Microsoft Download Center: VHDX Format Specification
This specification describes the VHDX virtual hard disk format that provides a disk-in-a-file abstraction. This specification assumes that you are familiar with hard disk technologies, including how hard disks interface with the operating system or a virtual machine and understand how data is accessed and laid out on the physical medium. This specification is released under the Microsoft Open Source Promise (OSP) initiative to help guide development of VHDX virtual hard disk format implementations that are compatible with those provided by Microsoft.