Microsoft has released a white paper clarifying how licensing for its current version of Windows Server works when paired with virtualisation software.
But users may face a completely new set of licensing rules when the next version, Windows Server 2008, is released later this year.
In a white paper, Licensing Microsoft Windows Server 2003 R2 to Run with Virtualization Technologies, Microsoft outlines clearly how to license the current version of the software - Windows Server 2003 R2 - for specific third-party virtualisation technologies, including VMware ESX, VMware Vmotion and SWsoft Virtuozzo. It also explains licensing for Microsoft's own System Center Virtual Machine Manager.
The document can be found on a web page that also includes previously released virtualisation calculators to help users determine the cost of Windows Server licensing in various virtualisation scenarios.
Virtualisation has complicated server operating system pricing because it allows more than one instance of server software to run on a single server. Traditional operating system pricing has been per server, assuming that only one version of an operating system can run on one piece of hardware. Virtualisation allows software to be emulated via a virtual machine, and so run without having to be physically installed.
But Windows Server 2008 will have virtualisation technology built-in, which could preclude the need for third-party virtualisation software.
"We believe most customers will choose to use Windows Server virtualisation, which comes as a role within Windows Server 2008," a Microsoft spokesperson said. "But customers still have the option to use third-party server virtualization software on Windows Server 2008."
According to the Microsoft white paper, Windows Server 2003 R2 Standard Edition (SE) allows users to run only one instance of the software in either a physical or virtual way on a server. Users need to assign an SE licence for each running instance of Windows Server on a system.
Users have more flexibility with the enterprise and datacentre editions of Windows Server 2003, however. They can run one physical instance of Windows Server and up to four simultaneous virtual instances with one Windows Server 2003 R2 Enterprise Edition licence, and can run unlimited physical or virtual versions of the software when Windows Server 2003 R2 Datacentre Edition is licensed for every physical processor in a server