The Present And Future of Xen


Over the past few months, a number of people have been vouching the idea that Xen’s development and adoption is slowing down because of the Citrix’ acquisition.

Paula over at ZDNet appears to have misunderstood what XenSource/Citrix explained her.

Citrix might have jumped some marketing hoops by first claiming to be an application delivery company, then trying to position itself next to VMWare as a virtualization company, and subsequently buying XenSource in order to reclaim its application delivery role while trying to benefit from the Xen brand.

But rest assured: they are still heavily backing Xen.

Paula proclaims:

“Open source backers will likely take another look at Xen and re-consider other open source firms embracing virtualization”

Well … it’s true different open source integrators were confused when XenSource decided to take their XenEnterprise 4.0 management GUI Windows-only, leaving the Linux Desktop users in the cold. Yes, some customers were lost, but the open source guys were never interested in a GUI that limited the functionality of an open and free product they were used to work with anyway, and neither were the kernel code contributors. So the question is if they were looking at XenSource in the first place.

At FOSDEM, we asked Ian Pratt how he feels about the evolution of the Xen open source community after the Citrix acquisition and if he thought the contributions from the community were slowing down. An excerpt from our interview:

“We certainly haven’t seen that , if you think about the life of the Xen project, there have been a number of significant changes. When we left the University to set up XenSource people were worried we might go of and take Xen in closed source or something, but we didn’t. It’s the same group of guys, basically myself, Keir Frasier, Steve Hand working on the project, and now many more of course. The Citrix acquisition of XenSource was obviously something we had to explain to people. I think the community has seen that nothing has changed . One of the things that we did do was just to provide greater transparency, set up , the Xen advisory board and the website. The advisory board has members from companies like Intel, AMD, HP, IBM, … big companies that are now contributing to Xen and have oversight from the advisory board, so I think the community is pretty happy and it’s going from strength to strength.”

In a video interview with Tarry Singh at VMworld Europe 2008, Simon Crosby stated:

“You have to understand that Xen is the foundation of the faith, we (Citrix) are first and foremost committed to this community and to that method of development for the server tools and Hypervisor. So one of the community questions was that Citrix was not known for open source and what was gonna happen with Xen.

It turned out, otherwise we would never have agreed to go, that Citrix has thrown a huge amount of money towards the open source community. We’ve setup – we’ve always wanted to do that, as a start-up we could never get there. is run by a charter committee of the major contributors and it has its own program management and it’s independent of us (Citrix) and that’s exactly the way we wanted it. Ian still leads the project, we still probably contribute about 60% of the code, but also all of these major partners deliver to us.

So the community is going from strength to strength, which is terrific.

We had 2 or 3 developer summits each year, at the one we had at the end of last year in Santa Clara we had more than 200 people attend.”

When comparing to, Simon replied:

“The difference is that Eclipse is an independent legal entity. With the Xen community, we discussed whether or not to do that and the cost of it and we decided that we would not do that, so we set up a steering committee which oversees several key components of Xen. First of all, the road map that advises the project on where to go, it sets the policy by which the Xen trademark is used and then that is all administered for the benefit of the community with the explicit admission by the advisory board by Citrix

It leaves us without the cost and the legal infrastructures of setting independent .org but with all principle and all the guiding.”


The Last Xen Summit was a 200-person conference, mainly Xen developers, with people from Sun, HP, Novell, RedHat, Virtual Iron, Oracle, Intel, AMD, Samsung, Solarflare, Google and of course Xensource/Citrix It’s too bad Ohloh doesn’t support Mercurial (the source managemt system) or we could have had real statistics on the Xen contributions but it’s fairly obvious most of the big players are contributing.

Which linux distro didn’t adopt Xen as a virtualization technology ? Xen Adoption in which distro ? True they are supporting other open source technologies apart from Xen and they are working on creating a uniform way to manage different virtualization techniques, but no matter how you look at it.. they all adopted Xen.

So let’s have a look at the companies that are adopting Xen in their products, starting with Citrix.

When XenEnterprise initially launched, Peter Levine told the world that their target audience was to provide easy-to-install (in less than 10 minutes as he could do himself) bare-metal virtualization for the Windows market. They were not planning a RedHat / MySQL style Xen distribution in free and commercial versions with support and updates, they went straight for a target audience that was used to buy proprietary software from a vendor, the Windows users.

Apart from Citrix, which is planning to launch their XenEnterprise 4.1 release in the next couple of months, amongst the first adopters were RedHat and Novell.


When Novell claimed first adoption , RedHat was saying Xen wasn’t stable enough yet. But today, Xen is a core part of both major Linux Distributors offerings. (and with them lots of other Linux distributions) The race however continues when RedHat and Novell started fighting over which version was about to offer better Window guest support :)

Amazon was also a really fast adopter. When Jeff Barr announced the Elastic Compute Cloud Beta, he told the world that they had built EC2 using Xen. In essance, Elastic Compute Cloud, or EC2, allows users to deploy server instances on-demand. Amazon isn’t selling Xen as a product, they are using Xen to provide the world with one of their most used services. Different startups and SMBs are using EC2 as their home on the web.

And let’s have a look at the people selling Xen implementations.

Virtual Iron came from a dubious non-Xen background and is now positioning its platform as an Enterprise-level platform for server virtualization and virtual infrastructure management. Their product consists of a Java-based Virtualization Manager that is used to manage the virtualization services that are deployed on bare-metal servers.

Back in 2007, Simon Crosby wrote:

“The Virtual Iron Hypervisor is not the Xen Hypervisor – it’s a proprietary product (some of which is open-sourced because they use bits & pieces of Xen code). Virtual Iron has not yet made any significant contributions back to the Xen community. Presumably they believe this gives them an edge in the market. Maybe it does. But if that’s the case, I don’t understand why they don’t just stand up and say so, rather than trying to jump on the Xen brand-wagon.”

Simon invited them multiple times to join the Xen community and it seems they accepted the invitation as the Virtual Iron people were even presenting on the last Xen summit. According to Wikipedia, today – as so many others – their platform is based on the Xen Hypervisor. The exact answer is probably somewhere in the source code.


When in November 2007, Oracle first launched their OracleVM there was a lot of fuzz because Oracle claimed both features other people didn’t have as well as better performance. Back then I blogged:

“First, seemingly Larry is claiming that his Xen package is better than others since he supports Live Migration and all the others don’t. I don’t know where he gets that idea.. I have to admit I don’t remember which year it was, but it was somewhere in December when I first started with Live Migration of Xen machines and it was also on a CentOS platform. No fancy GUI, no hardcover manuals that had it all documented. But fast and seamlessly working live migration, ready for everybody to use. Second, he is claiming that Xen was re-engineered by Oracle to be faster than the competition. The way you read it there is that Oracle took Xen, modified it then started redistributing it. Is that really what happened ? Are they redistributing the source, or are they violating the GPL ? Because if they are redistributing the source, everybody just got a faster Xen.”

Earlier, Charles Philips from Oracle had been telling GCN that cite “We’re big proponents of Linux and standard technologies, so we’re going to put the time toward Xen,” Phillips said. “Our strategy will be around Xen.” which didn’t surprise anybody as Oracle had been pushing before to get a single virtualization supportive interface into the Linux kernel.

The openSolaris people started out with building a DomU for Xen, at first, but what they really wanted was to run openSolaris for both their Dom0 and DomU. Today Sun is supporting the sun seemed to be working on Xen for Sparc but it seems they abandoned that effort.

Obviously Sun has is Solaris Zones technology but as running different isolated environments on one kernel is totally different from running different kernels Zones and xVM are obviously complementary technologies.

Toon Vanagt asked Ian Pratt :

“When I look at the Xen GPL License, I found it interesting that Xen is being renamed as xVM by Sun, OracleVM by Oracle. Oracle first announced OracleVM and then had to admit it was actually a tweaked Xen version. But they didn’t publish the tweaked code, did they?”

Ian’s reply:

“Oh no, they have. I mean, the fact is that there are lots of different vendors shipping Xen products that also they ship the Xen engine, pick up the Hypervisor the core engine and incorporate it into their own products. Obviously, the Linux vendors Novell and Redhat, there is Sun, there is obviously Xensource / Citrix , Virtual Iron, etc. Lots of different companies are doing that. Actually the GPL license means that any changes they make will go back into the main project. In reality, pretty much all those companies just pick it up as is. Take the latest stable release, which is maintained, they might add the odd little patch to it, but it really is all very clear, there is all uniformity in the Xen versions that are out there.”

“Most of those companies are very close to mainline Xen. They post a couple of patches in some cases, but not always. What they’ll be doing is taking Xen and it’s really on top of Xen , in the rest of their Virtualization stack that runs in user space, that’s where they’ll be probably doing their own of things, they’ll have their own management tools, they’ll have their own way of wanting to present virtualization to the user, so if you think about what the operating system vendors are typically doing is they want to expose virtualization using the same tools and user interfaces etc. they use for exposing other facilities in that operating system. Which if you think about it, is quite different from what companies like say Xensource is trying to do , which is to try and effectively build a virtual machine hosting appliance. You know, you just put the CD in the server, install it and just manage it from let’s say a windows GUI or a webinterface . So every company is bringing Xen to market in a different way for a different kind of user . And that is where the differentiation happens, but the core engine is the same throughout.”

So which Xen should you choose? One of the main decision points when choosing a Xen vendor is probably whether you want to virtualize Linux, Windows, or a mix. The different vendors have different relationships with Microsoft and will therefore be able to provide different levels of support and integration with their products. But one thing is certain: you have a lot more choice when going for a Xen alternative, than the other way around.

Building and maintaining a community is and will remain a difficult thing. Sun is learning that and Citrix / will also have to learn that. Six months from now, the story might be totally different. But today is growing stronger every day with corporate contributions from all over the planet.

About the author

Kris Buytaert is a long time Linux and Open Source Consultant active in Belgium , Europe and the rest of the universe. He is currently working for Inuits Kris is the Co-Author of Virtualization with Xen ,used to be the maintainer of the openMosix HOWTO and author of different technical publications. He is frequently speaking at, or organizing different international conferences He spends most of his time working on Linux Clustering (both High Availability, Scalability and HPC), Virtualisation and Large Infrastructure Management projects hence trying to build infrastructures that can survive the 10th floor test, better known today as the cloud while actively promoting the devops idea ! His blog titled "Everything is a Freaking DNS Problem" can be found at

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