Channel Insider content and product recommendations are editorially independent. We may make money when you click on links to our partners. Learn More.

Microsoft has decided to make its Virtual Hard Disk image format specification available to users at no cost and with the guarantee that it will never take legal action against them.

The VHD format will now be available to anybody wanting to use it under Microsoft’s Open Specification Promise, which the Redmond, Wash.-based software maker issued on its Interoperability Web page Sept. 12, when the company promised not to take any legal action against developers or customers who use any of 35 Web Service specifications.

Microsoft’s VHD format, which has been available since May 2005, captures the entire virtual machine operating system and the application stack in a single file.

“With the VHD format as a freely available specification, Microsoft is contributing to the continued expansion of the virtualization market by fostering interoperability among all commercial software solutions, including open source, Tom Robertson, the general manager for interoperability and standards at Microsoft, told eWEEK.

Click here to read more about how Microsoft promised not to sue developers over 35 Web Service specifications.

While the VHD format specification was previously available to anyone under a royalty-free license, the fact that users had to sign that agreement was seen as an impediment to its adoption, particularly by those in the open-source community, Jean Paoli, the general manager for interoperability and XML architecture at Microsoft, told eWEEK.

“By making the VHD specification available under the OSP, we are removing that adoption obstacle, and the format can be used in a commercial or noncommercial context, including in an open-source environment,” Paoli said.

But Microsoft’s VHD format is not the only one available. VMware released its core virtual machine format and specification license-free earlier this year. The technology lets customers manage, patch, update and back up virtual environments.

Dan Chu, VMware’s senior director of developer and ISV products, said at the time that the move was another step in the company’s push to create a larger ecosystem around virtualization.

Patrick O’Rourke, a senior product manager in Microsoft’s Windows Server division, told eWEEK Oct. 16 that the company’s goal was to make the benefits of virtualization available and usable to a broad audience.

“This means a virtualization solution that is inexpensive, easy to implement, manageable and secure. In addition to Microsoft’s VHD and VMware’s VMDK [Virtual Machine Disk Format], there is also a Xen format, and XenSource has licensed our VHD format,” he said.

There were tools in the market that converted VMDK-based virtual machines to VHD-based virtual machines, including Microsoft’s Virtual Server Migration Toolkit, Platespin’s PowerSDK and PowerConvert and LeoStream’s P>V Direct 3.0. The Platespin and Leostream applications also convert VHD-based virtual machines to VMDK-based ones, O’Rourke said.

“Further, Microsoft is already working with the industry in this area via the Virtual Server APIs and the VHD format. Windows customers and partners realize the value of standardizing on the Microsoft VHD format, as it is the Microsoft virtualization file format and offers migration across Virtual Server, Virtual PC, and the future Windows hypervisor,” he said.

To read more about Microsoft’s hypervisor technology, click here.

This latest interoperability move is one of five that Microsoft has announced over the past four months and follows customer feedback that interoperability is as important to the software giant as security and reliability, Robertson said.

“We know that heterogeneous IT systems are a reality, but how this all works together is everyone’s issue,” he said.

“Our goal is to provide interoperability by design, and we do that in four ways: making sure our products are interoperable out of the box; by working and collaborating with the community on ways to build bridges between technologies; by providing access to our technologies; and standardization, working in standards bodies to develop interoperable solutions and then deploy those,” Robertson said.

Next Page: Reaching out to the open-source community.

Microsoft has recently been reaching out to the open-source community to try to find ways to overcome the incompatibilities between software distributed under the GNU GPL (General Public License) and its own commercial software.

Can Windows and open source learn to play nice? Click here to read more.

Over the past four months Microsoft had announced a number of key interoperability activities focused on business and technical activities, including the establishment of an Interoperability Customer Executive Council, the Open XML Translator Project, and the strategic relationship with XenSource for the development of technology to provide interoperability between Xen-enabled Linux and Windows Server virtualization.

The decision to make the VHD format available under the OSP was welcomed by other players in the virtualization industry, including Virtual Iron Software, a company that chief marketing officer Mike Grandinetti says is committed to enabling the benefits of dynamic infrastructure and policy-based management, regardless of which virtualization technology a customer selects.

“By adopting Microsoft’s VHD format, Virtual Iron is leveraging a rapidly emerging industry standard. Our Virtual Infrastructure Management Platform, which supports advanced capabilities like the transparent migration of virtual servers between physical servers with zero downtime, will provide users with the ability to manage virtual machines in the VHD format,” he said.

Jens-Peter Seick, a vice president at Fujitsu Siemens Computers, said that “VHD will certainly set an important new standard in the area of server virtualization.”

The VHD file format has been adopted by more than 60 vendors, including Brocade Communications Systems, BMC Software, Diskeeper and Network Appliance, Robertson said.

Check out’s for Microsoft and Windows news, views and analysis.