====== Overview ======

KVM is a virtualization hypervisor for linux

====== qcow2 vs raw disk ======

As of November, 2012 and on ubuntu 12.04 disk I/O was actually better with a qcow2 image that had preallocate=metadata option done to it.  Raw disk I/O was slightly lower.  As cache has been disabled in both instances it's unclear why this is the case when all information online points to a raw disk image would be better.

====== Converting qcow2 and raw images ======

===== Convert qcow2 to raw =====

  * Shutdown the vm if it's running
  * ''qemu-img info /path/to/file.qcow2''
    * Note the size given for virtual size
  * ''sudo lvcreate -L 12345678b -n kvm_vm1 vg0'' where 12345678 is the size you got from above command. Remember the b at the end
  * ''sudo qemu-img convert /path/to/file.qcow2 -O host_device /dev/mapper/vg0-kvm_vm1''
  * ''virsh edit vm1'' and change it from this
<file xml>
    <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2' cache='none'/>
      <source file='/srv/kvm/vm1/disk0.qcow2'/>
      <target dev='vda' bus='virtio'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x0'/>
    </disk>
</file>
  * to this
<file xml>
    <disk type='block' device='disk'>
      <driver name='qemu' type='raw' cache='none'/>
      <source dev='/dev/mapper/vg0-kvm_vm1'/>
      <target dev='vda' bus='virtio'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x0'/>
    </disk>
</file>

===== Convert raw to qcow2 =====

  * Shutdown the vm if it's running
  * ''sudo lvcreate -L 12345678b -n kvm_vm1 vg0'' where 12345678 is the size you got from above command. Remember the b at the end
  * ''sudo qemu-img convert /dev/mapper/vg0-kvm_vm1 -O qcow2 -o preallocation=metadata /srv/kvm/vm1/disk0.qcow2''
  * ''virsh edit vm1'' and change it from this
<file xml>
    <disk type='block' device='disk'>
      <driver name='qemu' type='raw' cache='none'/>
      <source dev='/dev/mapper/vg0-kvm_vm1'/>
      <target dev='vda' bus='virtio'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x0'/>
    </disk>
</file>
  * To this:
<file xml>
    <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2' cache='none'/>
      <source file='/srv/kvm/vm1/disk0.qcow2'/>
      <target dev='vda' bus='virtio'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x0'/>
    </disk>
</file>

====== References ======

  * [[http://itscblog.tamu.edu/improve-disk-io-performance-in-kvm/|Improve disk I/O performance in KVM]]
  * [[https://www.nnbfn.net/2011/03/convert-kvm-qcow2-to-lvm-raw-partition/|Convert KVM qcow2 to LVM raw partition]]
  * [[https://help.ubuntu.com/community/KVM|Ubuntu wiki on KVM]]