Microsoft has confirmed that it is to eliminate a licensing restriction that prevented customers from moving virtualised applications to a different server more than once every 90 days.

The company also said it would begin providing new levels of technical support for applications running on virtual servers, but not those running on VMware's hypervisor.

Server virtualisation technologies allow virtual machines to be swapped from one physical server to another without disrupting end users. But under current Microsoft policies, customers must reassign a software licence in order to make such a move, and can only do so once every 90 days.

That 90-day restriction will be removed on 1 September for the most commonly used Microsoft server applications, including SQL Server 2008 Enterprise edition, Exchange Server 2007 Service Pack 1 Standard and Enterprise editions, Dynamics CRM 4.0 Enterprise and Professional editions, Office SharePoint Server 2007, and Microsoft System Center products. In all, 41 server applications are affected.

This move was anticipated by industry observers. Analysts believed the time restriction unnecessarily limited the system flexibility that makes server virtualisation so useful.

Not all server applications are gaining the new, more favorable licensing structure, however. Forrester analyst Christopher Voce said one noticeable absence iwas SQL Server Standard edition. Microsoft is apparently trying to give customers added incentive to upgrade to the more pricy SQL Server Enterprise edition, Voce says.

"This was an evolutionary change to the Microsoft licensing that everyone expected," Voce said. "They introduced the kind of flexibility that you would expect for Microsoft applications in virtual environments. But it's not all roses for certain customers."

Microsoft predicted that many customers will be able to reduce the number of licences they have to purchase, since they won't need licences for every machine a workload might possibly be moved to. "You don't have to have every target machine licensed ahead of time," says Zane Adam, senior director of integrated virtualisation at Microsoft.

With the change, Microsoft server apps running on virtual machines can be moved to any server within a "server farm" as often as customers want without accruing additional licensing fees.

Server farms are defined broadly. A customer, for example, can move a workload from a server in New York to one in Los Angeles without being charged an extra licence fee, Adam says. However, moving a workload from the United States to Europe would require an additional licence.

A 90-day restriction still applies to desktop applications, such as the Microsoft Office suite. Adam says Microsoft is leaving the desktop time limit in place because desktop programs don't require the same level of mobility as do server applications.

In addition to eliminating the 90-day rule for server applications, Microsoft also said it would provide technical support for applications running on several types of hypervisors. Previously, Microsoft did not offer the same level of support for products if they were running on virtual machines rather than physical servers. Now customers using any one of 31 server applications on virtual machines can expect full support.

But there's a catch: the policy applies only to Microsoft's Hyper-V virtualisation software and third-party hypervisors that have been certified under Microsoft's Server Virtualisation Validation Programme, which launched in June. So far, only Cisco, Citrix, Novell, Sun and Virtual Iron have started going through this validation process.

VMware, the x86 virtualisation market's dominant vendor, is not in the programme, Microsoft said, which means this enhanced level of support will not be extended to Microsoft products running on VMware servers. Adam said VMware had been invited into the programme, but had not participated.