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

[Max Reitz]

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.

Max

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