3 Creating An Image-Based Virtual Machine

I want to store the disk images of my image-based virtual machines in the directory /vm. Of course, you can use any other directory that has enough space left. 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 or if the directory /vm doesn't exist on your system, you can create it like this:

mkdir /vm

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

virt-install

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:

Would you like a fully virtualized guest (yes or no)? This will allow you to run unmodified operating systems. <-- no
 What is the name of your virtual machine? <-- vm01
 How much RAM should be allocated (in megabytes)? <-- 256
 What would you like to use as the disk (file path)? <-- /vm/vm01.img
 How large would you like the disk (/vm/vm01.img) to be (in gigabytes)? <-- 4
 Would you like to enable graphics support? (yes or no) <-- no
 What is the virtual CD image, CD device or install location? <-- http://wftp.tu-chemnitz.de/pub/linux/centos/5.3/os/x86_64

You should answer the first question about the fully virtualized guest with no because otherwise you will not see the CentOS installer - the installation of the guest system will seem to hang.

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 your CD/DVD drive (/dev/cdrom) or a mirror close to you where the installer can download all files needed for the installation of CentOS 5.3 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 5.3 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

name = "vm01"
uuid = "986d8b21-906f-8d64-bdc4-a6098e75cf23"
maxmem = 256
memory = 256
vcpus = 1
bootloader = "/usr/bin/pygrub"
on_poweroff = "destroy"
on_reboot = "restart"
on_crash = "restart"
vfb = [  ]
disk = [ "tap:aio:/vm/vm01.img,xvda,w" ]
vif = [ "mac=00:16:3e:08:fc:38,bridge=xenbr0" ]

Run

xm console vm01

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

xm list

The output should look like this:

[root@server1 ~]# xm list
Name                                      ID Mem(MiB) VCPUs State   Time(s)
Domain-0                                   0     3102     2 r-----     78.6
vm01                                       1      255     1 -b----    237.6
[root@server1 ~]#

To shut down vm01, do this:

xm shutdown vm01

To start vm01 again, run

xm create /etc/xen/vm01

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

ln -s /etc/xen/vm01 /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.

4 Creating An LVM-Based Virtual Machine

This chapter explains how you can set up LVM-based virtual machines instead of virtual machines that use disk images. Virtual machines that use disk images are very slow and heavy on disk IO.

In this example I'm using a CentOS 5.3 host with the LVM volume group /dev/VolGroup00 that has about 150GB of space. /dev/VolGroup00 contains two logical volumes, /dev/VolGroup00/LogVol00 and /dev/VolGroup00/LogVol01 that consume about 60GB of space - the rest is not allocated and can be used to create logical volumes for our virtual machines:

vgdisplay

[root@server1 ~]# vgdisplay
  --- Volume group ---
  VG Name               VolGroup00
  System ID
  Format                lvm2
  Metadata Areas        1
  Metadata Sequence No  14
  VG Access             read/write
  VG Status             resizable
  MAX LV                0
  Cur LV                3
  Open LV               3
  Max PV                0
  Cur PV                1
  Act PV                1
  VG Size               148.53 GB
  PE Size               32.00 MB
  Total PE              4753
  Alloc PE / Size       1906 / 59.56 GB
  Free  PE / Size       2847 / 88.97 GB
  VG UUID               5faE1k-DkMu-JUEk-K0JV-B9ta-Nyaf-n7tngf

[root@server1 ~]#

lvdisplay

[root@server1 ~]# lvdisplay
  --- Logical volume ---
  LV Name                /dev/VolGroup00/LogVol01
  VG Name                VolGroup00
  LV UUID                xA3e1Z-mEc9-rGT1-WcAu-TjF4-lbf3-6LvFaj
  LV Write Access        read/write
  LV Status              available
  # open                 1
  LV Size                992.00 MB
  Current LE             31
  Segments               1
  Allocation             inherit
  Read ahead sectors     auto
  - currently set to     256
  Block device           253:0

  --- Logical volume ---
  LV Name                /dev/VolGroup00/LogVol00
  VG Name                VolGroup00
  LV UUID                mUt9QZ-dluZ-a1ud-5RA7-gkVA-6Z0b-w0pvIa
  LV Write Access        read/write
  LV Status              available
  # open                 1
  LV Size                58.59 GB
  Current LE             1875
  Segments               1
  Allocation             inherit
  Read ahead sectors     auto
  - currently set to     256
  Block device           253:1

[root@server1 ~]#

I want to create the virtual machine vm02 now which uses the logical volume /dev/VolGroup00/vm02. I want the virtual machine to have a disk space of 10GB, so I create the logical volume /dev/VolGroup00/vm02 as follows:

lvcreate -L10G -n vm02 VolGroup00

Afterwards we can run

virt-install

again:

Would you like a fully virtualized guest (yes or no)? This will allow you to run unmodified operating systems. <-- no
 What is the name of your virtual machine? <-- vm02
 How much RAM should be allocated (in megabytes)? <-- 256
 What would you like to use as the disk (file path)? <-- /dev/VolGroup00/vm02
 Would you like to enable graphics support? (yes or no) <-- no
 What is the install location? <-- http://wftp.tu-chemnitz.de/pub/linux/centos/5.3/os/x86_64

As the disk file path, we specify our new volume group /dev/VolGroup00/vm02. Please note that virt-install doesn't ask for the disk space anymore because the disk space is determined by the size of the logical volume (10GB).

5 Links

• Xen: http://www.xensource.com/xen
• CentOS: http://www.centos.org