KVM virtualisation gets real momentum

…of the operating system. In the second VMware approach, used in the higher-end ESX Server product, a full-featured, heavyweight hypervisor governs access to underlying hardware.

Unlike Xen additions to Linux, the KVM patch slipped nearly instantly into the mainstream kernel maintained by Torvalds and a group of deputies.

"We did things the Linux way," Kivity said in an interview. "I am a long-time lurker on the Linux kernel mailing list, so I know what's important to the kernel maintainers and tried to get things right the first time. Where I got things wrong, I fixed them quickly."

He introduced KVM with source code, not words. "Kernel maintainers only take you seriously if the first word in a message is 'Patch'," Kivity said.

Torvalds, who accepted the first KVM patches in December, said the technology's lack of intrusiveness and complications led to its inclusion.

"One reason KVM was so easy to merge was that it was really fairly straightforward, from the kernel's point of view," Torvalds said. And KVM programmers were easier to deal with than Xen programmers, he added: "I think they just had a lot less politics, and very few general policy issues."

Social factors also shouldn't be discounted. KVM puts the Linux kernel squarely in the centre of the universe, whereas Xen shifts a lot of the brains of the computing operation to the separate hypervisor, so it's no surprise kernel engineers would be more interested in KVM.

Social dynamics may sound secondary to technical details, but in open-source programming they're closely related, Red Hat's Stevens said.

"I'm not separating the two. Technology... has to be done in a way that allows the community to build around it. KVM picked a technical approach that was clean and simple and easy to understand", and the programming interest followed, he added. Among those interested is Ingo Molnar, a top Red Hat programmer who has been improving KVM performance.

The importance of tight integration with the Linux kernel shouldn't be discounted, Stevens said.

"It's a more natural way to manage a community. We continue to bear the burden of merging Xen with the latest kernel. It's really expensive," Stevens said. "The developers are doing that work again and again and again — it takes weeks. They're always behind the latest kernel. That's what exciting about KVM: that work just goes away. Anything that doesn't check with Linux will be bounced or fixed right away."

For Qumranet, kernel integration means a lot of work is done for them, too. "Being part of the Linux kernel, KVM uses existing kernel components — for example, scheduler and memory manager — and saves overall programming resources, thus avoiding duplication of efforts for the open-source community," Schnaider said.

Xen programmers had originally planned to integrate their software with the Linux kernel, but have since backed away from that approach in favour of adding a hypervisor interface called paravirt-ops. That approach permits Linux to deal with other hypervisors, including VMware's.

"Xen is never going to be in the kernel, because it's not a kernel component," XenSource chief technology officer Simon Crosby said. "But the interface between Xen and the kernel, paravirt-ops — that's going in." The first components are expected in the upcoming 2.6.21 kernel, he added.

Bumps in the KVM road
For Crosby, KVM is a great idea — "Xen doesn't have a try-out mode" — but it has arrived late.

VMware did start off with a hosted model, but it now has moved to a true hypervisor. That's the preferred evolutionary direction, Crosby argued. Virtual machines are handy for developers who want to test new software in safe partitions, but hypervisors offer better performance, have security advantages, and juggle the competing needs of multiple virtual machines better, he said.

VMware has a similar belief in that evolutionary direction. Its higher-end and hypervisor-based ESX Server is the foundation of its Virtual Infrastructure software, which monitors a group of servers running virtual machines and shifts work from one to another according to preset rules.

"A hosted architecture works great but has not delivered what we call Virtual Infrastructure," said Raghu Raghuram, VMware's vice president of product and solutions marketing. "In order to do that, you need the separate hypervisor layer." However, KVM is both viable and helpful, he added.

Microsoft's next virtualisation technology, Viridian, is based on the notion of a hypervisor, while its existing Virtual Server is not. The Viridian technology is likely to arrive in a 2008 service pack for Windows "Longhorn" Server, an update to the operating system that is set for release…