Installing Xen virtualization on CentOS

Posted by Jerzy Seweryn on

Installing Xen On CentOS 5.0 (i386)


Version 1.0
Author: Falko Timme
Last edited 06/08/2007

This tutorial provides step-by-step instructions on how to install Xen (version 3.0.3) on a CentOS 5.0 system (i386).

Xen lets you create guest operating systems (*nix operating systems like Linux and FreeBSD), so called "virtual machines" or domUs, under a host operating system (dom0). Using Xen you can separate your applications into different virtual machines that are totally independent from each other (e.g. a virtual machine for a mail server, a virtual machine for a high-traffic web site, another virtual machine that serves your customers' web sites, a virtual machine for DNS, etc.), but still use the same hardware. This saves money, and what is even more important, it's more secure. If the virtual machine of your DNS server gets hacked, it has no effect on your other virtual machines. Plus, you can move virtual machines from one Xen server to the next one.

I will use CentOS 5.0 (i386) for both the host OS (dom0) and the guest OS (domU).

This howto is meant as a practical guide; it does not cover the theoretical backgrounds. They are treated in a lot of other documents in the web.

This document comes without warranty of any kind! I want to say that this is not the only way of setting up such a system. There are many ways of achieving this goal but this is the way I take. I do not issue any guarantee that this will work for you!

1 Preliminary Note


I use the following partitions on my CentOS 5.0 host system (dom0):

  • /boot 150 MB (ext3)

  • swap 1GB

  • / 3GB (ext3)

  • /vm the rest (ext3)


I will create the virtual machines in the /vm directory; of course, you can use any other directory that has enough space left, and you don't have to create a partition of its own for it. If you use another directory, replace /vm with your own directory in this tutorial.

If you want to save your virtual machines in /vm, too, but haven't created a partition for it of if the directory /vm doesn't exist on your system, you can create it like this:

[code lang="shell"]

mkdir /vm

[/code]

(Please note: You don't need a /boot partition, but then you have to keep in mind that the Grub stanzas I describe in this howto are slightly different. For example, when I write that I add






[...]
title CentOS (2.6.18-8.1.4.el5xen)
 root (hd0,0)
 kernel /xen.gz-2.6.18-8.1.4.el5
 module /vmlinuz-2.6.18-8.1.4.el5xen ro root=/dev/VolGroup00/LogVol00
 module /initrd-2.6.18-8.1.4.el5xen.img
[...]


to /boot/grub/menu.lst then you should probably use






[...]
title CentOS (2.6.18-8.1.4.el5xen)
 root (hd0,0)
 kernel /boot/xen.gz-2.6.18-8.1.4.el5
 module /boot/vmlinuz-2.6.18-8.1.4.el5xen ro root=/dev/VolGroup00/LogVol00
 module /boot/initrd-2.6.18-8.1.4.el5xen.img
[...]


in that file instead...)

2 Installing Xen


To install Xen, we simply run

[code lang="shell"]

yum install kernel-xen xen

[/code]

This installs Xen and a Xen kernel on our CentOS system. Afterwards, we can find our new Xen kernel (vmlinuz-2.6.18-8.1.4.el5xen) and its ramdisk (initrd-2.6.18-8.1.4.el5xen.img) in the /boot directory:

[code lang="shell"]

ls -l /boot/

[[email protected] ~]# ls -l /boot/
total 16327
-rw-r--r-- 1 root root 62154 Apr 9 16:30 config-2.6.18-8.1.1.el5
-rw-r--r-- 1 root root 61057 May 17 12:12 config-2.6.18-8.1.4.el5xen
-rw-r--r-- 1 root root 62150 Mar 16 01:19 config-2.6.18-8.el5
drwxr-xr-x 2 root root 1024 Jun 8 00:14 grub
-rw------- 1 root root 2318110 Apr 13 17:10 initrd-2.6.18-8.1.1.el5.img
-rw------- 1 root root 2320081 Jun 8 00:14 initrd-2.6.18-8.1.4.el5xen.img
-rw------- 1 root root 2318126 Apr 13 16:18 initrd-2.6.18-8.el5.img
drwx------ 2 root root 12288 Apr 13 16:05 lost+found
-rw-r--r-- 1 root root 80032 Apr 1 16:49 message
-rw-r--r-- 1 root root 83542 Apr 9 16:31 symvers-2.6.18-8.1.1.el5.gz
-rw-r--r-- 1 root root 84906 May 17 12:13 symvers-2.6.18-8.1.4.el5xen.gz
-rw-r--r-- 1 root root 83542 Mar 16 01:20 symvers-2.6.18-8.el5.gz
-rw-r--r-- 1 root root 884787 Apr 9 16:30 System.map-2.6.18-8.1.1.el5
-rw-r--r-- 1 root root 868062 May 17 12:12 System.map-2.6.18-8.1.4.el5xen
-rw-r--r-- 1 root root 884787 Mar 16 01:19 System.map-2.6.18-8.el5
-rw-r--r-- 1 root root 1765460 Apr 9 16:30 vmlinuz-2.6.18-8.1.1.el5
-rw-r--r-- 1 root root 2075341 May 17 12:12 vmlinuz-2.6.18-8.1.4.el5xen
-rw-r--r-- 1 root root 1765428 Mar 16 01:19 vmlinuz-2.6.18-8.el5
-rw-r--r-- 1 root root 274228 May 17 09:13 xen.gz-2.6.18-8.1.4.el5
-rwxr-xr-x 1 root root 608568 May 17 12:28 xen-syms-2.6.18-8.1.4.el5
[[email protected] ~]#

[/code]

Before we can boot the system with the Xen kernel, we must tell the bootloader GRUB about it. We open /boot/grub/menu.lst:

[code lang="shell"]

vi /boot/grub/menu.lst

[/code]

and add the following stanza above all other kernel stanzas:






[...]
title CentOS (2.6.18-8.1.4.el5xen)
 root (hd0,0)
 kernel /xen.gz-2.6.18-8.1.4.el5
 module /vmlinuz-2.6.18-8.1.4.el5xen ro root=/dev/VolGroup00/LogVol00
 module /initrd-2.6.18-8.1.4.el5xen.img
[...]


Then change the value of default to 0:






[...]
default=0
[...]


The complete /boot/grub/menu.lst should look something like this:






# grub.conf generated by anaconda
#
# Note that you do not have to rerun grub after making changes to this file
# NOTICE:  You have a /boot partition.  This means that
#          all kernel and initrd paths are relative to /boot/, eg.
#          root (hd0,0)
#          kernel /vmlinuz-version ro root=/dev/VolGroup00/LogVol00
#          initrd /initrd-version.img
#boot=/dev/sda
default=0
timeout=5
splashimage=(hd0,0)/grub/splash.xpm.gz
hiddenmenu
title CentOS (2.6.18-8.1.4.el5xen)
 root (hd0,0)
 kernel /xen.gz-2.6.18-8.1.4.el5
 module /vmlinuz-2.6.18-8.1.4.el5xen ro root=/dev/VolGroup00/LogVol00
 module /initrd-2.6.18-8.1.4.el5xen.img
title CentOS (2.6.18-8.1.1.el5)
 root (hd0,0)
 kernel /vmlinuz-2.6.18-8.1.1.el5 ro root=/dev/VolGroup00/LogVol00
 initrd /initrd-2.6.18-8.1.1.el5.img
title CentOS (2.6.18-8.el5)
 root (hd0,0)
 kernel /vmlinuz-2.6.18-8.el5 ro root=/dev/VolGroup00/LogVol00
 initrd /initrd-2.6.18-8.el5.img


Afterwards, we reboot the system:

[code lang="shell"]

shutdown -r now

[/code]

The system should now automatically boot the new Xen kernel. After the system has booted, we can check that by running

[code lang="shell"]

uname -r

[[email protected] ~]# uname -r
2.6.18-8.1.4.el5xen
[[email protected] ~]#

[/code]

So it's really using the new Xen kernel!

We can now run

[code lang="shell"]

xm list

[/code]

to check if Xen has started. It should list Domain-0 (dom0):

[code lang="shell"]

[[email protected] ~]# xm list
Name ID Mem(MiB) VCPUs State Time(s)
Domain-0 0 350 1 r----- 94.4
[[email protected] ~]#

[/code]

3 Creating A Virtual Machine


CentOS comes with a nice tool called virt-install with which we can create virtual machines for Xen. To start it, we simply run

[code lang="shell"]

virt-install

[/code]

The tools asks a few questions before it creates a virtual machine. I want to call my first virtual machine vm01, with 256MB RAM and a disk size of 4GB. I want to store it in the file /vm/vm01.img:

[code lang="shell"]

<span>What is the name of your virtual machine?</span> <span><-- vm01</span>
<span>How much RAM should be allocated (in megabytes)?</span> <span><-- 256 </span>
<span>What would you like to use as the disk (path)?</span> <span><-- /vm/vm01.img</span>
<span>How large would you like the disk (/vm/vm01.img) to be (in gigabytes)?</span> <span><-- 4</span>
<span>Would you like to enable graphics support? (yes or no)</span> <span><-- no</span>
<span>What is the install location?</span> <span><-- http://wftp.tu-chemnitz.de/pub/linux/centos/5.0/os/i386</span>

[/code]

The question about the graphics support refers to the installer, not the virtual machine itself! It is possible to start a graphical installer, but you'd have to connect to it via VNC. It's easier to use the text installer - it offers the same options, so I choose the text installer.

As install location, you should specify a mirror close to you where the installer can download all files needed for the installation of CentOS 5.0 in our virtual machine. You can find a list of CentOS mirrors here: http://www.centos.org/modules/tinycontent/index.php?id=13

After we have answered all questions, virt-install starts the normal CentOS 5.0 installer (in text mode) in our vm01 virtual machine. You already know the CentOS installer, so it should be no problem for you to finish the CentOS installation in vm01.

After the installation, we stay at the vm01 console. To leave it, type CTRL+] if you are at the console, or CTRL+5 if you're using PuTTY. You will then be back at the dom0 console.

virt-install has created the vm01 configuration file /etc/xen/vm01 for us (in dom0). It should look like this:

cat /etc/xen/vm01






# Automatically generated xen config file
name = "vm01"
memory = "256"
disk = [ 'tap:aio:/vm/vm01.img,xvda,w', ]
vif = [ 'mac=00:16:3e:13:e4:81, bridge=xenbr0', ]

uuid = "5aafecf1-dd66-401d-69cc-151c1cb8ac9e"
bootloader="/usr/bin/pygrub"
vcpus=1
on_reboot   = 'restart'
on_crash    = 'restart'


Run

[code lang="shell"]

xm console vm01

[/code]

to log in on that virtual machine again (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.

To get a list of running virtual machines, type

[code lang="shell"]

xm list

[/code]

The output should look like this:

[code lang="shell"]

[[email protected] xen]# xm list
Name ID Mem(MiB) VCPUs State Time(s)
Domain-0 0 259 1 r----- 1906.6
vm01 3 255 1 ------ 137.9
[[email protected] xen]#

[/code]

To shut down vm01, do this:

[code lang="shell"]

xm shutdown vm01

[/code]

To start vm01 again, run

[code lang="shell"]

xm create /etc/xen/vm01

[/code]

If you want vm01 to start automatically at the next boot of the system, then do this:

[code lang="shell"]

ln -s /etc/xen/vm01 /etc/xen/auto

[/code]

Here are the most important Xen commands:

[code lang="shell"]

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

[/code]

4 Links



link : http://www.howtoforge.com/centos_5.0_xen

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