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The wait debate

The two camps have debated VMI for months on Linux kernel mailing lists, with other developers from IBM, Red Hat, Oracle and elsewhere chiming in.

"Most of our interactions have been on that level," VMware senior director of research and development Jack Lo said.

VMware has reached out to the OSDL (Open Source Development Labs) and other unnamed parties to help set up a public forum for discussing the interface issue with Xen backers and anyone else who wants to show up.

But, after interviewing a number of people for this story, we get the sense that such a meeting is a concept at best right now. The parties involved proved reticent to nail down a time when they actually plan to sit down and don't seem all that determined to approach a debate until all the behind-the-scenes political posturing has taken place.

VMware has received praise for being so open with VMI and encouraging debate on the kernel mailing lists that include Xen backers. In addition, VMware has helped itself by setting up a slick VMI website that displays votes of support from the likes of IBM, HP and Novell.

The Xen camp, by contrast, has been criticized for taking the interface issue for granted and assuming its technology would go into Linux.

"This will take more work than the Xen people are anticipating," Morton said.

One Xen backer said a disconnect occurred where the Xen camp thought SuSE and Red Hat would use their close ties with the Linux kernel maintainers to sell the Xen interface. "As it turns out, that was a mistake. Morton wants more contact from us," he said.

On the mailings lists and in discussions, Xen advocates say VMware's VMI simply will not allow for the performance levels they would like to see out of virtualizaton. Xen, which already relies on the paravirtulization model, has learned a myriad of lessons about how a paravirtualized API should operate - skills that those like VMware and Microsoft that are just making their way toward paravirtualization have yet to acquire, Xen backers say.

In particular, they argue that VMI will not support I/O virtualization or SMPs.

VMware responded in a statement saying, "the VMI patches that we recently sent to the Linux kernel mailing list do include SMP support. I/O virtualization isn't in the spec today - it's an area of future work, and we're looking to collaborate with people on it."

More broadly, some Xen backers argue that VMI is simply a "crippled software version" of the virtualization hardware tools AMD and Intel have started building into their chips to make running virtual operating systems easier. That would make VMI rather redundant. Other VMI critiques include a proliferation of kernel hook points, a design that can't support optimizations Xen backers have found to be critical for top performance and weak semantics.

VMware counters by saying its approach requires less kernel changes than Xen, while delivering solid performance.

There are, in fact, laundry lists of other disputes - down to simple issues such as the interface name - that will have to be worked out.

Our sense is that VMware is honest about wanting an open interface but part of its openness stems from a desire to thwart Xen's apparent Linux edge. Xen 3.0 will be bundled with upcoming versions of Red Hat and SuSE, and Xen has a virtualization protocol license from Microsoft. VMware can't claim victories on either of those fronts and, as the clear market leader, is being pigeonholed to a degree by rivals. An open VMI interface could keep it closer to the Linux camp.

Xen, by contrast, wants to make the most of its open source ties and create the tightest possible bonds with Linux. Behind closed doors, some Xen backers say that Sun, Microsoft and Novell will refuse to support VMI. Such political manoeuvering shows how seriously Xen backers take this debate.

It's in Linux customers' best interests that all of this get worked out as soon as possible. The worst scenario would require ISVs and customers to certify applications for a host of paravirtualization interfaces.

As you can tell, however, it seems pretty unlikely that either VMware or Xen advocates will reach an agreement anytime soon, despite their promises to do so. And you can image how exciting the battle will become should the likes of Microsoft, IBM, SWsoft and others decide they want in on the issue as well. Yes, that's right. Microsoft may be a key player in a Linux kernel squabble.

You can expect more and more of these kinds of debates in the coming years, as server virtualization hots up even more. Microsoft, the open source player Xen and the proprietary middleman with all the market share VMware. Can you ask for anything more? ®

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