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I had to spend some time setting up Xen on one of the new Dell servers we bought and while there was some documentation around, I had to constantly refer to the different sources since there wasn’t a complete page which had everything I needed for my particular situation – I have a host machine with Ubuntu Feisty Fawn installed and wanted to install a couple of Xen guest domains on it. One other thing I found lacking in the documentation is the explanation of commands or answers to the question “so why the hell am I doing this?”. I’m not considering LVM at the moment and am happy with loopback disk file images too.
So I think it’s a good idea to write this down somewhere for the next time I have to do the same bloody thing over again on any new servers.
Some caveats before we start:
Some terms that deserve explaining (most guides out there just use “DomU” and “Dom0” without any ceremony, and assume you know what they mean):
OK let’s go!
First off, we need to install the Xen server on the host machine (or Dom0). Thankfully, there is a Xen server package in Feisty Fawn’s apt-get repository.
apt-get install ubuntu-xen-server
sudo xm listshould show you Domain-0 (which is the host machine that’s running Xen).
sudo xend restart
Now let’s create a base Xen image. We’ll use this as a template for any future images (domUs).
dd if=/dev/zero of=/xen-images/feisty_base.img bs=1024k count=4000 dd if=/dev/zero of=/xen-images/feisty_base_swap.img bs=1024k count=1000
These commands create image files of 4GB and 1GB for your virtual OS and its swap respectively. ‘count’ is the number of blocks (block size = 1024k).
Answer ‘yes’ to proceed anyway when it complains about the file not being a block device.
chmod 640 /xen-images/feisty_base*
mount -o loop /xen-images/feisty_base.img /xen-images/mnt
Now we have full access to the base image’s filesystem. What we need to do now is to put a base install of Feisty Fawn on it.
Next, we have to put Ubuntu onto the image and configure it to our liking.
apt-get install debootstrap
debootstrap feisty /xen-images/mnt
After this is done you should see the basic Ubuntu file hierarchy:
cp /etc/apt/sources.list /xen-images/mnt/etc/apt/
cp -a /lib/modules/2.6.19-4-server/ /xen-images/mnt/lib/modules/
libc6-xenpackage, which is a Xen-compatible glibc that allows applications to use Thread-Local Storage (TLS).
apt-get install libc6-xen
Taken from the Xen FAQ:
Some modern distributions ship with a ‘TLS’ version of glibc that is not fully compatible with Xen. To use Xen reliably and with maximum performance you must disable the incompatible glibc.
auto lo iface lo inet loopback # Uncomment this and fill up with your networks settings. #auto eth0 #iface eth0 inet static # address 192.168.0.201 # netmask 255.255.255.0 # broadcast 192.168.255.255 # gateway 192.168.0.1 # dns-nameservers 192.168.0.1
Notice how the network settings are commented out – this is because we are leaving the base image as a template from which other images would be made from.
127.0.0.1 localhost localhost.localdomain 127.0.0.1 guest # The following lines are desirable for IPv6 capable hosts ::1 ip6-localhost ip6-loopback fe00::0 ip6-localnet ff00::0 ip6-mcastprefix ff02::1 ip6-allnodes ff02::2 ip6-allrouters ff02::3 ip6-allhosts
/xen-images/mnt/etc/hostname. In this case I’m just naming it ‘guest’ – actual virtual machines would have sensible hostnames.
proc /proc proc defaults 0 0 /dev/hda1 / ext3 defaults,errors=remount-ro 0 1 /dev/hda2 none swap sw 0 0
Don’t worry, the /dev/hda* would be configured to point to the images you created, not to your actual physical harddisks.
/etc/xen/vm_feisty_base.config.example.sxp(you can name it whatever you like of course):
name = "feisty_base" kernel = "/boot/vmlinuz-2.6.19-4-server" ramdisk = "/boot/initrd.img-2.6.19-4-server" root = "/dev/hda1 ro" memory = 512 disk = ['file:/xen-images/feisty_base.img,hda1,w','file:/xen-images/feisty_base_swap.img,hda2,w'] # Network. hostname = "feisty_base" vif = ['bridge=xenbr0'] dhcp = "off" ip = "192.168.0.201" netmask = "255.255.255.0" gateway = "192.168.0.1"
An explanation of the configuration options:
xm create /etc/xen/vm_feisty_base.config.example.sxp -c
You should see a console (the ‘-c’ option connects you to the console immediately after booting up the virtual machine. Login as root (empty password) – you’ll want to change the password (with ‘passwd’).
/etc/network/interfacesto put in correct values so you can access the Internet from the virtual machine:
cp /etc/network/interfaces /etc/network/interfaces.bak vim /etc/network/interfaces
apt-get update apt-get install ubuntu-standard
apt-get install ssh
ifdown -a mv /etc/network/interfaces.bak /etc/network/interfaces
Ctrl-]. This will bring you back to dom0’s shell.
-wmeans “wait for the domU to shutdown completely”):
xm shutdown feisty_base -w
You’re done setting up the base image! Now you have a nicely configured base image.
The fruits of thy labor are almost at hand – with the base image, we can now make a copy of it for our first Xen virtual machine.
cp /xen-images/feisty_base.img /xen-images/yuki.img cp /xen-images/feisty_base_swap.img /xen-images/yuki_swap.img
You may want to resize the base image depending on your disk space needs. (And, of course, you can create your own swap image like we did earlier instead of copying the base image’s swap file.)
cp /etc/xen/vm_feisty_base.config.example.sxp /etc/xen/vm_yuki.config.sxp
/etc/xen/vm_yuki.config.sxp file, changing the values to suit your virtual server (you probably want to change at least the ‘name’, ‘disk’, ‘ip, and ‘hostname’ values).
name = "yuki" disk = ['file:/xen-images/yuki.img,hda1,w','file:/xen-images/yuki_swap.img,hda2,w'] hostname = "yuki" ip = "192.168.0.202"
xm create -c /etc/xen/vm_yuki.config.sxp
ifup -a ping google.com
ln -s /etc/xen/vm_yuki.config.sxp /etc/xen/auto/
Any Xen domain configuration placed in
/etc/xen/auto/ will be started up automatically on boot (and shutdown when the host machine is shutdown too).
That’s it! You have a functional Xen guest domain that starts up on boot and that can connect to the Internet. Oh before I forget, some useful commands when dealing with the Xen images:
xm create /path/to/config_file– Starts up the guest domain with the given Xen domU configuration file. (
xm create -cto login to the console immediately after booting the image.)
xm console yuki– Open the console for the ‘yuki’ Xen domain.
xm shutdown yuki– Shutdown the ‘yuki’ domU.
xm list– Lists all the Xen domains you have running.
Let’s say you wanna add 1GB of disk space to your virtual machine (named ‘yuki’).
Note that LVM is usually recommended for Xen setups, so you may wanna take a look at that instead.
dd if=/dev/zero of=/tmp/temp_expand bs=1024k count=1000
xm shutdown yuki
cat /tmp/temp_expand >> /xen-images/yuki.img
resize2fs -f /xen-images/yuki.img
xm create -c /etc/xen/vm_yuki.config.sxp
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