KVM and CentOS-6

CentOS 6 has native availability of KVM virtualization support and tools in the base distribution. Dell provides two whitepapers about how to use KVM in CentOS 6, part 1 and part 2.

See the meta packages contained in:

# yum grouplist | grep -i virt 

1. Host Setup

Install all the packages you might need.

yum -y install @virt* dejavu-lgc-* xorg-x11-xauth tigervnc \
libguestfs-tools policycoreutils-python bridge-utils 

If you have use any directories other than /var/lib/libvirt for kvm files, set the selinux context. In this example I use /vm to store my disk image files.

semanage fcontext -a -t virt_image_t "/vm(/.*)?"; restorecon -R /vm 

Allow packet forwarding between interfaces.

sed -i 's/^\(net.ipv4.ip_forward =\).*/\1 1/' /etc/sysctl.conf; sysctl -p 

Configure libvirtd service to start automatically and reboot.

chkconfig libvirtd on; shutdown -r now

Optionally you can set up bridging which will allow guests to have a network adaptor on the same physical lan as the host. In this example eth0 is the device to support the bridge and br0 will be the new device.

chkconfig network on
service network restart
yum -y erase NetworkManager
cp -p /etc/sysconfig/network-scripts/ifcfg-{eth0,br0}
sed -i -e'/HWADDR/d' -e'/UUID/d' -e's/eth0/br0/' -e's/Ethernet/Bridge/' \
/etc/sysconfig/network-scripts/ifcfg-br0
echo DELAY=0 >> /etc/sysconfig/network-scripts/ifcfg-br0
echo 'BOOTPROTO="none"' >> /etc/sysconfig/network-scripts/ifcfg-eth0
echo BRIDGE=br0 >> /etc/sysconfig/network-scripts/ifcfg-eth0
service network restart
brctl show

The host is now ready to start creating kvm guests.

2. Guest Setup

Since there are many options for setting up a guest, it is easier to have variables collect the information which will be used in a single command to create the guest. Several options are shown, and most can be adjusted as needed.

Start by reviewing the available OS variants.

virt-install --os-variant=list | more

Select one of the OS options:

OS="--os-variant=freebsd8"
OS="--os-variant=win7"
OS="--os-variant=win7 --disk path=/var/lib/libvirt/iso/virtio-win.iso,device=cdrom"
OS="--os-variant=win2k8"
OS="--os-variant=win2k8 --disk path=/var/lib/libvirt/iso/virtio-win.iso,device=cdrom"
OS="--os-variant=rhel6"

OS="--os-variant=rhel7" 

Select a network option, replacing the MAC address if needed:

Net="--network bridge=br0"
Net="--network model=virtio,bridge=br0"
Net="--network model=virtio,mac=52:54:00:00:00:00"
Net="--network model=virtio,bridge=br0,mac=52:54:00:00:00:00"

Select a disk option, replacing the filename and size with desired values:

Disk="--disk /vm/Name.img,size=8"
Disk="--disk /var/lib/libvirt/images/Name.qcow2,size=4"
Disk="--disk /var/lib/libvirt/images/Name.qcow2,sparse=false,size=8"
Disk="--disk /var/lib/libvirt/images/Name.qcow2,sparse=false,bus=virtio,size=8"
Disk="--disk vol=pool/volume"
Disk="--livecd --nodisks"
Disk="--disk /dev/mapper/vg_..."

Select a source (live cd iso, pxe or url):

Src="--cdrom=/var/lib/libvirt/iso/CentOS-6.6-x86_64.iso"
Src="--pxe"
Src="-l http://alt.fedoraproject.org/pub/fedora/linux/releases/20/Fedora/x86_64/os/"
Src="-l http://download.fedoraproject.org/pub/fedora/linux/releases/20/Fedora/x86_64/os/"
Src="-l http://ftp.us.debian.org/debian/dists/stable/main/installer-amd64/
Src="-l http://ftp.ubuntu.com/ubuntu/dists/trusty/main/installer-amd64/"
Src="-l http://download.opensuse.org/distribution/openSUSE-stable/repo/oss/"
Src="--location=http://mirror.centos.org/centos/6/os/x86_64"

Optionally add a URL for a kickstart file:

KS=""
KS="-x ks=http://ks.example.com/kickstart/c6-64.ks"

Optionally select a graphics option:

Gr=""
Gr="--graphics none"
Gr="--graphics vnc"
Gr="--graphics vnc,password=foo"
Gr="--graphics spice"

Select number of cpus:

Cpu="--vcpus=1"
Cpu="--vcpus=2"
Cpu="--vcpus=4"

Select amount of ram:

Ram="--ram=512" 

Ram="--ram=768"
Ram="--ram=1024"
Ram="--ram=2048"

Choose a name for the guest:

Name="myguest"

Create the guest:

virt-install $OS $Net $KS $Disk $Src $Gr $Cpu $Ram --name=$Name --cpu host --arch=x86_64

*If you got an error like MP-BIOS 8254 timer or host CPU does not support feature cx16, then you need --cpu host --arch=x86_64 option to solve these problems.

Note that it could take a considerable amount of time to complete, especially if you have chosen a large, non-sparse disk file on a slow harddrive. If you have selected an interactive installation, you will need to connect to the console to complete the installation.

*Connect to the console, using myhost as an example host:

virt-viewer --connect qemu_ssh://myhost/$Name

*Start the virtual OS:
virsh --connect qemu:///system start myhost

*If you would prefer a gui application:

virt-manager &

*Finally, you can set up this guest to start automatically whenever the host is booted:

virsh autostart $Name

3. Interact with your virtual guest OS

To view the running guest servers:
#virsh list -all

From your own desktop (with GUI), to establish a SSH tunnel for VNC connection to guest OS:

#ssh -X someuser@x.x.x.x -L 5900:127.0.0.1:5900

where x.x.x.x is the ip of the KVM host OS, who is hosting your guest OS. The VNC connection is running on port 5900 on your KVM host. Then:

#virt-manager &

If you are not using a kickstart file, then you need to interact with the guest OS. You may do it through VNC on KVM host after SSH-tunneling as shown above (remember to turn on ssh X11 forwarding on KVM host):

#yum install vnc-server

#virsh list --all

#virsh list --uuid

#virsh-viewer mygest 

#virsh shutdown myguest

#virsh start myguest

To edit the XML settings of the virtual guest: 

#virsh edit myguest 

If you like to automate the installation of guest OS, please use a kickstart file.

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To modify the XML file for virtual guests:
 virsh -c qemu:///session dumpxml myguest > ~/myguest.xml

Now you can edit this xml file (this trick didn't work in my case, though):

From   
<devices>
    <emulator>/usr/bin/kvm</emulator>
    <disk type='file' device='disk'>
      <driver name='qemu' type='raw'/>
      <source file='/storage/vmimages/Windows7_x64-01/qcow2-60GB_HDD1.qcow2'/>
      <target dev='hda' bus='ide'/>
    </disk>
To
<devices>
    <emulator>/usr/bin/kvm</emulator>
    <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2'/>
      <source file='/storage/vmimages/Windows7_x64-01/qcow2-60GB_HDD1.qcow2'/>
      <target dev='hda' bus='ide'/>
    </disk>

Open virt-manager and delete the virtual guest, but keep the image file on disk.

Then, place the modified XML file back:
virsh -c qemu:///session define ~/myguest.xml

Now this virtual guest will show up again in virt-manager.
Start it!

When you have a reboot failure from virtual hard drive

(i)Use .qcow2 as an extension of virtual disk file.
(ii)Do not use bridging. (NAT)

Net="--network model=virtio,network=default"

Then I got it rebootable on my machine.
DSL will not connect to network, but CentOS6 connected to LAN/WAN with no problem (CentOS6 was using a local VLAN on KVM host though, NAT)

3. Import from an existing virtual guest

 % virsh destroy myguest
 % virsh undefine myguest
 % virt-install --connect qemu:///system --ram 1024 -n rhel_64 -r 2048 --os-type=linux --os-variant=rhel5  --disk path=/root/RHEL-Server-5.8-64-virtio.qcow2,device=disk,bus=virtio,format=qcow2 --vcpus=2 --vnc --noautoconsole --import

where your existing virtual guest is provided by /root/RHEL-Server-5.8-64-virtio.qcow2. A new VM will be built on it.

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Thanks goes to ScottRobbins who contributed this guide. An earlier version of this article is found at http://home.roadrunner.com/~computertaijutsu/centoskvm.html