CBT allows a record of a virtual machine disk file (.VMDK) change to be accessed programmatically. This effectively treats a VMDK file like a block device, similar to that of a storage system. CBT makes data protection mechanisms for vSphere more efficient.