Linux kernel update provides open source virtualization users with a choice

Both KVM and Xen hypervisors are now included in every Linux kernel from 2.6.39 onward

During the week of Citrix Synergy, much of the Xen community was looking to the news coming out of the show around what Citrix and its ecosystem of partners were doing within the XenServer, XenDesktop, and cloud computing markets. However, there was an interesting event that may have been overlooked by the Xen community during that time. With news coming out about Linux 3.0, it was also announced that all key Xen code had been finally accepted into the Linux mainline kernel after almost eight years since Xen's first release.

For years, Oracle, Citrix, and other members of the community have been undertaking numerous efforts to fully integrate the various components of the open source Xen technology into the Linux mainline kernel so that each of the Linux distribution vendors could fully support and enable Xen in their solutions without requiring additional work and effort on their part or that of their users. It is that struggle that seems to have finally reached a conclusion.

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According to the latest news, the Linux mainline tree (2.6.39+) now contains literally every component needed for Linux to run both as a management domain kernel (Dom0) and as a guest (DomU).

Oracle's senior vice president of Linux and virtualization engineering, Wim Coekaerts explained things in a bit more detail within a recent Oracle blog post:

Xen has always used Linux as the management OS (Dom0) on top of the hypervisor itself, to do the device management and control of the virtual machines running on top of Xen. And for many years, next to the hypervisor, there was a substantial linux kernel patch that had to be applied on top of a linux kernel to transform into this "Dom0". This code had to constantly be kept in sync with the progress Linux itself was making and as such caused a substantial amount of extra work that had to be done.

The Xen team has been working hard for years to get everything together that they needed to run the Linux kernel as Dom0. Some of these components were added last year to handle memory management, grant table stuff, network pv driver and block pv driver back-end code, and now the final elements have been added to handle everything else.

Another addition that will make the mainline Linux kernel more Xen friendly is something called pvops. According to Coekaerts, pvops is a mode that will enable the kernel to switch between paravirtualization (pv), hardware virtualization (hvm), or paravirtual-hardware virtualization (pv-hvm) at boot time. Instead of having multiple kernel binaries, he explains, "There is just one and it will lay out its operations at boot time when it detects on what platform it runs."

Simon Crosby, CTO for the data center and cloud division at Citrix Systems, added another perspective to this announcement:

In practical terms, this means that every Linux kernel from 2.6.39 onwards, will contain every piece of code to seamlessly take the role of guest, host (in the case of KVM) or Domain 0 (in the case of Xen). For the key consumers of Xen, including open source powered clouds such as AWS, this achievement permits easy adoption of the latest and greatest Linux kernels, their drivers and other key architectural innovations, both to power the virtual infrastructure and as guest VMs. Whereas historically this required real engineering and code, this is no longer the case - it just runs.

So now you can use KVM as a component of your Linux distro, or you can use your Linux distro to run Dom0 (platform services) for Xen. This decoupling is unique to Linux, and to my mind gives Linux and the open source community the ability to support a far richer set of virtualized infrastructure abstractions.

One of the questions worth asking is if this Linux mainline win after eight years is too little, too late? Or is it just what the doctor ordered to allow the Xen community to further respond to the challenges that have been put forth by its open source virtualization rival Kernel-based Virtual Machine (KVM)?

The latest move does in theory put Xen back on a level playing field with the KVM hypervisor on at least one point. KVM has been enjoying the fact that it has been included in the Linux kernel going all the way back to 2007 with version 2.6.20; as you might expect, it has leveraged that fact as a key differentiator throughout its marketing material and competitive analysis against Xen and other hypervisor platforms.

While the community has been working diligently to package and update these patches to appease those in control of the Linux kernel mainline, it has received increased competition from KVM, which has had time to improve its core technology and scalability, and increase its number of enterprise-ready features. In addition to being championed by Red Hat and Ubuntu, KVM is also finding a nice home within the more modern and growing area of cloud computing with large cloud hosting providers such as IBM and The Planet.

KVM is also receiving additional support with the recently announced Open Virtualization Alliance, a consortium of software and hardware vendors such as Red Hat, HP, IBM, and Intel, who are intent on educating the market and encouraging organizations to adopt the KVM hypervisor technology and the ecosystem of technologies being developed around it.

But Xen is still the more mature platform and is currently ahead of KVM in terms of market deployment and use. The Xen hypervisor is also the foundation for many public and private cloud architectures currently in operation, as well as many of those being planned.

Xen continues to be a prominent part of the Citrix cloud story as well. During Citrix Synergy, the company announced its plans to launch Project Olympus by the end of the year, a commercialized version of the OpenStack open source cloud solution backed by partners such as Rackspace and Dell.

Both of these open source virtualization technologies are going to have an uphill battle when it comes to going against companies like the current virtualization market leader VMware and the Hyper-V creator Microsoft. But with both technologies now part of the Linux kernel and both technologies becoming a foundation for the growing cloud computing movement, it may be customer demand for openness in the cloud stack that helps them fight the existing powers that be.

At the end of the day, Simon Crosby puts it best when he writes: "Pick your preferred way to consume virtualization -- as an infrastructure abstraction independent of any OS (Xen), or as a component of your Linux distro (KVM)," and then adding "Ultimately the Linux and Xen communities have done the right thing for developers, customers and the market in general, permitting choice."

This article, "Linux kernel update provides open source virtualization users with a choice," was originally published at Follow the latest developments in virtualization at For the latest business technology news, follow on Twitter.

Copyright © 2011 IDG Communications, Inc.

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