KVM and Xen cofounders engage in war of words

Xen cofounder and project lead Ian Pratt said predictions about his virtualization hypervisor’s demise to the newer kernel-based virtual machine (KVM) technology are unfounded, a by-product of his competitors’ imaginations. “There’s no evidence for it.

Xen cofounder and project lead Ian Pratt said predictions about his virtualization hypervisor’s demise to the newer kernel-based virtual machine (KVM) technology are unfounded, a by-product of his competitors’ imaginations.

Ian Pratt

“There’s no evidence for it. The Xen community is alive and well. Xen is a true hypervisor architecture that’s better for scalability, security and availability,” said Pratt, chairman of Xen.org, a leading open source virtualization hypervisor project that is backed by IBM, Red Hat, Novell, Intel, Sun, Oracle, HP and Dell. Citrix's Xen-based XenServer began shipping factory integrated on Dell PowerEdge servers today.

“KVM is not a true hypervisor. It tries to add virtualization capabilities to the Linux kernel but it’s not a true hypervisor approach. It’s good for something like desktop Linux and a convenient way of doing simple virtualization but it’s not really a high end hypervisor,” Pratt said in an interview with this blogger. “Having true hypervisor architecture is clearly important on the server side.”

Them’s fightin’ words?

Maybe, but Pratt was responding to his KVM’s competitors' claims that Xen’s days are numbered because of KVM’s tight integration with the Linux kernel.

The war is heating up as KVM-based products begin hitting the market. The recently released Ubuntu 8.04 Linux distribution incorporates KVM and Qumranet – a KVM supporter that has been in development with its desktop product for more than a year – recently made available its SolidICE product and SPICE remote display protocol.  Xen.org sponsor Citrix ships XenServer now and will begin shipping SolidICE rival XenDesktop later this month.

“If Xen will die or not die, I don’t know. But KVM will take over and be the virtualization selection of choice,” said Benny Schnaider, CEO and co-founder  of Qumranet, whose first KVM-based virtualized desktop solution called SolidICE was recently released. 

Benny Schnaider

In an interview with Qumranet recently, Schnaider acknowledged that KVM is at its first stage of development but is being enhanced and will eventually be ideal for server, desktop, embedded and real time virtualization scenarios. The advantage KVM has over Xen is that it is integrated with the Linux kernel’s scheduler, which allows for sub 15 second switchover between guests and performance metrics that Xen cannot match, Schnaider claims.

“XenSource can’t do it because the scheduler they’re using is different and is independent of the Linux kernel,” said Schnaider, a former Cisco exec. “In the case of desktop virtualization, user response time is very important. And when you’re running and loading a server with 50 virtual machines, interactivity is very important."

Linux kernel developer and Linux Foundation Fellow Ted Ts’o claimed it’s inevitable that Red Hat and Novell will standardize on KVM because of its inclusion in the kernel. He said the Xen project was working to integrate with the kernel but enever finished the effort.

The Xen organization attempted to integrate its code with the Linux kernel but ran into some technical problems that would have compromised the performance of the hypervisor, Pratt said. He said integrating it with the kernel would have meant sacrificing key features of its platform. “You need to hold to your principles,” Pratt said.

He also said predictions about Xen’s demise by Linux kernel developers are inherently biased. “They have work going on with KVM and many folks see it as competitive to Xen and are more prone to be outspoken against Xen.”

Pratt also rejected out of hand Qumranet’s suggestions that open source developers who have worked on the Xen project for some time are not too thrilled with Citrix’s purchase of XenSource – the offspring of the Xen.org project – or the way Citrix is treating outsiders these days.

“I certainly haven’t been hearing grumblings from the community,” said Pratt, noting that open source projects routinely get complaints from developers whose patches don’t make it into the code but that’s business as usual.

Under Citrix’s umbrella, the Xen.org project now has a full time project manager and board that manages contributions to the Xen project. Pratt also said the organization conducts weekly or “fortnightly” (bi weekly) calls with major distributors and the Xen community is now “stronger” than ever. Pratt, who is also a lecturer at Cambridge University in England, where Xen was conceived, maintains there's plenty of room for two open source virtualization choices.