On 02/08/2017 02:16 AM, wayen wrote:
> @Thomas Huth, I just want to reduce the file size of qcow2 overlay image
> by this conversion.

Are you merely trying to sparsify the file (punch holes on the portion
of the file that is not mapped to disk), so that the apparent file size
is unchanged, but the actual disk usage is minimized? That's probably
already possible (virt-sparsify from libguestfs seems to be able to do
it; look at what steps it uses).

Or are you asking for a way to defragment the file, so that the apparent
size shrinks because all occupied clusters are now contiguous, so that
there are no longer any need for holes?  The end result is the same
amount of disk usage, but right now, the only way to defragment is to
convert from one image to a copy; we don't support in-place
defragmentation yet.  So far, no one has come up with a compelling
reason why it is needed, or a patch to implement it, but there's no
technical reason why it can't be done.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org