If you’re running qemu KVM on Ubuntu and want to take advantage of the qcow2 file format’s snapshotting capabilities and sparse disk population you can easily convert using the command line tool
First, make sure your virtual machine is turned off! Then you can navigate to the directory your virtual disks are stored in (usually /var/lib/libvirt). It’s probably a good idea to be a root user or otherwise sudo the following command
qemu-img convert -f raw -O qcow2 vm_hdd.img vm_hdd.qcow2
-f flag tells convert what format it’s reading. If you don’t provide it then it’ll guess based on the file extension.
-O flag tells convert what file format to output to, again if not provided it’ll guess based on the file extension.
Now you’ve got a qcow2 file, you’ll need to edit the VM configuration
virsh edit vm_name
this will open up an editor for your VM configuration. It’s an XML file, so it’s reasonably easy to follow. What you’re looking for is a disk section so you can change the file extension and disk type
<disk type='file' device='disk'> <driver name='qemu' type='raw' cache='none'/> <source file='/var/lib/libvirt/images/rhel62-2.img'/>
<disk type='file' device='disk'> <driver name='qemu' type='qcow2' cache='none'/> <source file='/var/lib/libvirt/images/rhel62-2.qcow2'/>
Note both “raw” and “img” have been changed to “qcow2” for this disk. Make sure you’ve picked the right disk to edit in the XML. It may be a good idea to take a backup first so you can fall back to the img file if needed!
That should be it, your VM should now boot with the new disk file. Once you’re sure it’s working you can delete the original (or keep it safe somewhere).
More information about KVM can be found on the RedHat website: https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/7/html/virtualization_deployment_and_administration_guide/index or the Ubuntu wiki https://help.ubuntu.com/community/KVM/Installation