====== KVM - Convert qcow2 to raw ======

Convert qcow2 disk images to raw disk images for performance.

The **qcow2** disk format has some decent features like encryption, compression and copy to write support.  However, the compression and the copy processes make it quite a bit slower than raw disk images.  Sometimes you want to convert the disk images so that the VM will perform better.

===== Benchmark =====

For me it has a performance boost. Using a RAID 1 setup with two 5900 RPM disks and the **deadline** on the host and the **noop** scheduler without caching on a raw image in the guest resulted in a boost over the default **deadline** scheduler on a qcow2 image on the guest:

Test command:

<code bash>
dd if=/dev/zero of=test bs=64k count=16k conv=fdatasync
</code>

Default Ubuntu 12.04 vmbuilder created vm on a qcow2 image without caching and with the deadline scheduler in the VM:

<code>
1073741824 bytes (1.1 GB) copied, 658.396 s, 1.6 MB/s
</code>

The same VM, disk image converted to raw image without caching and using the noop scheduler in the VM:

<code>
1073741824 bytes (1.1 GB) copied, 13.646 s, 78.7 MB/s
</code>

That's quite a performance boost.  The KVM host has the following result with the **dd** command:

<code>
1073741824 bytes (1.1 GB) copied, 10.4034 s, 103 MB/s
</code>


===== Converting the image =====

I'll convert the disk image for the example vm **vm1**.  Change the name and disk paths for your setup.

First shut down the VM:

<code bash>
virsh shutdown vm1
</code>

Then convert all the disk images using this command for each disk image:

<code bash>
qemu-img convert /var/lib/libvirt/images/vm1/ubuntu-kvm/tmp20ePgc.qcow2 /var/lib/libvirt/images/vm1/ubuntu-kvm/tmp20ePgc.raw
</code>

Edit the VM config:

<code bash>
virsh edit vm1
</code>

Change the **disk** section to point to the new raw image:

Change the lines **<driver name='qemu' type='qcow2'/>** and **<source file='/var/lib/libvirt/images/vm1/ubuntu-kvm/tmp20ePgc.qcow2'/>** to **<driver name='qemu' type='raw'/>** and **<source file='/var/lib/libvirt/images/vm1/ubuntu-kvm/tmp20ePgc.raw'/>**. Like so:

<code bash>
&lt;disk type='file' device='disk'&gt;
  &lt;driver name='qemu' type='raw' cache='none'/&gt;
  &lt;source file='/var/lib/libvirt/images/vm1/ubuntu-kvm/tmp20ePgc.raw'/&gt;
  &lt;target dev='hda' bus='ide'/&gt;
  &lt;address type='drive' controller='0' bus='0' unit='0'/&gt;
&lt;/disk&gt; 
</code>

That's it.  Start the VM:

<code bash>
virsh start vm1
</code>

If it all works, remove the qcow2 image:

<code bash>
rm /var/lib/libvirt/images/vm1/ubuntu-kvm/tmp20ePgc.qcow2
</code>

===== Disk scheduler =====

To change the default disk scheduler in your VM, you can use the following command:

<code bash>
echo $scheduler > /sys/block/$device/queue/scheduler
</code>

To check the current scheduler:

<code bash>
cat /sys/block/sda/queue/scheduler
</code>

Output:

<code>
noop [deadline] cfq
</code>

Here the **deadline** scheduler is being used.  To change it:

<code bash>
echo noop > /sys/block/sda/queue/scheduler
</code>

Now the **noop** scheduler is being used:

<code bash>
cat /sys/block/sda/queue/scheduler
[noop] deadline cfq
</code>

Remember to add this command to /etc/rc.local to make it survive a reboot. On ubuntu you can also define it in /etc/default/grub. Change GRUB_CMDLINE_LINUX_DEFAULT="quiet splash" to GRUB_CMDLINE_LINUX_DEFAULT="quiet splash elevator=noop" and run an update-grub2 to make it permanent