qemu-img convert documents the backing file and backing format options
as follows:
[-B backing_file [-F backing_fmt]]
whereas qemu-img create has this:
[-b backing_file] [-F backing_fmt]
That is, for convert, we document that -F cannot be given without -B,
while for create, way say that they are independent.
Indeed, it is technically possible to give -F without -b, because it is
left to the block driver to decide whether this is an error or not, so
sometimes it is:
$ qemu-img create -f qed -F qed test.qed 64M
Formatting 'test.qed', fmt=qed size=67108864 backing_fmt=qed [...]
And sometimes it is not:
$ qemu-img create -f qcow2 -F qcow2 test.qcow2 64M
Formatting 'test.qcow2', fmt=qcow2 cluster_size=65536 [...]
qemu-img: test.qcow2: Backing format cannot be used without backing file
Generally, it does not make much sense, though, and users should only
give -F with -b, so document it that way, as we have already done for
qemu-img convert (commit 1899bf47375ad40555dcdff12ba49b4b8b82df38).
Reported-by: Tingting Mao<timao@redhat.com>
Signed-off-by: Hanna Reitz<hreitz@redhat.com>