Some time ago I compared disk drivers performance in KVM. Today I compared different storage formats – raw and qcow2. Let’s have a look:
Test procedure: Create an empty 10 GB image, attach to VM using VirtIO driver, boot F20 Alpha Live x86_64, measure the time of installation. Repeat installation once again, this time reusing the existing image (instead of creating a new one). Do this for both formats.
raw 1st pass 2:36 raw 2nd pass 2:38 qcow2 1st pass 2:36 qcow2 2nd pass 2:44
As you can see, the results are very much the same. It seems it doesn’t matter much which format you use.
But, qcow2 format has some nice additional features, like copy-on-write cloning. If I need to test something very quickly in my existing VM and then revert the changes back, this is the easiest way:
$ cd /var/lib/libvirt/images $ mv f19.qcow2 f19.qcow2_orig $ qemu-img create -f qcow2 -b f19.qcow2_orig f19.qcow2 Formatting 'f19.qcow2', fmt=qcow2 size=10737418240 backing_file='f19.qcow2_orig' encryption=off cluster_size=65536 lazy_refcounts=off $ # Run the VM now and do your tasks $ mv f19.qcow2_orig f19.qcow2