"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 should not 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 favor 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 CTO 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 nice idea--"Xen doesn't
have a try-out mode"--but it's 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 virtualization 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 at the end of this year. Microsoft has been lagging rivals VMware and Xen in virtualization, which has given Linux something of an edge. Under that competitive pressure, Microsoft has linked up with XenSource to make sure Viridian can run versions of Linux that have been adapted for Xen.
Xen is more mature than KVM, Crosby argued, "not completely done, but the huge lift work to make a product that competes with VMware--that's done." KVM, he said, is in a more immature state than Xen was three years ago.
Holger Dyroff, vice president of product management for Novell's Suse Linux, estimates KVM will take one to two years to mature. Features he'd like to see are support for 64-bit virtual machines and for multiprocessor or multicore virtual machines, he said.
Qumranet's Kivity has a list of improvements under development, though, such as multiprocessor guest support and live migration that lets a running virtual machine be moved from one physical server to another. "We expect to have live migration working within a few days," Kivity said.
Another complication is that KVM could distract attention from Xen, which only made its commercial debut in Novell's Suse Linux Enterprise Server last July and is slated to arrive in Red Hat Enterprise Linux 5 with that product's March release.
Rusty Russell, a high-ranking Linux kernel developer who has mediated between Xen and VMware needs, and who wrote a hypervisor called lguest, doesn't see KVM as new competition for scarce developer resources.
"The main developers working on KVM are different from the ones working on Xen," he said. "KVM got people like legendary kernel developer Ingo Molnar excited about virtualization where he wasn't interested before, so it's looking like everyone's getting bigger slices of a growing pie."
But building, supporting and certifying a product imposes new constraints. While Novell is interested in KVM, it has no intention of supporting more than two virtualization technologies, Dyroff said. One of those technologies would be at a sub-operating system layer--as Xen is--and the other would compartmentalize a single operating system, as OpenVZ does.
Red Hat is warmer on KVM, but Stevens also is concerned about devoting resources to both Xen and the competitor. For example, the company is contemplating splitting the upcoming Fedora 7 Linux for enthusiasts into two versions, one with Xen and the other with KVM, Stevens said. That's because the company likes Fedora to track the mainstream Linux kernel, which now includes KVM. However, Xen uses an earlier kernel that doesn't have KVM built in.
Ironically, these complications could mean that the major beneficiary of KVM's open-source success, for now, could be its proprietary rival VMware, Haff said. "VMware will have to be loving it."
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