Changed Block Tracking

This feature, first available in vSphere 4, provides the foundation for incremental (or differential) backup of virtual disks. Your application can back up only changed data as indicated by the QueryChangedDiskAreas method. Virtual machines with virtual hardware version 7 and later support changed block tracking. These virtual machines contain changeTrackingSupported in the capability field of the VirtualMachine managed object. See Changed Block Tracking on Virtual Disks for details.