Andrew Hudson - Fedora™ Unleashed, 2008 edition

Moving your server farm from 15% to 80% or higher utilization either means your users get a much faster and more reliable service, or you suddenly have a huge amount of hard ware that you can sell or keep offline until needed. Either way, it is a huge cost savings, which makes virtualization the easiest way to cut your IT budget.

The method in which operating systems are kept isolated is quite simple: They really do each get a virtual machine. When you create a new VM, it is allocated a chunk of RAM all to itself, and lives completely self-contained from the outside world. VMware even has a virtual BIOS to complete the illusion. If your VM wants to communicate to other VMs on the same computer, it has to do so over a TCP/IP network connection, just like any other machine. In fact, when you install an operating system on a virtual machine, it cannot even tell that it is a virtual machine because it looks identical to raw hardware.

The problem with this type of virtualization is that it is very slow. For example, the VM has to access hardware frequently (to save files, show a display, and so on), but of course it cannot access the hardware directly because doing so would interfere with other VMs. Instead, its requests have to be translated and rerouted to the host operating system (OS), where they are handled. Similarly, if the VM tries to execute any special CPU instructions that would reveal that the VM is actually a virtual machine, the host OS has to stop those instructions and fake the answer so that the VM is not any wiser (a technique known as binary patching ).

The solution is to move from virtualization to paravirtualization — the technique that Xen uses. Linux is open source, so the Xen developers modified the Linux source code so that Linux becomes aware of its existence as a virtual machine. When the virtual Linux needs access to the hardware, it just asks Xen for permission. The difference is huge: You can expect a normal VM to run at about 50% of the speed of a native machine, whereas a Xen virtual machine can run up to about 95%, simply by removing the need for binary patching and other virtual hacks.

The downside to Xen is that the source code has to be patched, which rules out closed-source operating systems. VMware runs Windows XP on Linux out of the box, but Xen cannot. That said, the new chips from Intel and AMD include virtualization on hardware, which enables Xen to run unmodified Windows at full speed on top of Linux. Without this technology, Xen can use only a modified, open source distribution, such as Linux, NetBSD, or FreeBSD.

Xen is actually a very small operating system that has the sole goal of managing the resources of virtual machines. On top of the Xen OS runs what would previously have been called the host OS — the main operating system for the machine. Unlike VMware, the host OS (known as domain 0 or just dom0 in Xen terminology) is a virtual machine, but has special privileges assigned to it so that it is more responsive.

The domain 0 VM is where you control Xen, and where you start other guest VMs — known as unprivileged domains or domU . You can start as many as you want, with the only real limit being the amount of RAM in your machine. Because the domU OS is fully aware of its status as a Xen virtual machine, Xen lets you change the amount of RAM in a VM while it's running, with the exception that you can't go above the initial allocation of RAM.

Fedora Linux specifies a minimum system requirement of 256MB RAM, which means that if you are to run Fedora on top of Fedora, you need at least 512MB RAM. Keeping in mind that Xen uses a very small amount of RAM for itself, you should ideally have at least 768MB of RAM to be able to run two operating systems side by side at full speed.

Note that if you have tried Xen on old versions of Fedora, you no longer need to disable SELinux to get Xen to work.

The first step is to convert your current OS to a virtualized guest OS. This is actually a very easy thing to do because, as discussed, domain 0 has special privileges — such as the capability to access hardware directly. As a result, you do not have to reformat your machine: dom0 reads straight from the disk, uses the graphics card, uses the sound card, and so on.

To get started, go to Applications, Add/Remove Software. From the window that appears, choose List view, and then select the following packages: kernel-xen, vnc,and xen. The kernel-xenpackage provides a Linux kernel that is configured to run on top of Xen without any special privileges, as well as a Linux kernel designed to be used as dom0 so it can access hardware directly. The vncpackage is there to make VM management much easier. Finally, the xenpackage gives you all the tools you need to create and manage virtual machines. Along with these packages, there are several other dependencies that Fedora will automatically resolve for you, so just go ahead and install all the packages.

Because you have installed two new kernels, Fedora updates your Grub boot configuration to make them bootable, but leaves your original, non-Xen kernel as the default. Switch to root and bring up /boot/grub/grub.confin your favorite text editor. Look for the line "default=2"and change it to read "default=0". This might vary on your machine — set it to the position of the Xen hypervisor kernel in the grub.conf file, remembering that Grub counts from 0 rather than 1. That is, the first OS in the list is considered to be number 0. Note that you should not set the guest kernel as the default because it will not boot—it is designed only to be created on top of the hypervisor (dom0).

Save your changes and reboot, making sure that your new hypervisor kernel is the one that boots. Your system should restart as normal, and you will probably not notice anything different beyond a smattering of "XEN" at the very beginning of the boot phase. But when you are back in control, open a terminal and run uname -r — it should tell you that you are running the Xen hypervisor kernel.

At this point, you are already running as a virtual machine on top of the Xen kernel, but there is no way for you to communicate with the Xen kernel and thus manipulate the virtual machines on the system. To do that, you need to start the Xen daemon, which provides the link between dom0 (where you are working) and the Xen kernel underneath.

Run ps aux | grep xend — if you do not see xendin there, you need to start it yourself by switching to root with su -and then running service xend start . Now run the command xm list , which prints out a list of all the virtual machines that are running and how much RAM they have allocated — you should see Domain-0, which is your current system, in the list.

The output from xm listprobably shows that domain 0 is taking up all the RAM on your system, which means there is no room to create a new guest OS. Fortunately, you can resize that memory usage downward to make space: Run the command xm mem-set Domain-0 256 to have domain 0 use 256MB RAM. This is the bare minimum for a Fedora install, so expect some slowdown — if you have more than 512MB RAM, we recommend you allocate more to each VM.

Creating a domU VM on Fedora is handled with the xenguest-install.pyscript, which you should run as root. You are asked to do the following things: