Cloud service providers weigh Xen-to-KVM move

More cloud service providers are considering ditching Xen in favor of KVM, but the complicated Xen-to-KVM migration process has many in wait-and-see mode.

Cloud service providers are showing increased interest in the KVM hypervisor, given Xen's ongoing struggles to keep up with the latest versions of Linux.

Most service providers continue to run Xen which, as a bare-metal hypervisor lends itself to virtual private server deployments, and has a longer track record in the market, but the kernel-based virtual machine (KVM) hypervisor is on their radar -- and KVM support from heavyweights IBM and Red Hat doesn't hurt.

 With KVM, you just stick the CD in and it works.
Carl Meadows, The Planet,

Linode, a 15-employee company that operates five data centers between the U.S. and the U.K., has come to rely on automation built in-house around the open source Xen project. However, recent updates made to open source Xen -- to keep it current with the latest Linux kernel -- have not tested well in Linode's environment, said Christopher Aker, founder and CEO of the Absecon, N.J.-based company.

"The first few machines worked, but you get 20 to 30 machines in and it's a different story," he said. "I don't trust the initial build. We have to let things cook."

Linode already shifted platforms once, from user-mode Linux to Xen, and has kept Xen stable "just with elbow grease and trial and error," Aker said.

"It's premature to say let's bet the farm on KVM," Aker said. "I think Xen is going to reach a critical point where it moves off its old domain zero, or it will obsolete itself. If that happens, KVM would be a likely substitute out of necessity, but Linode has paid its [platform migration] dues. We're going to let someone else pay those dues this time."

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The Planet, a Houston-based cloud service provider, based its first public cloud service on an Ubuntu version of KVM after testing a number of hypervisors. The company has tested more than 1,000 servers running on KVM in beta, said Carl Meadows, senior product manager for cloud services.

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"We're seeing service providers that were Xen-open-source-based … [and] are rapidly having to shift over to [Citrix Systems'] XenServer," he said.

Rackspace, the second largest cloud provider after Amazon Web Services (AWS), said in May it will switch from XenSource to Citrix XenServer for better support. AWS still runs on Xen.

"Before long, it'll basically just be Amazon [on Xen]," Meadows said. "A lot of the open source guys are making the choice to either make the move to KVM or pony up the money and buy Citrix XenServer."

The Planet did quite a bit of testing on hypervisors, including Xen, XenServer, Oracle VM, and Parallels, Meadows said. Its managed hosting business also uses Hyper-V and VMware.

"What we found was basically that the Xen open source project was losing all momentum, and there's a lot of good reasons that it is as far as the maintainability of Xen … because it exists outside the kernel," Meadows said.

Because Xen is not part of the Linux kernel, patches and other operating system updates have to be re-coded to work with Xen, Meadows said. More recent versions of Xen claim to support unmodified guest OSes, but "it hasn't been seamless, as we've found in our testing," he said. "You've seen from the fact that a lot of Xen hosters don't offer Windows, and when they do, it's spotty. With KVM, pretty much any operating system we want to work with, you just stick the CD in and it works."

The devil you know
Other service providers remain reluctant to move to something relatively unproven and unknown in the market.

"I like KVM for desktop-type virtualization," said Matt Brown, senior systems engineer for an application service provider, via e-mail. "I don't know if I can see that taking off for pure production server use. … Xen is more of a well-thought-out, production-grade virtualized environment."

Practical day- to-day administration issues also have Brown shying away from KVM for production use.

"KVM is not the easiest thing to do if you don't know Linux," he said.

A shift in the cloud from Xen to KVM would mirror trends in the wider Linux community over the last few months, but like enterprise users at last month's Red Hat Summit in Boston, Meadows said management tools for KVM leave something to be desired.

"It's starting to flesh out now as more of the guys that had Xen-based platforms are starting to support KVM," he said. "But six or nine months ago, when we were first building out our stack, we ended up having to do a lot of build-it-yourself, because the management stacks weren't ready for prime time yet."

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