Server virtualization feels unstoppable these days. The worldwide market for virtualization platform software grew 69% last year to US$1.1 billion, according to market research firm IDC, which predicts average annual growth of 27% through 2011, when global sales are expected to total US$3.5 billion.

But several factors could still stall the technology's growth, analysts say. For instance, software licensing terms often remain too restrictive or expensive for users that want to run their databases or applications on virtualized servers. In addition, finding IT workers who have virtualization experience can be a challenge for companies.

Another factor that has been less recognized thus far is the dearth of formal support for virtualization on the part of application developers and other independent software vendors (ISVs).

When an application that hasn't been certified to run on virtual servers encounters technical issues that prevent it from working properly, a user can be left in a bind if none of the involved parties -- the virtualization or operating system vendors, or the ISV -- is willing to step up and fix the problem.

Such situations can result in "a lot of finger-pointing, which is what the user really wants to get around," said Christopher Voce, an analyst at Forrester Research.

The support issue hasn't come to the fore yet because early virtualization adopters have mostly been large enterprises with sufficient clout to demand personalized help from reluctant software vendors, said IDC analyst Michelle Bailey. "The basic message is, 'Either support me or I'll find some other ISV that will -- or maybe I'll go open-source,'" Bailey said.

But she added that as virtualization software trickles down to smaller and less cutting-edge companies, those users "will be more conservative in their approach and will be looking for compliance [from vendors] upfront."

Although applications generally will run on virtual servers, scalability and management problems persist, according to Dominic Sartorio, president of the Open Solutions Alliance, a consortium of commercial open-source software developers. And when a user deploys an application in a virtual environment, doing so may not invalidate the software vendor's support agreement but could limit it, Sartorio said.

Virtualization market leader VMware claims that more than 100 ISVs officially support their applications on its technology platform. But it also admits that vendors such as SAP AG, which last week announced that it will support its ERP and CRM apps on a variety of Windows and Linux servers running VMware's software, remain a rarity in terms of the breadth of their commitment.

A lack of virtualization know-how is one reason why many ISVs are lagging behind on support, claimed Parag Patel, vice president of alliances at VMware. "The problem is that ISVs didn't expect virtualization to take off the way it has in the past two years," he said.

Virtualization also multiplies the number of hardware combinations that software vendors need to test, adding to the time it takes to certify applications.

Conventional hypervisor-based virtualization "involves everything from the microprocessor to the handling of memory and peripherals," Voce said. "It's why certification from the application vendors is the slowest piece of the puzzle.