Thursday, April 19, 2012

Virtualization With KVM On A CentOS 6.2 Server


This guide explains how you can install and use KVM for creating and running virtual machines on a CentOS 6.2 server. I will show how to create image-based virtual machines and also virtual machines that use a logical volume (LVM). KVM is short for Kernel-based Virtual Machine and makes use of hardware virtualization, i.e., you need a CPU that supports hardware virtualization, e.g. Intel VT or AMD-V.
I do not issue any guarantee that this will work for you!

1 Preliminary Note

I'm using a CentOS 6.2 server with the hostname server1.example.com and the IP address 192.168.0.100 here as my KVM host.
I had SELinux disabled on my CentOS 6.2 system. I didn't test with SELinux on; it might work, but if not, you better switch off SELinux as well:
vi /etc/selinux/config
Set SELINUX=disabled...
# This file controls the state of SELinux on the system.
# SELINUX= can take one of these three values:
#     enforcing - SELinux security policy is enforced.
#     permissive - SELinux prints warnings instead of enforcing.
#     disabled - No SELinux policy is loaded.
SELINUX=disabled
# SELINUXTYPE= can take one of these two values:
#     targeted - Targeted processes are protected,
#     mls - Multi Level Security protection.
SELINUXTYPE=targeted
... and reboot:
reboot
We also need a desktop system where we install virt-manager so that we can connect to the graphical console of the virtual machines that we install. I'm using a Fedora 16 desktop here.

2 Installing KVM

CentOS 6.2 KVM Host:
First check if your CPU supports hardware virtualization - if this is the case, the command
egrep '(vmx|svm)' --color=always /proc/cpuinfo
should display something, e.g. like this:
[root@server1 ~]# egrep '(vmx|svm)' --color=always /proc/cpuinfo
flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall
 nx mmxext fxsr_opt rdtscp lm 3dnowext 3dnow pni cx16 lahf_lm cmp_legacy svm extapic cr8_legacy misalignsse
flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall
 nx mmxext fxsr_opt rdtscp lm 3dnowext 3dnow pni cx16 lahf_lm cmp_legacy svm extapic cr8_legacy misalignsse
[root@server1 ~]#
If nothing is displayed, then your processor doesn't support hardware virtualization, and you must stop here.
Now we import the GPG keys for software packages:
rpm --import /etc/pki/rpm-gpg/RPM-GPG-KEY*
To install KVM and virtinst (a tool to create virtual machines), we run
yum install kvm libvirt python-virtinst qemu-kvm
Then start the libvirt daemon:
/etc/init.d/libvirtd start
To check if KVM has successfully been installed, run
virsh -c qemu:///system list
It should display something like this:
[root@server1 ~]# virsh -c qemu:///system list
 Id Name                 State
----------------------------------

[root@server1 ~]#
If it displays an error instead, then something went wrong.
Next we need to set up a network bridge on our server so that our virtual machines can be accessed from other hosts as if they were physical systems in the network.
To do this, we install the package bridge-utils...
yum install bridge-utils
... and configure a bridge. Create the file /etc/sysconfig/network-scripts/ifcfg-br0 (please use the IPADDR, PREFIX, GATEWAY, DNS1 and DNS2 values from the /etc/sysconfig/network-scripts/ifcfg-eth0 file); make sure you use TYPE=Bridge, not TYPE=Ethernet:
vi /etc/sysconfig/network-scripts/ifcfg-br0
DEVICE="br0"
NM_CONTROLLED="yes"
ONBOOT=yes
TYPE=Bridge
BOOTPROTO=none
IPADDR=192.168.0.100
PREFIX=24
GATEWAY=192.168.0.1
DNS1=8.8.8.8
DNS2=8.8.4.4
DEFROUTE=yes
IPV4_FAILURE_FATAL=yes
IPV6INIT=no
NAME="System br0"
Modify /etc/sysconfig/network-scripts/ifcfg-eth0 as follows (comment out BOOTPROTO, IPADDR, PREFIX, GATEWAY, DNS1, and DNS2 and add BRIDGE=br0):
vi /etc/sysconfig/network-scripts/ifcfg-eth0
DEVICE="eth0"
NM_CONTROLLED="yes"
ONBOOT=yes
HWADDR=00:1E:90:F3:F0:02
TYPE=Ethernet
#BOOTPROTO=none
#IPADDR=192.168.0.100
#PREFIX=24
#GATEWAY=192.168.0.1
#DNS1=8.8.8.8
#DNS2=8.8.4.4
DEFROUTE=yes
IPV4_FAILURE_FATAL=yes
IPV6INIT=no
NAME="System eth0"
UUID=5fb06bd0-0bb0-7ffb-45f1-d6edd65f3e03
BRIDGE=br0
Restart the network...
/etc/init.d/network restart
... and run
ifconfig
It should now show the network bridge (br0):
[root@server1 ~]# ifconfig
br0       Link encap:Ethernet  HWaddr 00:1E:90:F3:F0:02
          inet addr:192.168.0.100  Bcast:192.168.0.255  Mask:255.255.255.0
          inet6 addr: fe80::21e:90ff:fef3:f002/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:103 errors:0 dropped:0 overruns:0 frame:0
          TX packets:79 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:7434 (7.2 KiB)  TX bytes:21398 (20.8 KiB)

eth0      Link encap:Ethernet  HWaddr 00:1E:90:F3:F0:02
          inet6 addr: fe80::21e:90ff:fef3:f002/64 Scope:Link
          UP BROADCAST RUNNING PROMISC MULTICAST  MTU:1500  Metric:1
          RX packets:19511 errors:0 dropped:0 overruns:0 frame:0
          TX packets:11592 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:25980124 (24.7 MiB)  TX bytes:1104371 (1.0 MiB)
          Interrupt:28 Base address:0x6000

lo        Link encap:Local Loopback
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:16436  Metric:1
          RX packets:7 errors:0 dropped:0 overruns:0 frame:0
          TX packets:7 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:3608 (3.5 KiB)  TX bytes:3608 (3.5 KiB)

virbr0    Link encap:Ethernet  HWaddr 6A:12:69:18:2B:05
          inet addr:192.168.122.1  Bcast:192.168.122.255  Mask:255.255.255.0
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:0 (0.0 b)  TX bytes:0 (0.0 b)

[root@server1 ~]#

3 Installing virt-viewer Or virt-manager On Your Fedora 16 Desktop

Fedora 16 Desktop:
We need a means of connecting to the graphical console of our guests - we can use virt-manager for this. I'm assuming that you're using a Fedora 16 desktop.
Become root...
su
... and run...
yum install virt-manager libvirt qemu-system-x86 openssh-askpass
... to install virt-manager.
(If you're using an Ubuntu 11.10 desktop, you can install virt-manager as follows:
sudo apt-get install virt-manager
)

4 Creating A Debian Squeeze Guest (Image-Based) From The Command Line

CentOs 6.2 KVM Host:
Now let's go back to our CentOS 6.2 KVM host.
Take a look at
man virt-install
to learn how to use virt-install.
We will create our image-based virtual machines in the directory /var/lib/libvirt/images/ which was created automatically when we installed KVM in chapter two.
To create a Debian Squeeze guest (in bridging mode) with the name vm10, 512MB of RAM, two virtual CPUs, and the disk image /var/lib/libvirt/images/vm10.img (with a size of 12GB), insert the Debian Squeeze Netinstall CD into the CD drive and run
virt-install --connect qemu:///system -n vm10 -r 512 --vcpus=2 --disk path=/var/lib/libvirt/images/vm10.img,size=12 -c /dev/cdrom --vnc --noautoconsole --os-type linux --os-variant debiansqueeze --accelerate --network=bridge:br0 --hvm
Of course, you can also create an ISO image of the Debian Squeeze Netinstall CD (please create it in the /var/lib/libvirt/images/ directory because later on I will show how to create virtual machines through virt-manager from your Fedora desktop, and virt-manager will look for ISO images in the /var/lib/libvirt/images/ directory)...
dd if=/dev/cdrom of=/var/lib/libvirt/images/debian-6.0.2.1-amd64-netinst.iso
... and use the ISO image in the virt-install command:
virt-install --connect qemu:///system -n vm10 -r 512 --vcpus=2 --disk path=/var/lib/libvirt/images/vm10.img,size=12 -c /var/lib/libvirt/images/debian-6.0.2.1-amd64-netinst.iso --vnc --noautoconsole --os-type linux --os-variant debiansqueeze --accelerate --network=bridge:br0 --hvm
The output is as follows:
[root@server1 ~]# virt-install --connect qemu:///system -n vm10 -r 512 --vcpus=2 --disk path=/var/lib/libvirt/images/vm10.img,size=12 -c /var/lib/libvirt/images/debian-6.0.2.1-amd64-netinst.iso --vnc --noautoconsole --os-type linux --os-variant debiansqueeze --accelerate --network=bridge:br0 --hvm


Starting install...
Allocating 'vm10.img'              |  12 GB     00:00
Creating domain...                 |    0 B     00:00
Domain installation still in progress. You can reconnect to
the console to complete the installation process.
[root@server1 ~]#

5 Connecting To The Guest

 
Fedora 16 Desktop:
The KVM guest will now boot from the Debian Squeeze Netinstall CD and start the Debian installer - that's why we need to connect to the graphical console of the guest. You can do this with virt-manager on the Fedora 16 desktop.
Go to Applications > System Tools > Virtual Machine Manager to start virt-manager:


When you start virt-manager for the first time, you will most likely see the following message (Unable to open a connection to the libvirt management daemon.). You can ignore this because we don't want to connect to the local libvirt daemon, but to the one on our CentOS 6.2 KVM host. Click on Close...


... and go to File > Add Connection... to connect to our CentOS 6.2 KVM host:


Select QEMU/KVM as Hypervisor, then check Connect to remote host, select SSH in the Method drop-down menu, type in root as the Username and the hostname (server1.example.com) or IP address (192.168.0.100) of the CentOS 6.2 KVM host in the Hostname field. Then click on Connect:


If this is the first connection to the remote KVM server, you must type in yes and click on OK:


Afterwards type in the root password of the CentOS 6.2 KVM host:


You should see vm10 as running. Mark that guest and click on the Open button to open the graphical console of the guest:


Type in the root password of the KVM host again:


You should now be connected to the graphical console of the guest and see the Debian installer:


Now install Debian as you would normally do on a physical system. Please note that at the end of the installation, the Debian guest needs a reboot. The guest will then stop, so you need to start it again, either with virt-manager or like this on our CentOS 6.2 KVM host command line:
CentOS 6.2 KVM Host:
virsh --connect qemu:///system
start vm10
quit
Afterwards, you can connect to the guest again with virt-manager and configure the guest. If you install OpenSSH (package openssh-server) in the guest, you can connect to it with an SSH client (such as PuTTY).


6 Creating A Debian Squeeze Guest (Image-Based) From The Desktop With virt-manager

 
Instead of creating a virtual machine from the command line (as shown in chapter 4), you can as well create it from the Fedora desktop using virt-manager (of course, the virtual machine will be created on the CentOS 6.2 KVM host - in case you ask yourself if virt-manager is able to create virtual machines on remote systems).
To do this, click on the following button:

The New VM dialogue comes up. Fill in a name for the VM (e.g. vm11), select Local install media (ISO image or CDROM), and click on Forward:


Next select Linux in the OS type drop-down menu and Debian Squeeze in the Version drop-down menu, then check Use ISO image and click on the Browse... button:


Select the debian-6.0.2.1-amd64-netinst.iso image that you created in chapter 4 and click on Choose Volume:


Now click on Forward:


Assign memory and the number of CPUs to the virtual machine and click on Forward:


Now we come to the storage. Check Enable storage for this virtual machine, select Create a disk image on the computer's hard drive, specify the size of the hard drive (e.g. 12GB), and check Allocate entire disk now. Then click on Forward:


Now we come to the last step of the New VM dialogue. Go to the Advanced options section. Select Host device vnet0 (Bridge 'br0'); that is the name of the bridge which we created in chapter 2. Click on Finish afterwards:


The disk image for the VM is now being created:


Afterwards, the VM will start. Type in the root password of the CentOS 6.2 KVM host:


You should now be connected to the graphical console of the guest and see the Debian installer:


Now install Debian as you would normally do on a physical system.


7 Managing A KVM Guest From The Command Line

 
CentOS 6.2 KVM Host:
KVM guests can be managed through virsh, the "virtual shell". To connect to the virtual shell, run
virsh --connect qemu:///system
This is how the virtual shell looks:
[root@server1 ~]# virsh --connect qemu:///system
Welcome to virsh, the virtualization interactive terminal.

Type:  'help' for help with commands
       'quit' to quit

virsh #
You can now type in commands on the virtual shell to manage your guests. Run
help
to get a list of available commands:
virsh # help
Grouped commands:

 Domain Management (help keyword 'domain'):
    attach-device                  attach device from an XML file
    attach-disk                    attach disk device
    attach-interface               attach network interface
    autostart                      autostart a domain
    blkiotune                      Get or set blkio parameters
    blockpull                      Populate a disk from its backing image.
    blockjob                       Manage active block operations.
    console                        connect to the guest console
    cpu-baseline                   compute baseline CPU
    cpu-compare                    compare host CPU with a CPU described by an XML file
    create                         create a domain from an XML file
    define                         define (but don't start) a domain from an XML file
    destroy                        destroy (stop) a domain
    detach-device                  detach device from an XML file
    detach-disk                    detach disk device
    detach-interface               detach network interface
    domid                          convert a domain name or UUID to domain id
    domjobabort                    abort active domain job
    domjobinfo                     domain job information
    domname                        convert a domain id or UUID to domain name
    domuuid                        convert a domain name or id to domain UUID
    domxml-from-native             Convert native config to domain XML
    domxml-to-native               Convert domain XML to native config
    dump                           dump the core of a domain to a file for analysis
    dumpxml                        domain information in XML
    edit                           edit XML configuration for a domain
    inject-nmi                     Inject NMI to the guest
    send-key                       Send keycodes to the guest
    managedsave                    managed save of a domain state
    managedsave-remove             Remove managed save of a domain
    maxvcpus                       connection vcpu maximum
    memtune                        Get or set memory parameters
    migrate                        migrate domain to another host
    migrate-setmaxdowntime         set maximum tolerable downtime
    migrate-setspeed               Set the maximum migration bandwidth
    reboot                         reboot a domain
    restore                        restore a domain from a saved state in a file
    resume                         resume a domain
    save                           save a domain state to a file
    save-image-define              redefine the XML for a domain's saved state file
    save-image-dumpxml             saved state domain information in XML
    save-image-edit                edit XML for a domain's saved state file
    schedinfo                      show/set scheduler parameters
    screenshot                     take a screenshot of a current domain console and store it into a file
    setmaxmem                      change maximum memory limit
    setmem                         change memory allocation
    setvcpus                       change number of virtual CPUs
    shutdown                       gracefully shutdown a domain
    start                          start a (previously defined) inactive domain
    suspend                        suspend a domain
    ttyconsole                     tty console
    undefine                       undefine an inactive domain
    update-device                  update device from an XML file
    vcpucount                      domain vcpu counts
    vcpuinfo                       detailed domain vcpu information
    vcpupin                        control or query domain vcpu affinity
    version                        show version
    vncdisplay                     vnc display

 Domain Monitoring (help keyword 'monitor'):
    domblkinfo                     domain block device size information
    domblklist                     list all domain blocks
    domblkstat                     get device block stats for a domain
    domcontrol                     domain control interface state
    domifstat                      get network interface stats for a domain
    dominfo                        domain information
    dommemstat                     get memory statistics for a domain
    domstate                       domain state
    list                           list domains

 Host and Hypervisor (help keyword 'host'):
    capabilities                   capabilities
    connect                        (re)connect to hypervisor
    freecell                       NUMA free memory
    hostname                       print the hypervisor hostname
    nodecpustats                   Prints cpu stats of the node.
    nodeinfo                       node information
    nodememstats                   Prints memory stats of the node.
    qemu-attach                    QEMU Attach
    qemu-monitor-command           QEMU Monitor Command
    sysinfo                        print the hypervisor sysinfo
    uri                            print the hypervisor canonical URI

 Interface (help keyword 'interface'):
    iface-begin                    create a snapshot of current interfaces settings, which can be later commited (iface-commit) or restored (iface-rollback)
    iface-commit                   commit changes made since iface-begin and free restore point
    iface-define                   define (but don't start) a physical host interface from an XML file
    iface-destroy                  destroy a physical host interface (disable it / "if-down")
    iface-dumpxml                  interface information in XML
    iface-edit                     edit XML configuration for a physical host interface
    iface-list                     list physical host interfaces
    iface-mac                      convert an interface name to interface MAC address
    iface-name                     convert an interface MAC address to interface name
    iface-rollback                 rollback to previous saved configuration created via iface-begin
    iface-start                    start a physical host interface (enable it / "if-up")
    iface-undefine                 undefine a physical host interface (remove it from configuration)

 Network Filter (help keyword 'filter'):
    nwfilter-define                define or update a network filter from an XML file
    nwfilter-dumpxml               network filter information in XML
    nwfilter-edit                  edit XML configuration for a network filter
    nwfilter-list                  list network filters
    nwfilter-undefine              undefine a network filter

 Networking (help keyword 'network'):
    net-autostart                  autostart a network
    net-create                     create a network from an XML file
    net-define                     define (but don't start) a network from an XML file
    net-destroy                    destroy (stop) a network
    net-dumpxml                    network information in XML
    net-edit                       edit XML configuration for a network
    net-info                       network information
    net-list                       list networks
    net-name                       convert a network UUID to network name
    net-start                      start a (previously defined) inactive network
    net-undefine                   undefine an inactive network
    net-uuid                       convert a network name to network UUID

 Node Device (help keyword 'nodedev'):
    nodedev-create                 create a device defined by an XML file on the node
    nodedev-destroy                destroy (stop) a device on the node
    nodedev-dettach                dettach node device from its device driver
    nodedev-dumpxml                node device details in XML
    nodedev-list                   enumerate devices on this host
    nodedev-reattach               reattach node device to its device driver
    nodedev-reset                  reset node device

 Secret (help keyword 'secret'):
    secret-define                  define or modify a secret from an XML file
    secret-dumpxml                 secret attributes in XML
    secret-get-value               Output a secret value
    secret-list                    list secrets
    secret-set-value               set a secret value
    secret-undefine                undefine a secret

 Snapshot (help keyword 'snapshot'):
    snapshot-create                Create a snapshot from XML
    snapshot-create-as             Create a snapshot from a set of args
    snapshot-current               Get or set the current snapshot
    snapshot-delete                Delete a domain snapshot
    snapshot-dumpxml               Dump XML for a domain snapshot
    snapshot-edit                  edit XML for a snapshot
    snapshot-list                  List snapshots for a domain
    snapshot-parent                Get the name of the parent of a snapshot
    snapshot-revert                Revert a domain to a snapshot

 Storage Pool (help keyword 'pool'):
    find-storage-pool-sources-as   find potential storage pool sources
    find-storage-pool-sources      discover potential storage pool sources
    pool-autostart                 autostart a pool
    pool-build                     build a pool
    pool-create-as                 create a pool from a set of args
    pool-create                    create a pool from an XML file
    pool-define-as                 define a pool from a set of args
    pool-define                    define (but don't start) a pool from an XML file
    pool-delete                    delete a pool
    pool-destroy                   destroy (stop) a pool
    pool-dumpxml                   pool information in XML
    pool-edit                      edit XML configuration for a storage pool
    pool-info                      storage pool information
    pool-list                      list pools
    pool-name                      convert a pool UUID to pool name
    pool-refresh                   refresh a pool
    pool-start                     start a (previously defined) inactive pool
    pool-undefine                  undefine an inactive pool
    pool-uuid                      convert a pool name to pool UUID

 Storage Volume (help keyword 'volume'):
    vol-clone                      clone a volume.
    vol-create-as                  create a volume from a set of args
    vol-create                     create a vol from an XML file
    vol-create-from                create a vol, using another volume as input
    vol-delete                     delete a vol
    vol-download                   Download a volume to a file
    vol-dumpxml                    vol information in XML
    vol-info                       storage vol information
    vol-key                        returns the volume key for a given volume name or path
    vol-list                       list vols
    vol-name                       returns the volume name for a given volume key or path
    vol-path                       returns the volume path for a given volume name or key
    vol-pool                       returns the storage pool for a given volume key or path
    vol-upload                     upload a file into a volume
    vol-wipe                       wipe a vol

 Virsh itself (help keyword 'virsh'):
    cd                             change the current directory
    echo                           echo arguments
    exit                           quit this interactive terminal
    help                           print help
    pwd                            print the current directory
    quit                           quit this interactive terminal


virsh #
list
shows all running guests;
list --all
shows all guests, running and inactive:
virsh # list --all
 Id Name                 State
----------------------------------
  3 vm11                 running
  - vm10                 shut off

virsh #
If you modify a guest's xml file (located in the /etc/libvirt/qemu/ directory), you must redefine the guest:
define /etc/libvirt/qemu/vm10.xml
Please note that whenever you modify the guest's xml file in /etc/libvirt/qemu/, you must run the define command again!
To start a stopped guest, run:
start vm10
To stop a guest, run
shutdown vm10
To immediately stop it (i.e., pull the power plug), run
destroy vm10
Suspend a guest:
suspend vm10
Resume a guest:
resume vm10
These are the most important commands.
Type
quit
to leave the virtual shell.


8 Creating An LVM-Based Guest From The Command Line

 
CentOS 6.2 KVM Host:
LVM-based guests have some advantages over image-based guests. They are not as heavy on hard disk IO, and they are easier to back up (using LVM snapshots).
To use LVM-based guests, you need a volume group that has some free space that is not allocated to any logical volume. In this example, I use the volume group /dev/vg_server1 with a size of approx. 465GB...
vgdisplay
[root@server1 ~]# vgdisplay
  --- Volume group ---
  VG Name               vg_server1
  System ID
  Format                lvm2
  Metadata Areas        1
  Metadata Sequence No  5
  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               465.27 GiB
  PE Size               4.00 MiB
  Total PE              119109
  Alloc PE / Size       24480 / 95.62 GiB
  Free  PE / Size       94629 / 369.64 GiB
  VG UUID               jk2N5a-pQdV-I49Y-UCR9-A0Js-jEe0-0y6Ipg

[root@server1 ~]#
... that contains the logical volumes /dev/vg_server1/lv_root with a size of approx. 50GB, /dev/vg_server1/lv_home with a size of approx. 40GB, and /dev/vg_server1/lv_swap (about 6GB) - the rest is not allocated and can be used for KVM guests:
lvdisplay
[root@server1 ~]# lvdisplay
  --- Logical volume ---
  LV Name                /dev/vg_server1/lv_root
  VG Name                vg_server1
  LV UUID                il99XH-Q7QV-R16x-Q51X-GOzp-8uq1-aMUb82
  LV Write Access        read/write
  LV Status              available
  # open                 1
  LV Size                50.00 GiB
  Current LE             12800
  Segments               1
  Allocation             inherit
  Read ahead sectors     auto
  - currently set to     256
  Block device           253:0

  --- Logical volume ---
  LV Name                /dev/vg_server1/lv_home
  VG Name                vg_server1
  LV UUID                2OqWO9-4P91-UDzK-K6i3-zgJz-vJWN-9OMXpi
  LV Write Access        read/write
  LV Status              available
  # open                 1
  LV Size                40.00 GiB
  Current LE             10240
  Segments               1
  Allocation             inherit
  Read ahead sectors     auto
  - currently set to     256
  Block device           253:2

  --- Logical volume ---
  LV Name                /dev/vg_server1/lv_swap
  VG Name                vg_server1
  LV UUID                f5e90C-DMt8-896t-MmNo-S2FM-7hSd-gtVGjy
  LV Write Access        read/write
  LV Status              available
  # open                 1
  LV Size                5.62 GiB
  Current LE             1440
  Segments               1
  Allocation             inherit
  Read ahead sectors     auto
  - currently set to     256
  Block device           253:1

[root@server1 ~]#
I will now create the virtual machine vm12 as an LVM-based guest. I want vm12 to have 20GB of disk space, so I create the logical volume /dev/vg_server1/vm12 with a size of 20GB:
lvcreate -L20G -n vm12 vg_server1
Afterwards, we use the virt-install command again to create the guest:
virt-install --connect qemu:///system -n vm12 -r 512 --vcpus=2 --disk path=/dev/vg_server1/vm12 -c /var/lib/libvirt/images/debian-6.0.2.1-amd64-netinst.iso --vnc --noautoconsole --os-type linux --os-variant debiansqueeze --accelerate --network=bridge:br0 --hvm
Please note that instead of --disk path=/var/lib/libvirt/images/vm12.img,size=20 I use --disk path=/dev/vg_server1/vm12, and I don't need to define the disk space anymore because the disk space is defined by the size of the logical volume vm12 (20GB).
Now follow chapter 5 to install that guest.

9 Converting Image-Based Guests To LVM-Based Guests

CentOS 6.2 Host:
No let's assume we want to convert our image-based guest vm10 into an LVM-based guest. This is how we do it:
First make sure the guest is stopped:
virsh --connect qemu:///system
shutdown vm10
quit
Then create a logical volume (e.g. /dev/vg_server1/vm10) that has the same size as the image file - the image has 12GB, so the logical volume must have 12GB of size as well:
lvcreate -L12G -n vm10 vg_server1
Now we convert the image:
qemu-img convert /var/lib/libvirt/images/vm10.img -O raw /dev/vg_server1/vm10
Afterwards you can delete the disk image:
rm -f /var/lib/libvirt/images/vm10.img
Now we must open the guest's xml configuration file /etc/libvirt/qemu/vm10.xml...
vi /etc/libvirt/qemu/vm10.xml
... and change the following section...
[...]
    
      
      
      
      

    
[...]
... so that it looks as follows:
[...]
    
      
      
      
      

    
[...]
Afterwards we must redefine the guest:
virsh --connect qemu:///system
define /etc/libvirt/qemu/vm10.xml
Still on the virsh shell, we can start the guest...
start vm10
... and leave the virsh shell:
quit

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