Re: [Qemu-devel] [Qemu-block] [PATCH v2 0/3] block: Warn about usage of growing formats over non-growable protocols

[Paolo Bonzini <pbonzini@redhat.com>]

On 06/05/2015 18:12, Max Reitz wrote:
> I very much think it would be worth fixing, if there wasn't the problem
> with legitimate use cases throwing unnecessary warnings.

Right.

> I remember having a discussion with Kevin about this series (v1)
> regarding qcow2 on LVM; I think my point was that the warning is
> basically still correct, or only needs rewording (oops, I guess I did
> forget that in v2). If you are using qcow2 on LVM, you need to know
> exactly what you are doing, so a warning about this is indeed
> appropriate (in my opinion, that is).

There's another thing to check.  In the BZ you linked you got an EINVAL
or EIO.  Why didn't you get an ENOSPC?  Can you check if virtio-scsi
gives ENOSPC?

If so, you could perhaps only warn for werror=report.  But even then,
there are legitimate cases where you want the guest to see the ENOSPC.
In fact, that's the reason why virtio-scsi converts ENOSPC to a SCSI
"SPACE ALLOCATION FAILED" sense code. :)

> So I think if we can word the warning in a way to make it clear that
> there are legitimate use cases, but you need to know what you are doing,
> I think it's worth having this warning. Users who know what they're
> doing won't be surprised or at least will know what it means, while
> users who don't know what it means most probably don't know what they're
> doing and thus the warning is appropriate for them.

I don't know...  But then, I'm not a maintainer of this code. :)

Paolo