Mental Ward

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KVM: Bare-Metal Hypervisor?

I need to turn to you, knowledgeable readers, for help in answering some questions. In a followup to yesterday's announcement by Red Hat about its virtualization roadmap, I asked the company some questions about the new Enterprise Hypervisor.

Specifically, I wanted some details about how KVM would work as a standalone hypervisor, since my understanding is that it's hosted inside the Linux kernel (i.e., a Type II hypervisor). The response I got from Navin Thadani, senior director, virtualization business at Red Hat, threw me for a bit of a loop. He says KVM is a bare-metal hypervisor (also known as Type I), and even tries to make the case that Xen is a hosted hypervisor. Here's his comment in full:

It is a myth that KVM is not a Type-1 hypervisor. KVM converts Linux into a Type-1 hypervisor. There is only one kernel that is used (and that is the Linux kernel, which has KVM included). On the flip side, I can make an argument that Xen is not a Type-1 hypervisor, because the CPU and memory is scheduled by the hypervisor, but IO is scheduled by Dom0, which is a guest (so it's not bare metal). In the KVM architecture, the CPU, memory, and IO are scheduled by the Linux kernel with KVM.

On the other hand, other folks fall into the "KVM is a hosted hypervisor" camp, exemplified by this snippet from Brian Madden:

Xen folks attack KVM, saying it's like VMware Server (the free one that was called "GSX") or Microsoft Virtual Server because it's really a Type 2 hypervisor that runs on top of another OS, rather than a "real" Type 1 hypervisor. KVM responds "So what? Why should we rewrite an OS from scratch when something like Linux is available? And if you want to use a KVM machine as a dedicated VM host, then fine, just don't install anything else on that box."

So, is KVM hosted or not? Is Xen hosted or not? Is Red Hat full of hot air, or are they onto something? I'll be honest and say that I just don't know. Thadani upset my hypervisor apple cart with this comment. But, with your help, I'll get to the bottom of it. Weigh in in the comments below, or e-mail me directly. As a follow-up, tell me what difference you think it makes in the grand scheme of things ("it" being whether KVM is hosted or not, and the same thing about Xen). I'll post comments as they come in. If you do not want your name used with your comments, let me know.

Posted by Keith Ward on 02/24/2009 at 12:48 PM


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