Virtualization With Xen 3.3.1 On Debian Etch - Page 3

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Submitted by falko (Contact Author) (Forums) on Tue, 2009-02-10 18:17. ::

5 Creating LVM-Based Virtual Machines

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 Debian Etch host with the LVM volume group /dev/vg0 that has about 50GB of space. /dev/vg0 contains two logical volumes, /dev/vg0/root and /dev/vg0/swap_1 that consume about 12GB of space - the rest is not allocated and can be used to create logical volumes for our virtual machines:

vgdisplay

server1:~# vgdisplay
  --- Volume group ---
  VG Name               vg0
  System ID
  Format                lvm2
  Metadata Areas        1
  Metadata Sequence No  3
  VG Access             read/write
  VG Status             resizable
  MAX LV                0
  Cur LV                2
  Open LV               2
  Max PV                0
  Cur PV                1
  Act PV                1
  VG Size               49.52 GB
  PE Size               4.00 MB
  Total PE              12678
  Alloc PE / Size       3072 / 12.00 GB
  Free  PE / Size       9606 / 37.52 GB
  VG UUID               iXaW0I-RkGD-WQlQ-dMaD-qfM5-xkvq-2w5uP5

server1:~#

lvdisplay

Now we edit /etc/xen-tools/xen-tools.conf. This file contains the default values that are used by the xen-create-image script unless you specify other values on the command line. I changed the following values and left the rest untouched:

vi /etc/xen-tools/xen-tools.conf

[...]
lvm = vg0
[...]
dist   = etch     # Default distribution to install.
[...]
gateway   = 192.168.0.1
netmask   = 255.255.255.0
broadcast = 192.168.0.255
[...]
passwd = 1
[...]
kernel      = /boot/vmlinuz-`uname -r`
initrd      = /boot/initrd.img-`uname -r`
[...]
mirror = http://ftp.de.debian.org/debian/
[...]
serial_device = hvc0 #default
[...]
disk_device = xvda #default
[...]

Make sure that you uncomment the lvm line and fill in the name of your volume group (vg0 in my case). At the same time make sure that the dir line is commented out!

dist specifies the distribution to be installed in the virtual machines (Debian Etch) (there's a comment in the file that explains what distributions are currently supported).

The passwd = 1 line makes that you can specify a root password when you create a new guest domain.

In the mirror line specify a Debian mirror close to you.

Make sure you specify a gateway, netmask, and broadcast address. If you don't, and you don't specify a gateway and netmask on the command line when using xen-create-image, your guest domains won't have networking even if you specified an IP address!

It is very important that you add the line serial_device = hvc0 (or serial_device = xvc0) because otherwise your virtual machines might not boot properly!

Now let's create our first guest domain, xen1.example.com, with the IP address 192.168.0.101:

xen-create-image --hostname=xen1.example.com --size=4Gb --swap=256Mb --ip=192.168.0.101 --memory=128Mb --arch=i386 --role=udev

Options that you specify on the command line override the settings in /etc/xen-tools/xen-tools.conf. Options that are not specified on the command line are taken from /etc/xen-tools/xen-tools.conf. Please make sure that you add --role=udev, or your virtual machine might not boot properly!

(To learn more about the available options, take a look at the xen-create-image man page:

man xen-create-image

)

The xen-create-image command will now create the xen1.example.com virtual machine for us. This can take a few minutes. The output should be similar to this one:

server1:~# xen-create-image --hostname=xen1.example.com --size=4Gb --swap=256Mb --ip=192.168.0.101 --memory=128Mb --arch=i386 --role=udev

General Information
--------------------
Hostname       :  xen1.example.com
Distribution   :  etch
Partitions     :  swap            256Mb (swap)
                  /               4Gb   (ext3)
Image type     :  full
Memory size    :  128Mb
Kernel path    :  /boot/vmlinuz-2.6.18.8-xen
Initrd path    :  /boot/initrd.img-2.6.18.8-xen

Networking Information
----------------------
IP Address 1   : 192.168.0.101 [MAC: 00:16:3E:1C:B9:F3]
Netmask        : 255.255.255.0
Broadcast      : 192.168.0.255
Gateway        : 192.168.0.1

Creating swap on /dev/vg0/xen1.example.com-swap
Done

Creating ext3 filesystem on /dev/vg0/xen1.example.com-disk
Done
Installation method: debootstrap
Done

Running hooks
Done

Role: udev
        File: /etc/xen-tools/role.d/udev
Role script completed.

Creating Xen configuration file
Done
Setting up root password
Enter new UNIX password:
Retype new UNIX password:
passwd: password updated successfully
All done

Logfile produced at:
         /var/log/xen-tools/xen1.example.com.log
server1:~#

As you see from the output, xen-create-image has created a new logical volume for our VM in the vg0 volume group, /dev/vg0/xen1.example.com-disk, for the VM's root filesystem. Take a look at

lvdisplay

and you will see that it has also created a second logical volume, /dev/vg0/xen1.example.com-swap, for the VM's swap:

server1:~# lvdisplay
  --- Logical volume ---
  LV Name                /dev/vg0/root
  VG Name                vg0
  LV UUID                17thCh-CgS8-ZvrJ-FOai-B1AX-89sv-xxq12a
  LV Write Access        read/write
  LV Status              available
  # open                 1
  LV Size                10.00 GB
  Current LE             2560
  Segments               1
  Allocation             inherit
  Read ahead sectors     0
  Block device           253:0

  --- Logical volume ---
  LV Name                /dev/vg0/swap_1
  VG Name                vg0
  LV UUID                u2EZyP-3Vp5-Lr32-nquG-ub5a-f0U5-VI3WRN
  LV Write Access        read/write
  LV Status              available
  # open                 1
  LV Size                2.00 GB
  Current LE             512
  Segments               1
  Allocation             inherit
  Read ahead sectors     0
  Block device           253:1

  --- Logical volume ---
  LV Name                /dev/vg0/xen1.example.com-swap
  VG Name                vg0
  LV UUID                No03G8-PzA3-67JH-m936-rUQY-rZFi-djIu21
  LV Write Access        read/write
  LV Status              available
  # open                 0
  LV Size                256.00 MB
  Current LE             64
  Segments               1
  Allocation             inherit
  Read ahead sectors     0
  Block device           253:2

  --- Logical volume ---
  LV Name                /dev/vg0/xen1.example.com-disk
  VG Name                vg0
  LV UUID                xDwpFO-XALQ-7I1N-TA3k-paYc-biG7-TjrNIM
  LV Write Access        read/write
  LV Status              available
  # open                 0
  LV Size                4.00 GB
  Current LE             1024
  Segments               1
  Allocation             inherit
  Read ahead sectors     0
  Block device           253:3

server1:~#

There should now be a xen1.example.com configuration file - /etc/xen/xen1.example.com.cfg. Open it and add the line extra = 'xencons=hvc0 console=hvc0' (or extra = 'xencons=xvc0 console=xvc0' if you use serial_device = xvc0 in /etc/xen-tools/xen-tools.conf) at the end so that the file looks as follows:

vi /etc/xen/xen1.example.com.cfg

#
# Configuration file for the Xen instance xen1.example.com, created
# by xen-tools 4.1 on Sun Feb  8 22:01:30 2009.
#

#
#  Kernel + memory size
#
kernel      = '/boot/vmlinuz-2.6.18.8-xen'
ramdisk     = '/boot/initrd.img-2.6.18.8-xen'

memory      = '128'

#
#  Disk device(s).
#
root        = '/dev/xvda2 ro'
disk        = [
                  'phy:/dev/vg0/xen1.example.com-disk,xvda2,w',
                  'phy:/dev/vg0/xen1.example.com-swap,xvda1,w',
              ]


#
#  Physical volumes
#



#
#  Hostname
#
name        = 'xen1.example.com'

#
#  Networking
#
vif         = [ 'ip=192.168.0.101,mac=00:16:3E:1C:B9:F3' ]

#
#  Behaviour
#
on_poweroff = 'destroy'
on_reboot   = 'restart'
on_crash    = 'restart'

extra = 'xencons=hvc0 console=hvc0'

If you don't add that line and you go to the console of the vm using the xm console command, the boot process seems to hang forever (while in fact, the vm is running perfectly fine - you can connect to it using SSH).

(If we had used disk images instead of logical volumes, the disk line would look similar to this one:

disk = [ 'file:/path/to/xen1.example.com/disk.img,xvda1,w', 'file:/path/to/xen1.example.com/swap.img,xvda2,w' ]

)

(Please note: if you have a dual-core or quad-core CPU and want the virtual machine to use all CPU cores, please add the line vcpus = '2' or vcpus = '4' to the configuration file.)

To start the virtual machine, run

xm create /etc/xen/xen1.example.com.cfg

Run

xm console xen1.example.com

to log in on that virtual machine (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 (192.168.0.101).

(If you see something like this during the boot process of the vm...

EXT3-fs: mounted filesystem with ordered data mode.

  ***************************************************************
  ***************************************************************
  ** WARNING: Currently emulating unsupported memory accesses  **
  **          in /lib/tls glibc libraries. The emulation is    **
  **          slow. To ensure full performance you should      **
  **          install a 'xen-friendly' (nosegneg) version of   **
  **          the library, or disable tls support by executing **
  **          the following as root:                           **
  **          mv /lib/tls /lib/tls.disabled                    **
  ** Offending process: modprobe (pid=1757)                    **
  ***************************************************************
  ***************************************************************

Continuing...

... run the following command in the vm:

mv /lib/tls /lib/tls.disabled

)

To get a list of running virtual machines, type

xm list

The output should look like this:

server1:~# xm list
Name                                        ID   Mem VCPUs      State   Time(s)
Domain-0                                     0   874     1     r-----   1367.1
xen1.example.com                             2   128     1     ------     33.3
server1:~#

To shut down xen1.example.com, do this:

xm shutdown xen1.example.com

If you want xen1.example.com to start automatically at the next boot of the system, then do this:

ln -s /etc/xen/xen1.example.com.cfg /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.

A list of all virtual machines that were created with the xen-create-image command is available under

xen-list-images

server1:~# xen-list-images
Name: xen1.example.com
Memory: 128
IP: 192.168.0.101
server1:~#

To learn more about what you can do with xen-tools, take a look at this tutorial: http://www.howtoforge.com/xen_tools_xen_shell_argo