Yesterday I posted a blog post about the new recommendations about Virtual Disks files in Hyper-V called VHD and VHDX.
Today I saw a tweet from Niklas Akerlund who posted a link to the new VHDX Format Specification v1.00.
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.
Download: Hyper-V VHDX Format Specification v1.00
VHDX is the new format which is currently supported by Windows 8 and Windows Server 2012.