3 Creating Image-Based Virtual Machines

We will use xen-tools to create virtual machines. xen-tools make it very easy to create virtual machines - please read this tutorial to learn more: http://www.howtoforge.com/xen_tools_xen_shell_argo. The xen-tools package got installed as a dependency when we installed the ubuntu-xen-server package, so we don't need to install it anymore.

Now we edit /etc/xen-tools/xen-tools.conf. This file contains the default values that are used by the xen-create-image script unless you specify other values on the command line. I changed the following values and left the rest untouched:

vi /etc/xen-tools/xen-tools.conf

[...]
dir = /home/xen
[...]
memory = 256Mb
[...]
dist   = lenny     # Default distribution to install.
[...]
gateway   = 192.168.0.1
netmask   = 255.255.255.0
broadcast = 192.168.0.255
[...]
passwd = 1
[...]
kernel      = /boot/vmlinuz-2.6.27-11-server
initrd      = /boot/initrd.img-2.6.27-11-server
[...]
mirror = http://ftp.de.debian.org/debian/
[...]
serial_device = hvc0
[...]
disk_device = xvda #default
[...]

In the kernel and initrd lines, we don't use our dom0 kernel that we built in the last chapter (2.6.27.5) because this kernel does not work in a domU. Please use one of the default Ubuntu kernels (take a look at the output of

ls -l /boot

), e.g. 2.6.27-11-server! The default Ubuntu kernels are built to work in Xen guests, so there's no need to build a special domU kernel.

The dir line specifies where the virtual machine images will be stored. dist specifies the distribution to be installed in the virtual machines (Debian Lenny) (there's a comment in the file that explains what distributions are currently supported).

The passwd = 1 line makes that you can specify a root password when you create a new guest domain. In the mirror line specify a Debian mirror close to you.

Make sure you specify a gateway, netmask, and broadcast address. If you don't, and you don't specify a gateway and netmask on the command line when using xen-create-image, your guest domains won't have networking even if you specified an IP address!

It is very important that you add the line serial_device = hvc0 (not xvc0 or tty1!) because otherwise the console of your virtual machines might not work properly!

Before we go on, we must create the directory where the virtual machine images should be stored:

mkdir /home/xen

Now let's create our first guest domain, xen1.example.com, with the IP address 192.168.0.101:

xen-create-image --hostname=xen1.example.com --size=4Gb --swap=256Mb --ip=192.168.0.101 --arch=amd64 --role=udev

Options that you specify on the command line override the settings in /etc/xen-tools/xen-tools.conf. Options that are not specified on the command line are taken from /etc/xen-tools/xen-tools.conf. Please make sure that you add --role=udev, or your virtual machine might not boot properly!

(To learn more about the available options, take a look at the xen-create-image man page:

man xen-create-image

)

The xen-create-image command will now create the xen1.example.com virtual machine for us. This can take a few minutes. The output should be similar to this one:

root@server1:~# xen-create-image --hostname=xen1.example.com --size=4Gb --swap=256Mb --ip=192.168.0.101 --arch=amd64 --role=udev

General Information
--------------------
Hostname       :  xen1.example.com
Distribution   :  lenny
Partitions     :  swap            256Mb (swap)
                  /               4Gb   (ext3)
Image type     :  sparse
Memory size    :  256Mb
Kernel path    :  /boot/vmlinuz-2.6.27-11-server
Initrd path    :  /boot/initrd.img-2.6.27-11-server

Networking Information
----------------------
IP Address 1   : 192.168.0.101 [MAC: 00:16:3E:1E:C4:E1]
Netmask        : 255.255.255.0
Broadcast      : 192.168.0.255
Gateway        : 192.168.0.1


Creating partition image: /home/xen/domains/xen1.example.com/swap.img
Done

Creating swap on /home/xen/domains/xen1.example.com/swap.img
Done

Creating partition image: /home/xen/domains/xen1.example.com/disk.img
Done

Creating ext3 filesystem on /home/xen/domains/xen1.example.com/disk.img
Done
Installation method: debootstrap
Done

Running hooks
Done

Role: udev
        File: /etc/xen-tools/role.d/udev
Role script completed.

Creating Xen configuration file
Done
Setting up root password
Enter new UNIX password:
Retype new UNIX password:
passwd: password updated successfully
All done


Logfile produced at:
         /var/log/xen-tools/xen1.example.com.log
root@server1:~#

There should now be a xen1.example.com configuration file - /etc/xen/xen1.example.com.cfg. Take a look at it to become familiar with virtual machine configuration files:

vi /etc/xen/xen1.example.com.cfg

#
# Configuration file for the Xen instance xen1.example.com, created
# by xen-tools 3.9 on Fri Feb 13 00:28:34 2009.
#

#
#  Kernel + memory size
#
kernel      = '/boot/vmlinuz-2.6.27-11-server'
ramdisk     = '/boot/initrd.img-2.6.27-11-server'
memory      = '256'

#
#  Disk device(s).
#
root        = '/dev/xvda2 ro'
disk        = [
                  'file:/home/xen/domains/xen1.example.com/swap.img,xvda1,w',
                  'file:/home/xen/domains/xen1.example.com/disk.img,xvda2,w',
              ]


#
#  Hostname
#
name        = 'xen1.example.com'

#
#  Networking
#
vif         = [ 'ip=192.168.0.101,mac=00:16:3E:1E:C4:E1' ]

#
#  Behaviour
#
on_poweroff = 'destroy'
on_reboot   = 'restart'
on_crash    = 'restart'

(Please note: if you have a dual-core or quad-core CPU and want the virtual machine to use all CPU cores, please add the line vcpus = '2' or vcpus = '4' to the configuration file.)

Before we start the virtual machine for the first time, we must first mount its storage device and copy the domU kernel to it (the xen-create-image command copies the dom0 kernel 2.6.27.5 to the guest, but as I mentioned before, our domU kernel is different from the dom0 kernel (2.6.27-11-server in this example)):

mount -o loop /home/xen/domains/xen1.example.com/disk.img /mnt
cd /mnt/lib/modules/
cp -pfr /lib/modules/2.6.27-11-server/ .
cd
umount /mnt

To start the virtual machine, run

xm create /etc/xen/xen1.example.com.cfg

Run

xm console xen1.example.com

to log in on that virtual machine (type CTRL+] if you are at the console, or CTRL+5 if you're using PuTTY to go back to dom0), or use an SSH client to connect to it (192.168.0.101).

To get a list of running virtual machines, type

xm list

The output should look like this:

root@server1:~# xm list
Name                                        ID   Mem VCPUs      State   Time(s)
Domain-0                                     0  3499     2     r-----   2826.0
xen1.example.com                            22   256     1     -b----      5.8
root@server1:~#

To shut down xen1.example.com, do this:

xm shutdown xen1.example.com

If you want xen1.example.com to start automatically at the next boot of the system, then do this:

ln -s /etc/xen/xen1.example.com.cfg /etc/xen/auto

Here are the most important Xen commands:

xm create -c /path/to/config - Start a virtual machine.
xm shutdown <name> - Stop a virtual machine.
xm destroy <name> - Stop a virtual machine immediately without shutting it down. It's as if you switch off the power button.
xm list - List all running systems.
xm console <name> - Log in on a virtual machine.
xm help - List of all commands.

A list of all virtual machines that were created with the xen-create-image command is available under

xen-list-images

root@server1:~# xen-list-images
Name: xen1.example.com
Memory: 256
IP: 192.168.0.101
root@server1:~#

To learn more about what you can do with xen-tools, take a look at this tutorial: http://www.howtoforge.com/xen_tools_xen_shell_argo