This is a very short quick setup on how to get KVM (The Linux Kernel Virtual Machine hypervisor) server up and running.

Why KVM?

KVM is a hypervisor, just like VmWare ESX, Microsoft’s Hyper-V and XEN. The great thing (as usual) about KVM, is that it’s part of Linux, meaning its free, and it’s performance is excellent. Using it in a production environment as a standalone hypervisor is an excellent choice, has a low host server footprint (in terms of performance needs and disk), and can be administered easily with other tools like virt-manager.

Assumptions

You want to run a KVM server with at least one virtual machine guest,
Your KVM server gets an ip address in your network,
Your virtual machine(s) get an ip address from your network – so you can use bridging instead of natting (using NATting instead of bridging is an easy task but not part of this howto),
You can use lvm for disk space allocation on your KVM master (using other disk space allocations methods like image files is easy, too, but not part of this howto) – note you will have to install this prior to going through the below, if it is not already installed on your server.
You are using Ubuntu 10.04 or newer. It should be up-to-date Ubuntu server with network connectivity and access via ssh.

Acknowledgements

This is a short guide which needs to be revamped and will do soon, however in its current state, it was taken mainly wholesale from www.screenage.de, so big props to the orginal author, ccm. Minor amendments have been made for readability.

Get the network up and running

For the bridged network you need to install the bridge utilities and change your network configuration. First install the package:

$ sudo apt-get install bridge-utils

Now add a bridge named „br0“ (this has only be done once):

$ sudo brctl addbr br0

Now change your /etc/network/interfaces so it uses the bridge br0. This step actually sets up br0 instead of eth0. Think of eth0 as being just a physical transport added to the virtual bridge interface.

# The loopback network interface
auto lo
iface lo inet loopback
 
auto eth0
iface eth0 inet manual
 
auto br0
iface br0 inet static
address 192.168.1.100
netmask 255.255.255.0
network 192.168.1.0
broadcast 192.168.1.255
gateway 192.168.1.1
bridge_ports eth0
bridge_fd 9
bridge_hello 2
bridge_maxage 12
bridge_stp off

Please make sure you don’t forget setting your „eth0“ to „iface eth0 inet manual“ as shown above. This is needed as you want to prevent eth0 to fetch an address via dhcp but still want it to be there for your bridge as it is the physical layer. After you setup the bridge either restart your network (sudo /etc/init.d/networking restart) or reboot your server. If you are accessing your server already by ssh be warned that a misconfiguration might lock you out.

Install KVM

Now it’s time to install kvm and some usefull helper applications:

$ sudo apt-get install qemu-kvm ubuntu-vm-builder uml-utilities \
  virtinst

That’s all: You already have a kvm server now. Time to…

Install your first virtual machine

We are going to setup a 100Gb logical volume for the guest, download Ubuntu and create a machine with 2Gb of Ram and 4 cores:

# create an empty 100Gb logical volume
sudo lvcreate --size 100G vg0 --name guest1
# download Ubuntu iso (or use one you already have)
$ wget http://..../
# create machine
$ sudo virt-install --connect qemu:///system -n guest1 -r 2048 \
 --vcpus=4 -f /dev/mapper/guest1 --network=bridge:br0 \
 --vnc --accelerate -v -c ./SOMEUBUNTUISO.iso \
 --os-type=linux --os-variant=ubuntuKarmic --noautoconsole
# please note: "ubuntuKarmic" is currently the most recent
# virt-install defaults scheme - just use this if in doubt.

Get a VNC connection

KVM uses VNC to give you ca graphical interface to your machine. The good thing about this is, that it enables you to use graphical installers (and yes, even Windows) without problems. As even Ubuntu server boots into a graphical mode in the beginning – it’s great to use VNC here.

I assume you are working on a remote server. KVM gives every guest it launches a new vnc instance with a new, incremented port. It starts with 5900. So let’s tunnel via ssh:

ssh user@remotekvmhost -L 5900:localhost:5900

You connect to your remote kvm host via ssh and open a ssh tunnel fort port 5900. Now start your prefered VNC client locally and let it connect to either display „0“ or port 5900 which means the same in VNC (duh…).

From now on you should see your server on a VNC display. Install it like you’d install every other server. The networking is bridged, so you could even use dhcp if that is offered in your network.

Please make sure, you install the package „acpi“ inside your kvm guest, otherwise you won’t be able to stop the guest from the master (as it is done via acpi):

# make sure, "acpi" is installed in the *guest* machine
sudo apt-get install acpi

After installation you can manage your kvm gues by using the following commands:

# list running instances
$ virsh list
# start an instance
$ virsh start INSTANCENAME
# stop an instance politely
$ virsh stop INSTANCE
# immediatly destroy a running instance
$ virsh destroy INSTANCE
# edit the config file for an instance
$ virsh edit INSTANCE

Mounting the LVM volumes

As you might have noticed, your virtual guest’s lvm volumes cannot be mounted directly in the master as they contain their own partition table. If you need access to the guest’s filesystem from the master, though, you have to create some device nodes. There is a great tool called „kpartx“ than can create and delete device nodes for you. It’s as easy as this:

# install kpartx
$ sudo install kpartx
# make sure, virtual gues is switched off!
# create device nodes
$ sudo kpartx -a /dev/mapper/guest1
# check /dev/mapper for new device nodes and mount/unmount them
# after you are done, delete the nodes
$ sudo kpartx -d /dev/mapper/guest1

Please note, this methods also works with other block devices like image files containing partition tables. You only might run into trouble, when your lvm volume contains it’s own lvm. If that is the case, play around with pvscan, vgscan and lvscan after using kpartx. Be brave but be warned that backing up data is always a great idea.

Alternative Management Interfaces

In case you really need a gui for your management needs, check „virt-manager“. You can install this on your desktop and remotely manage running instances:

$ sudo install virt-manager

You should check RedHat’s „Virtual Machine Manager“ page, though. It might be a good idea to manually compile and install a more recent version and rely on the setup howtos. Personally I prefer using plain text console here, as it helps being able to act quite fast and from everywhere when problems occur and of course, you don’t have the overhead of running X11 on the server.

Conclusion

Nowadays it’s fairly easy setting up a KVM server. As KVM/libvirt enabled guests are quite fast, it’s a nice and easy way for even hosting virtual machines. I run about a dozen virtual machines and three hardware servers for two years now without any serious problems.