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Xen Overview

Modern computers are sufficiently powerful to use virtualization to present the illusion of many smaller virtual machines (VMs), each running a separate operating system instance. Successful partitioning of a machine to support the concurrent execution of multiple operating systems poses several challenges. Firstly, virtual machines must be isolated from one another: it is not acceptable for the execution of one to adversely affect the performance of another. This is particularly true when virtual machines are owned by mutually untrusting users. Secondly, it is necessary to support a variety of different operating systems to accommodate the heterogeneity of popular applications. Thirdly, the performance overhead introduced by virtualization should be small.

Xen is a virtual machine monitor for x86 that supports execution of multiple guest operating systems with unprecedented levels of performance and resource isolation. Xen is Open Source software, released under the terms of the GNU General Public License. We have a fully functional ports of Linux 2.6 running over Xen, and regularly use it for running demanding applications like MySQL, Apache and PostgreSQL. Any Linux distribution (RedHat, SuSE, Debian, Mandrake) should run unmodified over the ported OS.

In addition to Linux, members of Xen's user community have contributed or are working on ports to other operating systems such as NetBSD (Christian Limpach), FreeBSD (Kip Macy) and Plan 9 (Ron Minnich).

Different types of virtualization offered by Xen

There are two different types of virtualization offered by Xen:

Para-virtualization

A term used to describe a virtualization technique that allows the operating system to be aware that it is running on a hypervisor instead of base hardware. The operating system must be modified to accommodate the unique situation of running on a hypervisor instead of basic hardware.

Hardware Virtual Machine

A term used to describe an operating system that is running in a virtualized environment unchanged and unaware that it is not running directly on the hardware. Special hardware is required to allow this, thus the term HVM.

(Source: What is Xen Hypervisor, www.xen.org)

Domain 0 (Host) Installation

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 kernel and xen-tools. This can be done by a metapackage:

apt-get install xen-linux-system

To get Xen HVM support Xen 4.0 Wiki

apt-get install xen-qemu-dm-4.0

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 the default kernel to boot in two ways:

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

grep menuentry /boot/grub/grub.cfg

Count to the number of the first Xen kernel, starting at 0, and enter it in the default file. Note that this procedure is not robust against the installation or removal of new kernel versions which will cause the Xen entries to get different numbers, for this reason reordering the priorities using dpkg-divert as above is preferable.

After either of these procedures, do an update to the GRUB configuration:

update-grub

If you wish to improve this process there is an open bug discussing the default values after a Xen installation. #603832

To avoid getting boot entries for each virtual machine you install on a volume group, disable the GRUB OS prober.

/!\ ToDo: does this problem still happen and under what circumstances? Bug number?

Note that if you are running a computer with multi-boot with for example Windows, this will also remove the entries for it, which might not be what you wish for.

Edit /etc/default/grub and add:

# Disable OS prober to prevent virtual machines on logical volumes from appearing in the boot menu.
GRUB_DISABLE_OS_PROBER=true

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

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=""

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.

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)

Serial console access

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


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