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 virtualisation 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 virtualisation technologies, Dyroff said. One of those technologies would be at a sub-operating system layer -- as Xen is -- and the other would compartmentalise 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.
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