Re: [Qemu-devel] [Qemu-block] [PATCH v4] qemu-img: Check for backing image if specified during create

[Nir Soffer]

On Wed, Jul 12, 2017 at 8:00 PM Max Reitz <address@hidden> wrote:

> On 2017-07-12 16:56, Kevin Wolf wrote:
> > Am 12.07.2017 um 16:42 hat Eric Blake geschrieben:
> >> On 05/17/2017 07:38 AM, Max Reitz wrote:
> >>
> >>>>>>>>>> Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1213786
> >>>>>>>>>>
> >>>>>>>>>> Or, rather, force the open of a backing image if one was
> specified
> >>>>>>>>>> for creation. Using a similar -unsafe option as rebase, allow
> qemu-img
> >>>>>>>>>> to ignore the backing file validation if possible.
> >>>>>>>>>>
> >>>>>>>>
> >>
> >>>>> I suspect this is because the backing file is opened somewhere and
> >>>>> trying to open it breaks now with the locking series applied.
> >>>>
> >>>> I think we can legitimately set force-shared=on for opening the
> backing
> >>>> file when testing whether the file exists.
> >>>
> >>> For testing whether the file exists, probably. I wouldn't call it
> >>> legitimately because I don't like making that the default at all, but
> it
> >>> should work.
> >>>
> >>> But then we have to differentiate between testing whether the file
> >>> exists and opening it to read its size; I'm even more wary regarding
> the
> >>> latter.
> >>>
> >>> All in all, I've stated my opinion on this matter on IRC: I don't know
> >>> how much we need this patch. It's nice to have, it's convenient to know
> >>> you have mistyped the backing filename before you try to use the image
> >>> in qemu, but it's not critical. I don't consider the current behavior a
> >>> bug per-se, it's just counterintuitive behavior (that will not cut your
> >>> head off if you don't know it, though!).
> >>>
> >>
> >> The fact that this topic came up on the mailing list again means we
> >> should probably consider something along these lines for 2.10.
> >>
> >>> (Also, we have a lot of counterintuitive behavior. See this:
> >>>
> >>> $ qemu-img create -f qcow2 sub/backing.qcow2 64M
> >>> Formatting 'sub/backing.qcow2', fmt=qcow2 size=67108864 encryption=off
> >>> cluster_size=65536 lazy_refcounts=off refcount_bits=16
> >>> $ qemu-img create -f qcow2 -b sub/backing.qcow2 sub/top.qcow2
> >>> qemu-img: sub/top.qcow2: Could not open 'sub/sub/backing.qcow2': No
> such
> >>> file or directory
> >>>
> >>> Yeah, sure, you'll know after you've done the mistake once. But the
> same
> >>> applies here, too, doesn't it...?)
> >>>
> >>> So for me, it's a question of convenience: How much does the current
> >>> behavior annoy users? How much annoyance will changing the behavior
> >>> bring? I'm not (or rather, no longer) convinced the former outweighs
> the
> >>> latter, especially since it means having to change management tools
> >>> (which create external snapshots with qemu-img create while the backing
> >>> image is in use by qemu).
> >>>
> >>> If you think otherwise, well, you're the main maintainer. :-)
> >>>
> >>> Max
> >>>
> >>>
> >>> PS: Can there be a middle ground? Like just printing a warning if the
> >>> backing file cannot be opened and we don't absolutely have to?
> >>
> >> Middle ground may be harder, but may be nicer (especially since we are
> >> talking about tightening behavior, making things that used to work now
> >> fail is not nice if there is not a transition period where it first just
> >> warns).
> >
> > We can do the nowadays usual thing: Add a -u option, print a deprecation
> > warning if we don't have -u and can't open the backing file, and put
> > it into the list of things to remove in 3.0.
>
> Works for me.
>

Should work for oVirt.

When we consume the -u/--unsafe option, should we use qemu-img version,
or maybe we need an official api for detecting capabilities?

    qemu-img caps --format json
    {
        "unsafe-create": True,
        ...

Nir