In a white paper published Friday, the said Microsoft is using Windows licensing terms to prevent customers from taking full advantage of virtualization.
"Microsoft is trying to restrict customers' flexibility and freedom to choose virtualization software by limiting who can run their software and how they can run it," VMware said.
The software giant rejected VMware's assertions.
"Microsoft believes the claims made in VMware's white paper contain several inaccuracies and misunderstandings of our current license and use policies, our support policy and our commitment to technology collaboration," Mike Neil, Microsoft's general manager of virtualization strategy, said in a statement Tuesday. "We believe that we are being progressive and fair with our existing licensing and use policies and creating a level playing field for partners and customers," he said
Neil added that he believes Microsoft and VMware will be able to settle their differences.
Virtualization lets a single computer run multiple operating systems simultaneously, in compartments called virtual machines. That sharing lets companies use their computers more efficiently. The ultimate promise of virtualization is broader than that, however: by moving virtual machines from one server to another, companies can create data centers that can automatically adjust to shifting work demands or hardware outages.
The Microsoft-VMware debate spotlights the difficult adjustments necessary, as the industry moves into a world where a new software layer breaks the hard link between operating systems and the hardware they use.
There are technical challenges. Software can't be as tightly tied to hardware such as hard drives or network cards, and an operating system might be moved from an overtaxed dual-processor server at one moment to a more powerful eight-processor machine the next.
And there are business challenges too, as companies such as Microsoft and VMware jockey for technology leadership and customer account control.
It's easy to envision virtualization technology as part of the operating system, but VMware has made a strong business selling it as a separate component. Right now, it is moving to deliver higher-level management features that take on products from Microsoft and others.
But Microsoft is building virtualization into the next version of Windows Server, code-named, and that could be a minefield for the software company. It has already faced antitrust challenges by building its Internet Explorer browser and Windows Media Player software into the operating system.
VMware stopped short of accusing Microsoft of violating antitrust law--which prohibits a company with a monopoly in one market from using that power to extend into another market--but it came close.
"Microsoft is leveraging its ownership of the market-leading operating system and numerous applications that are market leaders in their respective categories...to drive customers to use Microsoft virtualization products," VMware said. Microsoft needs make sure it's possible for Windows and Windows-based software "to be created, licensed, supported and distributed equivalently on Microsoft or non-Microsoft system virtualization stacks."
VMware leveled several charges at Microsoft:
Microsoft sharedinterface details at a hardware conference but isn't permitting other virtualization companies to use them. The exception is .
"Microsoft licensing policies ask for permanent assignment of operating system licenses to hardware and then restrict the movement of those operating system licenses, even for virtualized environments that can be moved seamlessly from machine to machine," VMware said.
In particular, Microsoft permits Windows Server licenses to be moved only once every 90 days, VMware said. That breaks VMware's VMotion technology (which moves running virtual machines from one computer to another) and the higher-level Distributed Resource Manager (which balances software work with available hardware by moving virtual machines with VMotion).
Virtual machines stored in Microsoft's VHD format may not be converted into other virtual machine formats, including VMware's.
Microsoft provides support for virtualization only to customers with premier support contracts.
Neil also defended Microsoft's position in a statement and in a blog entry.
"In the end, customers with mixed environments expect it all to work together," Neil said. For that reason, Microsoft is sharing interface details for its current Virtual Server 2005 product and has begun sharing preliminary details with its upcoming Windows Server virtualization technology, code-named Viridian, he said.
Mac users have been in for a tough haul with Windows Vista's virtualization policies. While Intel-based Macs have made it technically reasonable to run Windows through virtualization, and Parallels' software makes it relatively easy, Microsoft has made it expensive to do so legally. Running Vista on a Mac through virtualization requires a full, not an upgrade, version of Windows. Plus, for it, and not the far less pricy Home Basic or Home Premium varieties.
Neil, though, argued that most consumers just aren't ready for virtual machines.
"Virtualization is a new technology for consumers, and one that isn't mature enough yet from a security perspective for broad consumer adoption," he wrote. "Our security and data protection features can potentially be subverted by a malicious virtualization layer. We're working with the hardware and software industry to improve the security of virtualization technologies and we will evolve our licensing policies as virtualization becomes more widely used on client systems."