XEN, Hardware Virtualization – The Wave of Future

Let me introduce you to my new favorite piece of open source software: Xen.

Xen is a virtualization operating system that allows you to share hardware between multiple OS’es. The concept isn’t really new, as there are several commercial as well as open source implementations of this concept already.

In the past I have used Linux-Vserver which is a set of kernel patches that create security contexts inside the kernel that different versions of Linux can co-exist in. It works very well and I have been using it for over 2 years in production. The main drawback is that it only works with Linux based OS’es. The same with Jail in FreeBSD which I have also used for a while, but again is specific to the OS, in this case FreeBSD.

Now I have never used VMWare, but VMWare uses a “core” application that the guest OS’es run in. This works very well with all current OS’es from Windows, Linux to *BSD. I have heard that there is some performance degradation with this method, but I have never used it so I don’t really know for certain.

Now enter Xen. Xen works much the same way as VMWare, but is more lightweight. Xen loads up as a base kernel that in turn loads a Dom0 kernel. The Dom0 is the OS that runs with higher priviledges than any other guest OS on the system. The purpose of the Dom0 is to allow an interface between the guest OS’es and the Xen kernel (to start and stop guest OS’es, create new ones, etc).

The biggest limitation for Xen currently is that in order to run under Xen, an OS must be compiled to run under Xen. Currently there are hardware compile options for Linux (patchset), FreeBSD, and NetBSD (both support Xen as a native hardware architecture, like i386, or PowerPC). You will notice that Windows is missing from this list, and that is because while Xen will run Windows as a guest OS, there is currently no way the average Joe can get access to the code to Windows to make the changes necessary to get Windows to run under Xen. The Xen guys have gotten Windows to run under Xen by modifying the Windows source, and I am betting by signing NDAs and giving away their first born children, so we know that Windows will work under Xen with some modifications.

That is the basics of Xen. Now we turn an eye to the future and find out what is on the horizon.

Here is where things get interesting, both Intel and AMD are planning on offering a new technology into their CPUs that supports virtualization. Intel calls theirs VT (Virtualization Technology – catchy right?). Currently (as of the last processor table guide from Intel) two processors support the VT technology, the Pentium 4 662 and 672. I am assuming (using my secret decoder ring) that the XX2 designator will indicate the presence of VT for the chip. I have looked around, but I haven’t found a single place that sells these processors yet so who knows how much they will cost.

I don’t have any details on AMDs VT offering yet.

So what is so special about VT? Well Xen 3.0 (released December 2005) supports this new feature (VmWare also supports this feature). With support for VT technology it is now possible to run guest OS’es in Xen that are not compiled for the Xen architecture…. Hello Windows!

So lets stop and think about this for a minute, it will now be possible to purchase a fairly large server (lots of RAM and disk space) and run Linux, FreeBSD and yes, even Windows side by side by side.

It seems that this next innovation might just be a great way to improve the over all cost of operating multiple servers with multiple OS’es. Only time, and probably usability tools will tell.

Update: I want to thank Karsten Kruse for quoting me in an article done on installing XEN under NetBSD. If you want to know how to install XEN on NetBSD you can read Karsten’s post about how to do it here.

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