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Red Hat Wants Xen In Linux Kernel
DIY News writes "Red Hat is aggressively pushing to get Xen virtualization technology included in the Linux kernel as quickly as possible. This move comes as Microsoft is pushing its own virtualization products and recently relaxed some of its licensing requirements around Windows Server 2003 to facilitate more pervasive adoption and use of those technologies."
Well, Xen is free, and Intel/AMD hardware solutions are comming soon, which will allow Xen to run Windows unmodified. So, once everyone is upgraded to the new CPU's, virtualization will become a basic standard feature for everyone. MS can compete by giving their solution away for free, but either way, it doesn't get better than free for the consumer.
Since you've received few answers to you actual question, getting xen into the kernel means the xen patches required to run the linux kernel in a xen hypervisor (both as a guest and a host) will be a part of the mainstream kernel and be able to be built trivially. RedHat ships 3 different kernels now with FC4. One is a normal kernel available in both smp and non-smp configurations. Then we have the XenU kernel, which is a kernel designed to boot in a guest xen session. The Xen0 kernel is the kernel that you'd actually boot on top of xen and use as your main OS.
Once the Xen0 kernel is running on top of xen, you have basically a normal linux kernel running that does all the hardware support. Then you load up Xen guest machines running the Xen0 kernel and these run their in their own virtual machines complete with their own disk images and linux distro. So xen doesn't really have anything to do with running elf binaries on the other machine. If you ran FreeBSD in the guest, it would run those binaries inside of that OS and that libc. When Xen 3.0 comes out, if you have the new intel or amd chips that support on-chip virtualization, then Windows XP can even run as a guest underneath the linux kernel-Xen0 host.
I still remeber reading that the whole x86 architecture didn't meet the requirements for virtualization, meaning that this recent trend is probably the result of VMware figuring out some "tricks to make it work", and then everyone else jumping on the bandwagon.
In any case, if you really want to learn about the fundamental concepts behind virtualization, I strongly recommend reading the following paper: Formal Requirements for Virtualizable Third Generation Architectures
Yes, it was published in 1974, but most of the concepts are still very applicable and make a lot of sense. (though the architecture examples are obviously dated)
This is a very good paper which lays out all the ground rules. Sure, it may sound a bit academic in terminology and explanation, but it is still quite readable.
Don't let that comment fool you though. Xen is much, much more. What if you organization had 4 distinct sites they wanted to host on one server? Start up 4 virtual machines, and back up their running state from time to time. If one goes down, just restart it from your clean backup IN SECONDS. Better yet, do it automatically. At the same time, Xen enforces separation from the host OS that the virtual machines are running on, so you don't have to worry about it being compromised (well, not in any way anyone has been able to demonstrate or even postulate yet).
Multiple OSes at once? [..] I never got the whole concept of virtual servers
Its mainly an enterprise play.
If you're an old-timer UNIX admin, you may have difficulty understanding the point of server virtualization (i.e. multiple OS instances). In UNIXland, it has been normal and customary for several completely unrelated applications to run under the same OS instance, together servicing thousands of users. That never worked well in Windowsland. That being said, it didn't stop manufacturers from making staggering improvements in performace and capabilities of Intel servers. Companies grew to expect single Intel boxes to perform at the levels of large UNIX servers. The only way to achieve that in recent years has been to use industrial strength virtualization technology (basically, ESX). The boxes are beefier than ever, and ESX isn't cheap, but it works wonders.
For $50k, you can run 50 VMware guests on one very beefy box (not counting SAN), but you'd want a second for failover. For $75k, you can run about 100 guests on 14 blades in 7U (again, not counting SAN) and have the guests automatically migrate to the blades most able to run their workload at that moment in time. Ask a blade to come down for maintenance, and all the guests scatter to other blades before the blade powers off. Replaced a dead blade with a blank? Your systems management policies detect the new blank and automatically install ESX on it so guests can migrate back and evenly spread out the load.
Sounds crazy, I know, but that's a taste of what we're doing in the enteprise space these days.
Intelligent Life on Earth
The full x86 architecture is not suitable for virtualization, because there are a few instructions which fail silently when run from user level.
VMware uses various techniques to get around this, including full simulation and binary re-writing.
Xen uses another approach, where they port to an instruction set that is basically x86 without the problematic instructions. This approach requires that the guest OS's be modfied.
This will all change with the new virtualization instructions being added by both AMD and Intel. Once that is in place, Xen will be able to run unmodified guest OS's (such as Windows, for instance). There will be a speed hit though, so modified guests will be prefered if speed is an issue.