Re: [PATCH v3] qcow2: Forbid discard in qcow2 v2 images with backing files

[Eric Blake]

On 3/27/20 1:43 PM, Alberto Garcia wrote:
> On Fri 27 Mar 2020 07:13:04 PM CET, Eric Blake wrote:
> > > +for qcow2_compat in 0.10 1.1; do
> > > +    echo "# Create an image with compat=$qcow2_compat without a backing file"
> > > +    _make_test_img -o "compat=$qcow2_compat" 128k
> > > +
> > > +    echo "# Fill all clusters with data and then discard them"
> > > +    $QEMU_IO -c 'write -P 0x01 0 128k' "$TEST_IMG" | _filter_qemu_io
> > > +    $QEMU_IO -c 'discard 0 128k' "$TEST_IMG" | _filter_qemu_io
> > > +
> > > +    echo "# Read the data from the discarded clusters"
> > > +    $QEMU_IO -c 'read -P 0x00 0 128k' "$TEST_IMG" | _filter_qemu_io
> > > +done
> >
> > Should this loop also inspect qemu-img map output?
>
> I guess we can, although here the image is completely unallocated in
> both cases.

Which shows that even for v2 images, discard DOES do something when 
there is no backing file (even if it is now a no-op when there is a 
backing file after this patch).

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org