Howto Setup and Run Xen

This is an extremly brief and quick Xen tutorial. There are lots of them already, see your GNU/Linux distribution’s wiki, HowtoForge or other places for a starter guide. This particular HowTo deals with setting up Xen as easy as possible using Ubuntu 8.04 LTS as host operating system (dom0 in Xen terms) and Ubuntu 9.04 as guest operating systems (domU in Xen lingo).

Ubuntu 8.04 LTS comes with a Xen kernel that can run as dom0. This is quite important, so pay attention to setting that up correctly. For this I recommend the HowtoForge Tutorial.

Note: you will need to upgrade the default Xen 3.2 to 3.3 to be able to run the latest distributions as guest OS. See this Ubuntu question for more insight. To get Xen 3.3, simply activate the Hardy backports repository in your /etc/apt/sources.list and, as usual, pay attention to your /boot/grub/menu.lst so that it points out the Xen 3.3 image in the defaults= option.

When done with the basic setup you should install pygrub on your host, this is a truly magic piece of software that makes it possible to boot all guests using their own kernel and modules. See the Debian wiki for some details on install and setup. Then ignore what you read and do like this:

# /var/lib/xend/domains/example/config.sxp - Xen domU example.
# Note the order of partitions in disk=[], the first listed partition
# is the partition where your /boot (grub) resides. 
bootloader  = '/usr/bin/pygrub'

builder     = 'linux'
memory      = '2048'
root        = '/dev/xvda1 ro'

disk        = [
  'phy:/dev/disk/by-uuid/df3418f5-2fc3-443c-8e64-4395828dc678,xvda1,w',
  'phy:/dev/disk/by-uuid/acecf01e-c4d9-4a7b-b18a-681c69f71173,xvda2,w',
]

name        = 'example'

vif         = [ 'bridge=eth1' ]
dhcp        = "off"
ip          = "12.34.56.78"
netmask     = "255.255.255.252"
gateway     = "12.34.56.77"
hostname    = "example"

on_poweroff = 'destroy'
on_reboot   = 'restart'
on_crash    = 'restart'
vcpus       = 1
extra       = 'xencons=tty1'

When installing grub on your guest, don’t pay any attention to Grub complaining about not being able to map /dev/xvda1 to a BIOS disk. pyGrub doesn’t care about such wordly things as physical disks. Just make sure the /boot directory’s partition (often /) is the first listed in the disk=[] array above.

You may need to edit the update-grub script before you install grub, it is located at /usr/sbin/update-grub. Search for in_xen, a fairly long way down. Make sure it’s set to in_xen=1 before the big if[]-clause that depends on it. Without this fix the Xen able kernels in Ubuntu 9.04, and later, are not detected.

Now run update-grub and answer yes to the questions. Take good care to verify that the script actually finds one active (server) kernel and adds it properly to your /boot/grub/menu.lst file.

Another common problem is the lack of a console login. Usually you don’t need one, but if you’d like to check your domU from within your dom0 you need to add the file “console” to /etc/event.d

# console - getty
#
# This service maintains a getty on the Xen serial console
# from the point the system is started until it is shut
# down again.

start on stopped rc2
start on stopped rc3
start on stopped rc4
start on stopped rc5

stop on runlevel 0
stop on runlevel 1
stop on runlevel 6

respawn
exec /sbin/getty 38400 console

That should do the trick! If it doesn’t, then there’s plenty of help to find in the Debian and Ubuntu wikis, see links above. Good luck!