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Xen Overview

Xen is an open-source (GPL) type-1 or baremetal hypervisor, which makes it possible to run many instances of an operating system or indeed different operating systems in parallel on a single machine (or host)

Some of Xen's key features are:

See the Xen Overview on the Xen wiki for more information.

Guest types

Xen supports running two different types of guests: Paravirtualization (PV) and Full or Hardware assisted Virtualization (HVM). Both guest types can be used at the same time on a single Xen system. It is also possible to use techniques used for Paravirtualization in an HVM guest: essentially creating a continuum between PV and HVM. This approach is called PV on HVM. Again see the Xen Overview on the Xen wiki for more information.

Domain 0

Xen has a special domain called domain 0 which contains drivers for the hardware, as well as the toolstack to control VMs. Domain 0 is often referred to as dom0.

Domain 0 (Host) Installation

Initial Host Installation

Before installing Xen you should install Debian on the host machine. This installation will form the basis of Domain 0.

Installing Debian can be done in the usual way using the DebianInstaller. See the Debian Release Notes for more information on installing Debian.

In order to install Xen you will either a 32-bit PC (i386) or 64-bit PC (amd64) installation of Debian. Although it is recommended to always run a 64-bit hypervisor note that this does not mean one has to run a 64-bit domain 0. It is quite common to run a 32-bit domain 0 on a 64-bit hypervisor (a so-called "32on64" configuration).

In general you can install your Domain 0 Debian as you would any other Debian install. However the Xen wiki has some Host OS Installation Considerations which may be of interest. The main thing to consider is the partition layout of the host since this will have an impact on the available guest disk configurations.

If you have already installed Debian then continue on to the next section.

Installing Xen Packages

The Xen and debootstrap software in Squeeze (Debian 6.0) are very much newer than that in Lenny. Because of that, working with Xen becomes a lot easier.

The setup described here is tested for Debian Squeeze and Ubuntu Maverick virtual machines, but should work for a lot more.

First install the hypervisor, xen aware kernel and xen tools. This can be done by a metapackage:

apt-get install xen-linux-system

To get Xen HVM support on Wheezy the qemu device model package, which provides the necessary emulation infrastructure for an HVM guest, is also required.

apt-get install xen-qemu-dm-4.0

This is no longer needed in Wheezy since the device model is part of the Xen packages.

Prioritise Booting Xen Over Native

Debian Squeeze uses Grub 2 whose default is to list normal kernels first, and only then list the Xen hypervisor and its kernels.

You can change this to cause Grub to prefer to boot Xen by changing the priority of Grub's Xen configuration script (20_linux_xen) to be higher than the standard Linux config (10_linux). This is most easily done using dpkg-divert:

dpkg-divert --divert /etc/grub.d/08_linux_xen --rename /etc/grub.d/20_linux_xen

to undo this:

dpkg-divert --rename --remove /etc/grub.d/20_linux_xen

After any update to the Grub configuration you must run:

update-grub

Configure Networking

In order to give network access to guest domains it is necessary to configure the domain 0 network appropriately. The most common configuration is to use a software bridge.

It is recommended that you manage your own network bridge using the Debian network bridge. The Xen wiki page Host Configuration/Networking also has some useful information. The Xen supplied network scripts are not always reliable and will be removed from a later version. They are disabled by default in Debian's packages

Other configuration tweaks

Configure Boot Parameters

You may also want to pass some boot parameters to Xen when starting up in normal or recovery mode. Add these variables to /etc/default/grub to achieve this:

# Xen boot parameters for all Xen boots
GRUB_CMDLINE_XEN="something"
# Xen boot parameters for non-recovery Xen boots (in addition to GRUB_CMDLINE_XEN)
GRUB_CMDLINE_XEN_DEFAULT="something else"

After editing GRUB configuration, you must apply it by running:

update-grub

Configure Domain 0 Memory

The /etc/xen/xend-config.sxp config file has options to set the memory and CPU usage for your dom0, which you might want to change. To reduce dom0 memory usage as it boots, use the dom0_mem kernel option in the aforementioned GRUB_CMDLINE_XEN variable. Xen wiki also advise to disable dom0 memory ballooning and set minimal memory in /etc/xen/xend-config.sxp (1024M is an example) :

(dom0-min-mem 1024)
(enable-dom0-ballooning no)

Configure guest behaviour on host reboot

By default, when Xen dom0 shuts down or reboots, it tries to save the state of the domUs. Sometimes there are problems with that - it could fail because of a lack of disk space in /var, or because of random software bugs. Because it is also clean to just have the VMs shutdown upon host shutdown, if you want you can make sure they get shut down normally by setting these parameters in /etc/default/xendomains:

XENDOMAINS_RESTORE=false
XENDOMAINS_SAVE=""

Enable Serial Console

To get output from GRUB, the Xen hypervisor, the kernel and getty (login prompt) via both VGA and serial console to work, here's an example of the right settings on squeeze:

Edit /etc/default/grub and add:

GRUB_SERIAL_COMMAND="serial --unit=0 --speed=9600 --word=8 --parity=no --stop=1"
GRUB_TERMINAL="console serial"
GRUB_TIMEOUT=5
GRUB_CMDLINE_XEN="com1=9600,8n1 console=com1,vga"
GRUB_CMDLINE_LINUX="console=tty0 console=hvc0"

Here's what I used to configure the serial console (for a Supermicro X8STi-F motherboard with IPMI and SOL):

GRUB_CMDLINE_XEN="loglvl=all guest_loglvl=all com1=115200,8n1,0x3e8,5 console=com1,vga"
GRUB_CMDLINE_LINUX="console=hvc0 earlyprintk=xen"

In /etc/inittab you need at least these lines:

1:2345:respawn:/sbin/getty 38400 hvc0
2:23:respawn:/sbin/getty 38400 tty1
# NO getty on ttyS0!

This way, tty1 will show up at the VGA output, and the hvc0 will show up at the serial console.

To keep both Xen and dom0 kernel output on the same tty, just omit the "vga"-related settings from the above setup.

If you need to debug Xen and see a crash dump of the kernel, you can do it using IPMITool if your server has SOL:

ipmitool -I lanplus -H server-ip-address -U your-username sol activate | tee my-log-file.txt

Installation as a DomU (guest)

Using xen-tools

xen-tools is a set of scripts which can easily create fully configured Xen guest domains.

Once you have installed dom0 you can install xen-tools on your host with:

apt-get install xen-tools

Then you can create virtual machines with this command:

xen-create-image --hostname <hostname> --ip <ip> --vcpus 2 --pygrub --dist <lenny|squeeze|maverick|whatever>

To configure xen-tools, you can edit /etc/xen-tools/xen-tools.conf which contains default values that the xen-create-image script will use. The xen-create-image(8) manual page contains information on the available options.

Possible problems and bugs

Using Debian Installer

The Xen wiki page Debian Guest Instalation Using DebianInstaller contains instructions on how to install Xen DomU from Lenny onwards using ?Debian Installer.

Upgrading/transition

See also: Debian Release Notes

Upgrading a server to Squeeze that uses both Lenny Dom0 and DomU's is fairly straightforward. There are a few catches that one needs to be aware of however: Reference

Note on kernel version compatibility

The new 2.6.32 kernel images have paravirt_ops-based Xen dom0 and domU support.

When you create an image for a modern Debian or Ubuntu domU machine, it will include a kernel that has pv_ops domU support, it will therefore not use a Xen kernel, but the "stock" one, as it is capable of running on Xen's hypervisor.

Possible problems and bugs

Older Releases

Xen Installation on Debian 5.0 ( Lenny )

Xen Installation on Debian 4.0 ( Etch )

The page DebianInstaller/Xen contains instructions on how to install Xen Dom0 and Etch DomU with DebianInstaller.

Package maintenance

Debian's Xen packages are maintained by the pkg-xen project. (developers' mailing list)

The Debian Developer's Package Overview page lists source packages that are maintained by the team.

Common Errors

dom0 automatic reboots

Edit /etc/default/grub and add the "noreboot" option to GRUB_CMDLINE_XEN, for example:

GRUB_CMDLINE_XEN="noreboot"

Error "Device ... (vif) could not be connected"

You need to configure some basic networking between dom0 and domU.

The recommended way to do this is to configure bridging in /etc/networking/interfaces. See BridgeNetworkConnections and/or the Xen wiki page Host Configuration/Networking for details.

'clocksource/0: Time went backwards'

If a domU crashes or freezes while uttering the famous lasts words 'clocksource/0: Time went backwards' see Xen/Clocksource.

PV drivers on HVM guest

It may be possible to build the PV drivers for use on HVM guests. These drivers are called unmodified_drivers and are part of the xen-unstable.hg repository. You can fetch the repository using mercurial thus:

The drivers reside under xen-unstable.hg/unmodified_drivers/linux-2.6. The README in this directory gives compilation instructions.

A somewhat dated, detailed set of instructions for building these drivers can be found here:

http://wp.colliertech.org/cj/?p=653

Resources