Citrix Open Sources Their VHD Implementation

One sticking point I’ve had with Citrix since the XenSource acquisition has been the perception of a failure to give back to the open source Xen community. Note that I said perception. It appeared, following the XenSource acquisition, that Citrix was all about using open source Xen as a base but failing to return any of enhancements they made to the code base. No, I don’t have any concrete examples; again, this was the perception.

It appears that Citrix is now taking steps to remedy that perception. In a blog entry posted last night, Simon Crosby announced that Citrix has open sourced their optimized VHD support. This means that XenServer’s robust VHD implementation is now available to any developer under the BSD license. In case you don’t already know, VHD is the same virtual disk technology Microsoft uses in Hyper-V, and which Microsoft is using even more extensively in future versions of Windows.

In my opinion, this is an excellent move. It addresses the perception of failing to give back to the open source community, and it puts what appears to be a valuable piece of technology into the open source world. Making XenServer’s VHD implementation available for other open source developers to use in their projects puts VHD on the fast track to being the de facto virtual disk standard. Assuming that other virtualization platforms adopt VHD support—and I’m not sure why many of them wouldn’t adopt VHD support, except for VMware—we’ve now removed a huge barrier to interoperability. That’s a good thing.

Not being a lawyer, I’m a bit worried about the compatibility of the BSD license—which is generally regarded as quite generous—and the Microsoft Open Specification Promise, but I’ll leave that for others to hash out.

It will be interesting to see if Citrix also open sources some of their other XenServer-related technologies. Time will tell…

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  1. Simon Crosby, CTO, VMD, Citrix’s avatar


    The Microsoft Open Specification Promise specifically permits implementation of the technologies in open source.

    Of course in terms of Xen, our continued contribution to the open sourcing of all core component technologies in the hypervisor remains unwavering. This is just as true on the client, where the Xen Client Intiative, and our own project Independence are, with the community, driving toward the delivery of an industry standard reference open source client hypervisor with broad ISV and IHV support.

    The reason that Xen is so much bigger than Citrix XenServer, or any other implementation is that it is an engine, not a car. If we built a complete car in open source, it would certainly not fit everyone’s needs. The implementation of Xen in (say) EC2 is quite different in the surrounding infrastructure from the implementation of Xen in Linux, or indeed XenServer. That’s the whole point – keep the community focussed just on the engine, and allow anyone to deliver that engine in whatever chassis they think is appropriate to their environment. The next Xen developer Summit will be held at Oracle, in the bay area, next week. I expect we’ll see the same rich set of organizations (about 50) and developers who have made Xen such a great continued success story.


  2. slowe’s avatar


    If indeed Citrix is committed to open sourcing “all core component technologies,” then I look forward to your future open source announcements.

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