HOWTO create an LVM-based virtual machine with KVM/libvirt

A quick google didn't turn up any well-researched benchmarks for performance of VM image files vs. KVM-based VMs (see here for an attempt), but it makes sense to me to eliminate the file loopback overhead by using KVM-based VMs.

There are a few good HOWTOs out there, which I have boiled down into basics:

First, build your VM using JeOS

sudo vmbuilder kvm ubuntu --dest=/data/kvm/temp-ubuntu-amd64 --bridge=br0 --mem=2048 -c mybox.vmbuild.cfg

I tried the vmbuilder '--raw' option and pointed it at my LVM, but vmbuilder seemed to silently ignore it. So we will have to convert the image file instead.

The "raw" output option for qemu-img should do the trick, but I believe I hit a known bug, because I got:

qemu-img: Error while formatting '/dev/vg01/mybox'

Using "host_device" worked (you could also just convert to raw then dd):

qemu-img convert disk0.qcow2 -O host_device /dev/vg01/mybox

You then need to update your KVM definition file to point the hard disk at the logical volume.