Re: [PATCH v3 4/4] qemu-img: Deprecate use of -b without -F

[Eric Blake]

On 3/10/20 4:47 AM, Kashyap Chamarthy wrote:

> > To provoke the warning during convert, you'll
> > have to also pass -B (or -o backing_file), without -o backing_fmt (since
> > convert lacks the -F shorthand).
>
> Hmm, I tried the following way, but it doesn't provoke the warning:
>
>      $> ~/build/qemu/qemu-img convert -B ./base.raw -O qcow2 overlay1.qcow2 
> flattened.qcow2
>
>      $> ~/build/qemu/qemu-img info flattened.qcow2
>      image: flattened.qcow2
>      file format: qcow2
>      virtual size: 4 GiB (4294967296 bytes)
>      disk size: 196 KiB
>      cluster_size: 65536
>      backing file: ./base.raw
>      Format specific information:
>          compat: 1.1
>          lazy refcounts: false
>          refcount bits: 16
>          corrupt: false
>
> What am I missing?

Rather, it looks like my patch is missing a warning on that path.  I'll 
have to investigate, for v4.

>              - - -
>
> <digression>
>
> Ah, didn't realize the inconsistency of 'convert' lacking the '-F'
> shorthand ... which reminds me, there are at least _three_ ways that I
> know of, to specify backing file format with 'create':
>
>      $ qemu-img create -f qcow2 -o 'backing_file=./base.raw,backing_fmt=raw' 
> ./overlay1.qcow2
>      $ qemu-img create -f qcow2 -b ./base.raw -o backing_fmt=raw overlay1.qcow2
>      $ qemu-img create -f qcow2 -b ./base.raw -F raw ./overlay1.qcow2
>
> I'm wondering about the consistency of having all the above three
> supported for other operations too.  Now I at least know 'convert' lacks
> the "-F".

The -o forms (backing_file= and backing_fmt=) always work.  Various 
commands then have additional shorthand: -b/-F for create, -B for 
convert.  You're right that we aren't very consistent, but I'm reluctant 
to change the inconsistencies in this patch (at one point in the past, 
we tried to get rid of the shorthand and force all users to go through 
-o, but that broke too many clients that were depending on the 
undocumented shorthand, so we documented the existing shorthand instead).

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