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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