Have you heard about Xen virtualization and want to get some hands-on experience? Do you want to experiment with Linux clustering but only have a single computer to devote to the cause? If you answered yes to either of these questions, keep reading.

In this article, I briefly introduce the concepts of Xen virtualization and Linux clustering. From there, I show you how to set up multiple operating systems on a single computer using Xen and how to configure them for use with clustering. I should point out that a cluster implemented in this manner does not provide the computational power of multiple physical computers. It does, however, offer a way to prototype a cluster as well as provide a cost-effective development environment for cluster-based software. Even if you're not interested in clustering, this article gives you hands-on experience using Xen virtualization.

Xen is an open-source software project that provides high-performance, resource-managed virtualization on the x86 processor architecture. It allows multiple operating system instances to run concurrently on a single physical computer. Xen manages the computer's hardware resources so they are shared effectively among the operating system instances, called domains. So what does all this mean? It means that by using Xen virtualization, you can run and use several different operating systems at the same time on a single computer.

Xen supports a variety of operating systems, which increases its applicability for running many popular applications on whichever OS you desire. Xen currently supports Linux kernels through an approach called paravirtualization in which the operating system is modified to become aware of Xen. Other ports have been developed or are in progress for other operating systems, such as NetBSD and FreeBSD. Intel and AMD also are developing new virtualization technology in their processors to allow an OS to run on Xen without modification. That means even Microsoft Windows will be an OS option on Xen in the near future.

Xen virtualization provides many exciting benefits over traditional single OS computers, including server consolidation, application mobility, secure computing and research/testing. What sets Xen apart from other virtualization technologies, however, is its performance. In fact, running over Xen, Linux's performance in a variety of benchmarks is consistently close to native Linux.

So how is a Xen system structured? Each Xen system has a single privileged OS, called Domain-0, that is responsible for starting and managing the other unprivileged OS instances. Domain-0 is the OS that boots when you start your computer, and it has the tools necessary to manage other domains.

Clustering essentially is the unification of multiple independent computers into a single system through the use of software and networking. By definition, a cluster must have at least two computers (called nodes), one master and one slave. The master node typically has a job scheduler that schedules the work to be done by the slave nodes. As the name implies, the slave nodes perform whatever work has been scheduled for them to do. The nodes of the cluster communicate using a message passing interface. Two of the most commonly used message passing interfaces are PVM and MPI. For this tutorial, we are using PVM 3.4 to implement clustering.

For the tutorial that follows, the master node is the privileged Xen Domain-0 and the slave nodes are the unprivileged Xen Domains. This approach allows us to create and use as many slave nodes as we want.

The first step to getting started with Xen is to install it. This process involves several steps:

I do not cover these first three steps here, because adequate documentation already is available on-line describing this process. A good place to start is the Xen User Manual. In addition, a wide variety of tutorials and troubleshooting articles are available on-line. Be sure to find one that matches the Linux distribution you are using as Domain-0. During the installation process, you need to consider which Linux kernels are appropriate for your use. The Xen binary release contains pre-compiled kernels for both the privileged and unprivileged domains. If you require anything special that's not supported by these kernels, you may need to compile a custom kernel with Xen support. On my system, I compiled my own Domain-0 kernel but used the stock kernel for the unprivileged domains. Another important consideration for this tutorial is to leave an extra, unused partition on your hard drive for later use with the unprivileged domains.

For the tutorial that follows, I assume you have Xen 2.0.7 installed and can boot Domain-0. You also must be able to start xend, the Xen control daemon, which must be running in order to start and manage other domains.

Xen 3.0 was released recently. If you are more daring, you might consider installing that version instead, although you may not be able to follow along with this article as easily. I also assume that your Domain-0 is a Debian-based distribution. If not, you simply need to use whatever mechanism your distribution provides for downloading and installing packages, typically where the apt-get command is used below.

Now that we have all the formalities out of the way, let's get started by configuring Domain-0. The first thing we need to do is install PVM 3.4.5 on our master node.

Installing PVM on the Master Node

If you run into any challenges during this process, the $PVM_ROOT/Readme file contains a lot of useful information.

Now that we have Domain-0 configured, we can begin installing and configuring the unprivileged Xen domains. We start by creating a minimal Debian Sarge domain as a template for all future unprivileged domains, which will save us a considerable amount of time down the road. The filesystems for the unprivileged domains should be set up as 1GB LVM volumes in a volume group named VG. Now, you may be thinking "What is LVM?" LVM stands for logical volume management, and it allows you to create, delete and resize multiple logical volumes without having to repartition your hard drive, which is extremely useful for our situation. For the sections that follow, you need to have LVM2 installed. Go ahead and try running the following commands. If they aren't available, install LVM2 using apt-get install lvm2.

Creating the Filesystems

This is where the extra hard drive partition comes into play. We start by initializing the extra partition, call it /dev/hda6, for use by LVM. We then create a new volume group on that partition:

# pvcreate /dev/hda6
# vgcreate VG /dev/hda6

You can check the man pages for the pvcreate and vgcreate commands to find out what's going on behind the scenes. We now are ready to create the filesystems for our first unprivileged domain. The naming scheme I use is Debian_Slave#_Root and Debian_Slave#_Swap, where # is replaced with the number of the slave and Root and Swap denote the root and swap filesystems, respectively. So, let's create and initialize the root and swap filesystems:

# lvcreate -L1024M -n Debian_Slave1_Root VG
# lvcreate -L64M -n Debian_Slave1_Swap VG
# mke2fs -j /dev/VG/Debian_Slave1_Root
# mkswap /dev/VG/Debian_Slave1_Swap

The first lvcreate command creates a logical volume in the existing "VG" volume group with a size of 1,024MB and a name of "Debian_Slave1_Root". An ext3 filesystem is created on this volume using the mke2fs command. The second lvcreate command creates a smaller logical volume that is set up as a swap area using the mkswap command.

Now that the filesystems are created and initialized, we can mount the new root filesystem at /mnt/xen and install a minimal Debian Sarge root filesystem on it. The debootstrap utility allows you to do exactly this:

# apt-get install debootstrap
# mkdir /mnt/xen
# mount /dev/VG/Debian_Slave1_Root /mnt/xen
# debootstrap --arch i386 sarge /mnt/xen http://www.uk.debian.org/debian

Now we need to configure certain files manually in the new filesystem, which should be mounted at /mnt/xen. The files that need to be modified are listed below, followed by the text they should contain:

Creating the Domain Config File

The unprivileged domain now must have a Xen configuration file created so Xen knows how to boot it. A sample configuration file is shown below. The actual values may be adjusted as necessary to reflect your own system and network configuration. You can find more details about the domain configuration file and the available options in the Xen User Manual. I save my configuration files in the /etc/xen directory. The following file is saved as debian_slave1.conf:

name="debian_slave1"
memory=64
kernel="/boot/vmlinuz-2.6.11-xenU"
nics=1
vif=[ 'mac=aa:00:00:00:00:01, bridge=xen-br0' ]
disk=[ 'phy:VG/Debian_Slave1_Root,sda1,w', 'phy:VG/Debian_Slave1_Swap,sda2,w' ]
root="/dev/sda1 ro"

Booting the Slave

The minimal Debian Sarge domain now is ready to boot. If your Domain-0 currently is taking up all RAM on the machine, you may need to free some RAM for use with your new domain before booting it. Type xm help balloon for information on how to do this. Alternately, you can modify your bootloader configuration to start Domain-0 using a specified amount of RAM. The Xen User Manual shows how to do this for GRUB. Once that's taken care of, boot the new domain using the command:

# xm create /etc/xen/debian_slave1.conf -c

The xm tool provides a command-line interface for managing Xen. In this case, it boots the new Debian Sarge OS for the first time, turning the current console session into the console of the debian_slave1 domain. Log in as root and change the root password using the passwd command.

Verifying Network Connectivity

You now should verify your network connectivity from the unprivileged domain's console. If you encounter errors at any of these steps, you need to troubleshoot the network configuration for the unprivileged domain before proceeding.

Start by trying to ping Domain-0 using its IP address, for example, ping 192.168.0.200. If Domain-0 has a firewall enabled, you may need to disable it or come up with a workaround. Next, check Internet connectivity, which is needed to install additional required packages, for example, ping 4.2.2.1.

Finally, check whether DNS resolution is working correctly, for example, ping www.google.com. On my machine, I received an unknown host error message. I found that this can be resolved by running the command apt-get install bind. Unfortunately, without DNS resolution working, apt-get is not able to download and install the package using the Web sites currently specified in the /etc/apt/sources.list file. To work around this, manually look up the IP address for ftp.debian.org and use it in the following command to download and install the BIND package:

      # wget ftp://128.101.80.133/debian/pool/main/b/bind/bind_8.4.6-1_i386.deb
      # dpkg -i bind_8.4.6-1_i386.deb

Installing PVM

You now are ready to install PVM on the slave. From the debian_slave1 console, perform the following steps:

1. Install the packages necessary for compiling PVM:

      # apt-get update
      # apt-get install make build-essential m4

2. Compile and install PVM using the same process described above for the master node (in the section "Installing PVM on the Master Node").

3. Install an SSH client and sshd server:

      # apt-get install ssh

The SSH configuration asks the following questions. Answer them as indicated below:

4. Set up SSH to accept connections from the master without a password:

At this point, you have completed the construction of your first unprivileged domain. Congratulations! In the next installment of this article, we'll look at how we easily can create additional unprivileged domains using the one we just created as a template. We'll then move on to configuring the PVM cluster and testing it using an open-source parallel ray tracer.