Adoption of virtual appliances could be a threat to sales of operating systems from the likes of Microsoft, a co-founder of VMware claimed.

Mendel Rosenblum told an audience at LinuxWorld in San Francisco yesterday that a virtual appliance, which bundles applications with only the OS code needed to make it run, can then be deployed on a virtualised server regardless of the host OS. Rosenblum is also chief scientist of virtualisation at VMware.

Asked if he thinks the virtual appliance model is a threat to OS vendors, principally Microsoft, Rosenblum said, "If I were in their shoes ... I think the answer is yes."

The virtual appliance bundle is similar to the software appliance, but is specifically designed to run on a virtualised server.

A virtualisation "hypervisor," like VMware's, separates the server hardware from the OS and the rest of the software stack. The hypervisor creates multiple logical servers on one physical server, allowing it to run multiple applications and use more of the server's capacity.

Rosenblum sees the virtual appliance as an alternative to making applications fit ever larger and more complex operating systems, simplifying the software writing process by taking out parts of the OS not needed to run that application, or even creating a customised OS.

"What I consider predictive [of the threat] is their complexity," he said. "They've built this OS to the point that they basically have no choice but to actually go and put in a hypervisor. All the OS vendors want to own the hardware connection, but I see there is going to be a big battle over who gets control over the hardware."

The open-source Linux OS is "well positioned to be the OS of choice" for a virtual appliance, said Rosenblum, because its licensing model is very friendly to this type of use. A software developer can bundle the software with parts of Linux easily, while commercial OS vendors like Microsoft would "look askance" at something like that.

"VMware feels at home in the Linux community," said Rosenblum in his keynote address, a remark that one listener described later as "gratuitous."

Mike Grandinetti, chief marketing officer for Virtual Iron, a small virtualisation software vendor, says 90 percent of VMware's virtualisation runs on Microsoft Windows machines, not Linux. He described Rosenblum's address honouring Linux as "a revisionist view of history."

Virtual Iron's software products are based partly on the open-source Xen kernel for virtualisation, which makes them far less expensive than VMware's products, Grandinetti said. XenSource is another Xen-based virtualisation vendor.

Although VMware is the dominant player in the virtualisation space, there is still a lot of market to pursue. Only about 2- to 3-percent of servers industry-wide are virtualised, less than the 5- to 7-percent range forecast by major industry analysts, he said.

"We're just getting started," Grandinetti said.

Rosenblum was a last-minute substitute for VMware president Diane Green, who is currently visiting with investment analysts discussing VMware's pending initial public offering (IPO) of stock. The stock is expected to begin selling on 14 August, though Rosenblum has to keep quiet about it.

"We're excited and hopeful," was all he could say.