US Patent 7447854 Tracking and replicating changes to a virtual disk

A mechanism is provided to maintain multiple copies of a virtual disk associated with a virtual machine by a synchronization function. The content of a first virtual disk is maintained, as a copy, at a second virtual disk by maintaining a record of the changes made to the first virtual disk and transferring only the changed data to the second. A disk change map represents the changed portions of the first virtual disk and is used to determine the content that must be transferred. Only data that differs between the two virtual disks is transferred thereby reducing the amount of data that needs to be transmitted.