Re: [PATCH v4 07/30] qcow2: Document the Extended L2 Entries feature

[Eric Blake]

On 4/14/20 1:23 PM, Eric Blake wrote:
> On 4/14/20 1:16 PM, Alberto Garcia wrote:
> > On Thu 09 Apr 2020 05:12:16 PM CEST, Eric Blake <address@hidden> 
> > wrote:
> > > Hmm - raw external files are incompatible with backing files.
> >
> > Pre-existing, but I just realized that we are not checking that in
> > qcow2_do_open(), only on _create().
> >
> > I suppose that if we find such an image we should either
> >
> >     a) Show an error message and abort.
> >     b) Clear the 'raw data file' bit and proceed as if it was unset.
>
> I would favor a).  Such an image was (hopefully) created externally, and 
> not by qemu; therefore refusing to open it will call attention to the 
> image (and it's creation process) being broken, rather than risking 
> silent corruption of whatever the external process thought it was 
> accomplishing by creating an image like that.

Also, 'qemu-img check' should flag the problem, and I'd be okay with 
'qemu-img check -r all' repairing the problem by method b) (because then 
the user is explicitly opting in to having qemu change the image in 
order to maximize the amount of data that qemu can then extract from the 
image).

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