Mendel Rosenblum, Stanford University operating system researcher and chief scientist at VMware, is an approachable, brainy uncle kind of figure. For example, he pauses to think about a question instead of just automatically answering it.

When we sat down together at VMware's VMworld user group meeting in Los Angeles a few weeks ago, I asked him where virtualization's impact will fall most heavily on the data center.

He paused to think and said, the advent of virtualization's hypervisor "means a big change in the role of the operating system."

"Until now, the operating system has been the only piece of software that's been viewed as an extension of the hardware," he said. On most computers, the operating system is the software given the right to talk directly to the hardware resources and that means it had to be a general purpose system--all things to all applications.

"That leads to problems in security, reliability, manageability and performance," says Rosenblum.

Now the operating systems role is being taken over by the hypervisor, which is a kind of kernel of an operating system that talks to the hardware and arbitrates between many demands of running virtual machines.

If the hypervisor speaks to the hardware, the operating system is freed up to re-orient itself. It can switch from being geared to run many applications to being optimized for just one. An operating system/application combination that's been converted into a virtual machine file format is what's known as a virtual appliance.

"The virtual appliance operating system is much smaller and more application specific. It's much better tuned and much less complex," he said. Some security loopholes are closed off. Needless parts are chucked out.

And because hypervisors speak a kind of Esperanto to the rest of the software on a server, the operating system used by each virtual machine can be different and still get what it needs from the hardware. A hypervisor can provide hardware services to one application that runs best with Windows and another that runs best with Linux.

It doesn't necessarily sound like a big change. But Rosenblum understands how it will ripple through the data center. Software can be moved off one piece of hardware and onto another without going through the programmer pain of being tweaked and rewritten for new hardware. Instead of needing to be operating system experts, they need to be virtual machine experts.

"Somebody else builds your software infrastructure. You just drop it in," said Rosenblum.