Xen Beginners Guide - Xen

7/29/2019 Xen Beginners Guide - Xen

1/12

Xen Beginners Guide

From Xen

Welcome!

This guide was written to introduce beginners to basic Xen concepts and allow you to get started with Xen

with no prior knowledge. Some prior Linux experience is required however, knowledge of networking, lvm

and grub will go a long way!

By completing this guide you will have installed a fully functional Xen hypervisor and started your first

guest operating systems, connected them to your network and have been introduced to fundamental concepts

such as virtual machine storage and virtual networking. To make this process easy we will be using a Linux

distribution called Debian. Debians current stable release Squeeze ships with support for Xen 4 and

everything you need to get started!

Though this guide looks long at first dont be daunted, it is very indepth and comprehensive but doesnt

expect you to know all that much beforehand. The goal instead is to teach you all the things you need to

know to build a functioning Xen hypervisor. :)

Contents

1 What is Xen all about?

2 Preparation

2.1 Enable virtualization support in BIOS2.2 Download and burn Debian Squeeze Installer CD

2.3 Quick intro to Debian

3 Installing Debian Squeeze

4 Setup LVM storage for guests

5 Setup Linux Bridge for guest networking

6 Installing Xen

7 Configure GRUB to start Xen

8 Basic Xen commands

9 Creating a Debian PV (Paravirtualized) Guest

10 Creating a Windows HVM (Hardware Virtualized) Guest11 Conclusion

What is Xen all about?

Xen is a Virtual Machine Monitor (VMM) also known as a hypervisor, this is a software system that allows

the execution of multiple virtual guest operating systems simultaneously on a single physical machine. Xen

is known as a Type 1 or bare-metal hypervisor, meaning that it runs directly ontop of the physical machine

as opposed to within an operating system.

Guest virtual machines running on Xen are known as domains and a special domain known as dom0 is

responsible for controlling the hypervisor and starting other guest operating systems. These other guest

Beginners Guide - Xen http://wiki.xen.org/wiki/Xen_Beginners_Guide#Basic_Xen_commands

12 11/03/2013 12:21 p.m.


2/12

operating systems are called domUs, this is because these domains are unprivileged in the sense they

cannot control Xen or start/stop other domains.

Xen supports 2 primary types of virtualization, para-virtualization and hardware virtual machine (HVM)

also known as full virtualization. Para-virtualization uses modified guest operating systems that we refer

to as enlightened guests. These operating systems are aware that they are being virtualized and as such dont

require virtual hardware devices, instead they make special calls to Xen that allow them to access CPUs,

storage and network resources.

In contrast HVM guests need not be modified as Xen will create a fully virtual set of hardware devices for

this machine that resemble a physical x86 computer. This emulation requires much more overhead than the

paravirtualisation approach but allows unmodified guest operating systems like Microsoft Windows to run

ontop of Xen. HVM support requires special CPU extensions - VT-x for Intel processors and AMD-V for

AMD based machines. This technology is now prevalent and all recent servers and desktop systems should

be equipped with them.

A third type of virtualization though not discussed in this guide is called PVHVM or Para-virtualisation on

HVM which is a HVM domain with paravirtualized storage, network and other devices. This provides the

best of both worlds by reducing expensive emulation but providing hardware accelerated CPU and memoryaccess. A brief look at Xen architecture To understand how storage, networking and other resources are

delivered to guest systems we need to quickly delve into how the different bits of Xen interact.

The dom0 forms the interface to the hypervisor, through special instructions the dom0 communicates to Xen

and changes the configuration of the hypervisor. This includes instantiating new domains and related tasks.

Another crucial part of the dom0s role is that it is the primary interface to the hardware. Xen doesntcontain device drivers, instead the devices are attached to dom0 and you can use standard Linux drivers.

Dom0 then shares these resources with guest operating systems through a number of backend deamons.

Each para-virtualized subsystem in Xen consists of 2 parts: 1) the aforementioned backend that lives in

dom0, and 2) the frontend driver within the guest domain. The backend is effectively a deamon which

uses special ring buffer based interfaces to transfer data to guests, be it to provide a virtual hard-disk,

Ethernet adapter of even a generic SCSI device. The frontend driver then takes this stream of data and

converts it back into a device within the guest operating system.

The two important paravirtualisation systems are: net-back/net-front, and blk-back/blk-front - which are

the paravirtualized networking and storage systems, respectively.

You can read more about how Xen is architected, paravirtualization and the benefits of such here:

Preparation

This guide requires a number of items, this checklist is what you will need:

64bit x86 computer with atleast 1GB of RAM (this can be a server,desktop or laptop!)

(Optional) VT-d or AMD-V support

Sufficient storage space for your dom0 and whatever guests you want to install

A CD burner + blank CD (you can use a USB but this is not covered here)


12 11/03/2013 12:21 p.m.


3/12

Internet access for downloading and installing Debian

(Optional) Windows Server 2008R2 installation ISO, a trial copy is sufficient

(Optional) VNC client for installing HVM domain

Enable virtualization support in BIOS

NOTE: This is optional and not required for PV guests

In order to support HVM guests we need to ensure that virtualization extensions are enabled in the BIOS. If

you dont wish to start a HVM guest you can skip this step but it is still highly recommended. If your system

doesnt support these extensions you cannot use Xen to virtualize unmodified operating systems, however

para-virtualization will work fine.

The virtualization option appears differently in different BIOS builds but generally it is refered to as Enable

Virtualisation Technology or Enable Intel VT for Intel chipsets, however in some cases it can be listed as

Vanderpool Technology. Oftentimes this option can be found under the Advanced Chipset Features

menu in the BIOS. Similar also for AMD.

Consult your motherboard documentation for more assistance in enabling virtualization extensions on your

system.

Download and burn Debian Squeeze Installer CD

Download the ISO image from this URL:

http://cdimage.debian.org/debian-cd/current/amd64/iso-cd/

The netinst image is sufficient for our purposes.

Burn the ISO to disk using your computers standard utilities. I recommend wodim on Linux or the built in

ISO burning feature in Windows.

Quick intro to Debian

Debian is a simple, stable and well supported Linux distribution. It has included Xen support since Debian

3.1 Sarge released in 2005. The current stable release Debian 6.0 Squeeze ships with Xen 4.0.1 and a

Xen enabled Linux 2.6.32 kernel.

Debian uses the simple Apt package management system which is both powerful and simple to use.

Installing a package is as simple as the following example:

aptitude install htop

Where htop was the application desired to install.

Simple tasks such as configuring startup scripts, setting up the network etc are covered by this tutorial so

dont worry if you havent used Debian before!

Many popular distributions are based off of Debian and also use the Apt package manager, if you have used

Ubuntu, Linux Mint or Damn Small Linux you will feel right at home.


12 11/03/2013 12:21 p.m.


4/12

Installing Debian Squeeze

Boot the Debian Squeeze Installer CD Insert the Debian CD and configure the CDROM drive as your

default boot device in the BIOS or use the system bootmenu if your BIOS supports it (usually F12).

You should see a menu, choose the default Install option to begin the installation process. Install the

system The Debian installer is very straight forward. Follow the prompts until you reach the diskpartitioning section.

Choose advanced/custom, we are going to configure a few partitions here, one for /boot another for /, one

more for swap and a final partition to setup as an LVM volume group for our guest machines.

First create the /boot partition by choosing the disk and hitting enter, make the partition 300MB and format

it as ext2, choose /boot as the mountpoint.

Repeat the process for / but of course changing the mountpoint to / and making it 15GB or so large. Format

it as ext3.

Create another partition approximately 1.5x the amount of RAM you have in size and elect to have it used as

a swap volume.

Finally create a partition that consumes the rest of the diskspace but dont format it or assign a mount point.

We should now have a layout that looks like this assuming your disk device is /dev/sda :

sda1 - /boot 200MB

sda2 - / 15GB

sda3 - swap

sda4 - reserved for LVM

When you reach the package selection stage only install the base system, we wont require any GUI or other

packages for this guide.

Continue through the installer then reboot and login at the prompt as root.

Setup LVM storage for guests

LVM is the Linux Logical Volume manager. It is a technology that allows Linux to manage block devices in

a more abstract manner.

LVM introduces the concept of a logical volume, effectively a virtualized block device composed of

blocks written to 1 or more physical devices. These blocks dont need to be contiguous unlike proper disk

partitions.

Because of this abstraction logical volumes can be created, deleted, resized and even snapshotted without

affecting other logical volumes.

LVM creates logical volumes within what is called a volume group, which is simply a set of logical volumes

that share the same physical storage, known as physical volumes.

The process of setting up LVM can be summarized as allocating a physical volume, creating a volume group


12 11/03/2013 12:21 p.m.


5/12

on top of this, then creating logical volumes to store data.

Because of these features and superior performance over file backed virtual machines I recommend the use

of LVM if you are going to store VM data locally.

Now lets install LVM and get started!

Install LVM with aptitude by running this command:

aptitude install lvm2

Now that we have LVM installed lets configure it to use /dev/sda4 as its physical volume

pvcreate /dev/sda4

Ok, now LVM has somewhere to store its blocks (known as extents for future reference). Lets create a

volume group called vg0 using this physical volume:

vgcreate vg0 /dev/sda4

Now LVM is setup and initialized so that we can later create logical volumes for our virtual machines.

For the interested below is a number of useful commands and tricks when using LVM.

Create a new logical volume:

lvcreate -n -L

For example, creating a 100 gigabyte volume called database-data on a volume group called vg0.

lvcreate -ndatabase-data -L100G vg0

You can then remove this volume with the following:

lvremove /dev/vg0/database-data

Note that you have to provide the path to the volume here.

If you already have a volume setup that you would like to copy, LVM has a cool feature that allows you to

create a CoW (copy on write) clone called a snapshot. This means that you can make an "instant" copy that

will only store the changes compared to the original. There are a number of caveats to this that will be

discussed in a yet unwritten article. The most important thing to note is that the "size" of the snapshot is only

the amount of space allocated to store changes. So you can make the snapshot "size" a lot smaller than the

source volume.

To create a snapshot use the following command:

lvcreate -s /dev/vg0/database-data -ndatabase-backup -L5G

Once again note the use of the full path.


12 11/03/2013 12:21 p.m.


6/12

Setup Linux Bridge for guest networking

Next we need to setup our system so that we can attach virtual machines to the external network. This is

done by creating a virtual switch within dom0 that takes packets from the virtual machines and forwards

them onto the physical network so they can see the internet and other machines on your network.

The piece of software we use to do this is called the Linux bridge and its core components reside inside theLinux kernel. In this case the bridge is effectively our virtual switch. Our Debian kernels are compiled

with the Linux bridging module so all we need to do is install the control utilities.

aptitude install bridge-utils

With bridge-utils installed we now have a utility called brctl this utility talks to the Linux bridging module

to setup new bridges and attach physical or virtual interfaces to these bridges.

brctl can be used to create a bridge as such, where is the name of the bridge:

brctl addbr

And interfaces can be added to that bridge by running the following:

brctl addif

Instead of calling brctl directly we are instead going to configure our bridge through Debians networking

infrastructure which can be configured through

/etc/network/interfaces

Open this file with the editor of your choice (more editors can be installed with aptitude) Nano is installed

by default if you selected the minimal install

nano /etc/network/interfaces

Depending on your hardware you probably see a file pretty similar to this:

auto lo

iface lo inet loopback

auto eth0

iface eth0 inet dhcp

This file is very simple. Each stanza represents a single interface. Breaking it down auto eth0 means that

eth0 will be configured when ifup -a is run (which is run a boot time) what this means is that the interface

with automatically be started/stopped for you. iface eth0 then describes the interface itself, in this case it

merely specifies that it should be configured by DHCP - we are going to assume that you have DHCP

running on your network for this guide. If you are using static addressing you probably know how to set thatup. We are going to edit this file so it resembles such:

auto lo


12 11/03/2013 12:21 p.m.


7/12

iface lo inet loopback

iface eth0 inet manual

auto xenbr0

iface xenbr0 inet dhcp

bridge_ports eth0

This will setup the bridge xenbr0 and add eth0 to the bridge for us. The equivalent commands would be:

brctl addbr xenbr0

brctl addif xenbr0 eth0

dhclient xenbr0

Except this will now be done automatically on boot and completely managed by Debian.

This method of network configuration is best practice and the automated scripts called by xend are now

deprecated, we will discuss this later once we have Xen installed.

Installing Xen

The Debian Xen packages consists primarily of a Xen enabled Linux kernel, the Xen hypervisor itself, a

modified version of QEMU that support Xens HVM mode and a set of userland tools.

All of this except QEMU can be installed via an Apt meta-package callled xen-linux-system. A

meta-package is basically a way of installing a group of packages automatically. Apt will ofcourse resolve

all dependencies and bring in all the extra libraries we need.

Lets install the xen-linux-system metapackage:

aptitude -P install xen-linux-system

Next we will install the Xen QEMU package so that we can boot HVM guests later (this is optional but

highly recommended)

aptitude install xen-qemu-dm

Now have Xen, a Xen kernel and the userland tools installed, almost ready to go.

Configure GRUB to start Xen

Because Xen starts before your operating system we need to change how your systems boot process is setup.

The bootloader installed during installation called GRUB is what tells your computer which operating

system to start and how.

GRUB2 configuration is stored in the file /boot/grub/grub.cfg However we arent going to edit this file

directly, as it changes every time we update our kernel. Debian configures GRUB for us using a number of

automated scripts that handle upgrades etc, these scripts are stored in /etc/grub.d/* and are configured via

/etc/default/grub


12 11/03/2013 12:21 p.m.


8/12

We are going to change the order of the operating systems so that our Xen system is the default option. By

executing the below command we are moving Xen to a higher priority than default Linux so that it gets the

first position in the boot menu.

mv -i /etc/grub.d/20_linux_xen /etc/grub.d/09_linux_xen

We then generate the /boot/grub/grub.cfg file by running the command below:

update-grub

We can now reboot and the default boot option will be our Xen dom0 running ontop of Xen!

See also

Xen GRUB Boot Options

Basic Xen commandsBefore we dive into creating some guest domains we will quickly cover some basic Xen commands using

the xm utiltiy. Going forward it is likely that xm will be deprecated as previously mentioned but once xl is

stable and part of a supported release this guide will be rewritten to reflect that.

So lets start with simple stuff!

xl info

This returns the information about the Xen hypervisor and dom0 including version, free memory etc.

xl list

Lists running domains, their IDs. memory, state and CPU time consumed

xl top

Shows running domains in real time and is similar to the top command under Linux. This can be used tovisualize CPU, memory usage and block device access.

We will cover some more commands during the creation of our guest domains.

See also

Xen Man Pages

Creating a Debian PV (Paravirtualized) Guest

PV guests are notoriously different to install. Due to the nature of enlightened systems they dont have the

usual concepts of a CD-ROM drive installer analagous to their physical counterparts. However, luckly

enough there does exist some tools that help us prepare images or effectively snapshots of the operating


12 11/03/2013 12:21 p.m.


9/12

systems that are able to run inside of Xen domains.

Debian contains a number of tools for creating Xen guests. The easiest of which is known as xen-tools. This

software suite manages the downloading and installing of guest operating systems including both Debian

and RHEL based DomUs. In this guide we are going to use xen-tools to prepare a Debian Squeeze

paravirtualized domU.

Xen-tools can use LVM storage for storing the guest operating systems, in this guide we created the volume

group vg0 in the Setting up LVM Storage section.

When guests are paravirtualized there is no BIOS or bootloader resident within the guest filesystem and

for a long time guests were provided with kernels external to the guest image. This however is bad for

maintainability (guests cannot upgrade their kernels without access to the dom0) and is not as flexible in

terms of boot options as they must be passed via the config file.

The Xen community wrote a utility known as pygrub which is a python application for PV guests that

enables the dom0 to parse the GRUB configuration of the domU and extract its kernel, initrd and boot

parameters. This allows for kernel upgrades etc inside of our guest machines along with a GRUB menu.

Using pygrub or the stub-dom implementation known as pv-grub is best practice for starting PV guests. Insome cases pv-grub is arguably more secure but as it is not included with Debian we wont use it here

though it is recommended in production environments where guests cannot be trusted.

Apart from this PV guests are very similar to their HVM and physical OS counterparts. Configuring

xen-tools and building our guest First lets install the xen-tools package:

aptitude install xen-tools

We can now create a guest operating system with this tool. It effectly automates the process of setting up a

PV guest from scratch right to the point of creating config files and starting the guest. The process can besummarized as follows:

Create logical volume for rootfs

Create logical volume for swap

Create filesystem for rootfs

Mount rootfs

Install operating system using debootstrap (or rinse etc, only debootstrap covered here)

Run a series of scripts to generate guest config files like fstab/inittab/menu.lst

Create a Xen config file for the guest

Generate a root password for the guest systemUnmount the guest filesystem

These 9 steps can be carried out manually but the manual process is outside the scope of this guide. We

instead will execute the below command:

xen-create-image --hostname=tutorial-pv-guest \

--memory=512mb \

--vcpus=2 \

--lvm=vg0 \

--dhcp \

--pygrub \

--dist=squeeze

This command instructs xen-create-image (the primary binary of the xen-tools toolkit) to create a guest

domain with 512MB of memory, 2 vcpus, using storage from the vg0 volume group we created, use DHCP


12 11/03/2013 12:21 p.m.


10/12

for networking, pygrub to extract the kernel from the image when booted and lastly we specify that we want

to deploy a Debian Squeeze operating system.

This process will take a few minutes after which you can start the guest with:

xl create -c /etc/xen/tutorial-pv-guest.cfg

The -c in this command tells xm that we wish to connect to the guest virtual console. Which is aparavirtualised serial port within the domain that xen-create-image configured to listen with a getty. This is

analgous to running:

xl create /etc/xen/tutorial-pv-guest.cfg && xm console tutorial-pv-guest

You can leave the guest virtual console by pressing ctrl+] and re-enter it by running the xl console

command.

You can later shutdown this guest either from within the domain or from dom0 with the following:

xl shutdown tutorial-pv-guest

That completes our section on setting up your first paravirtualized domain! If you dont have any interest in

setting up a HVM domain then no need to read any further but it is highly recommended!

Creating a Windows HVM (Hardware Virtualized)

GuestHVM guests are quite a bit different to their PV counterparts. Because they require the emulation of

hardware there is more moving pieces that need to be configured etc.

The main point worth mentioning here is that HVM requires the emulation of ATA, Ethernet and other

devices, while virtualized CPU and Memory access is performed in hardware to achieve good performance.

Because of this the default emulated devices are very slow and we generally try to use PV drivers within

HVM domains. We will be installing a set of Windows PV drivers that greatly increase performance once

we have our Windows guest running.

This extra emulation is provided by a Xen modified version of QEMU we should have installed this earlierbut in case you skipped that step install the Xen QEMU package now:

aptitude install xen-qemu-dm

Once the necessary packages are installed we need to create a logical volume to store our Windows VM

hard disk, create a config file that tells Xen to start the domain in HVM mode and boot from the DVD in

order to install Windows.

First, create the new logical volume - name the volume "windows", set the size to 20GB and use the volume

group vg0 we created earlier.

lvcreate -nwindows -L20G vg0


f 12 11/03/2013 12:21 p.m.


11/12

Next open a new file with your text editor of choice:

nano windows.cfg

Paste the config below into the file and save it, NOTE this assumes your Windows iso is located in /root/

with the filename windows.iso.

kernel = "/usr/lib/xen-4.0/boot/hvmloader"

builder='hvm'

memory = 4096

vcpus=4

name = "ovm-1734"

vif = ['bridge=xenbr0']

disk = ['phy:/dev/vg0/windows,hda,w','file:/root/windows.iso,hdc:cdrom,r']

acpi = 1

device_model = 'qemu-dm'

boot="d"

sdl=0

serial='pty'

vnc=1

vnclisten=""

vncpasswd=""

You can then start the domain and connect to it via VNC from your graphical machine.

xl create windows.cfg

The VNC display should be available on port 5900 of your dom0 IP, for instance using gvncviewer:

gvncviewer :5900

Once you have installed Windows by formatting the disk and by following the prompts the domain will

restart - however this time we want to prevent it booting from DVD so destroy the domain with

xl destroy windows

Then change the boot line in the config file to read boot="c"' restart the domain with

xl create windows.cfg

Reconnect with VNC and finish the installatation. When this process is complete you should then procede to

download the GPLPV drivers for Windows by James Harper.

Signed drivers can be obtained from here:

http://wiki.univention.de/index.php?title=Installing-signed-GPLPV-drivers

Many thanks for Univention for making signed drivers available to the Xen community and ofcourse a

massive thanks to James for all his work on making Windows on Xen such a smooth experience. On

finalizing the installation and rebooting you should notice much improved disk and network performanceand Xen will now be able to gracefully shutdown your Windows domains.


f 12 11/03/2013 12:21 p.m.


12/12

Conclusion

That concludes our introduction to Xen, by now you can setup both PV and HVM domains on a bare dom0

hypervisor!

You can now move onto building your own guest images or try out some prebuilt Guest VM Images.

Retrieved from "http://wiki.xen.org/wiki/Xen_Beginners_Guide"

Categories: Xen | Beginners | Tutorial | Debian

This page was last modified on 23 January 2013, at 23:38.

Xen.org's servers are hosted with RackSpace, monitoring our

servers 24x7x365 and backed by RackSpace's Fanatical Support.


Documents

Xen Beginners Guide - Xen