Windows 8 and Windows Server 2012 Hyper-V support the new VHDX Virtual Hard Disk format. If you are interested in the technical details just became available and you can download the VHDX specification:
http://www.microsoft.com/en-us/download/details.aspx?id=29681

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.

 

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