Hoping to up interest in its virtualization wares, Microsoft today freed up one of its key specifications used to manipulate virtual servers and applications.

Redmond has pushed the Virtual Hard Disk (VHD) Image Format specification under the umbrella of its Open Specification Promise (OSP). That means that Microsoft will license out VHD for free and promises not to sue companies which build their own products around the spec.

Microsoft has shipped the VHD format since May of 2005 and uses it to combine data about a virtual operating system and application into a single file. Software makers and customers can then shove that file from server to server.

"We are focused on delivering interoperability by design,” said Bob Muglia, a Microsoft SVP. "By having the VHD specification available under the OSP, the technology is viable for any development or business model."

Microsoft had already granted a VHD license to rival/partner XenSource. The smaller software maker used the technology to move virtual machines created on Microsoft servers over to the open source Xen. VMware failed to acquire a VHD license, and we'll be watching to see if it picks up the spec now.

VMware, which pulled in $189m last quarter, has dominated the server virtualization market and forced Microsoft into some unusual behavior. Microsoft, for example, has decided to give away its server virtualization products for free, has teamed with the open source player XenSource and has opened its server licensing policy on the Data Center version of Windows Server to allow for free virtual OS licenses.

Microsoft plans to revamp its virtualization play months after Longhorn Server is released by shipping software similar to what VMware and Xen offer today.

The likes of Virtual Iron, Network Appliance, BMC and Brocade use VHD today. ®

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