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Re: [Qemu-discuss] [Qemu-devel] Supporting unsafe create when backing fi

From: Kevin Wolf
Subject: Re: [Qemu-discuss] [Qemu-devel] Supporting unsafe create when backing file is not accessible
Date: Mon, 17 Jul 2017 14:19:10 +0200
User-agent: Mutt/1.5.21 (2010-09-15)

Am 17.07.2017 um 13:36 hat Eric Blake geschrieben:
> On 07/14/2017 02:13 PM, Nir Soffer wrote:
> > Eric, we are more concerned about using the current qemu version.
> > 
> > We can use the fact that providing both size and backing format,
> > qemu does not open the backing file, but this is not documented, and
> > we don't want to base oVirt code on undocumented behavior.
> > 
> > What we would like to have is:
> > - qemu blessing for using this undocumented behaviour
> But how are you going to get that? By the time someone writes a
> documentation patch, it won't land until qemu 2.10, but by then, they
> might as well have written the -u patch instead.

In fact, soft freeze is tomorrow, so if we want to have -u in 2.10, it
would be good to have a patch ready today.

> Older versions will continue to have the older behavior, unless someone
> backports a patch to give them the newer behavior - but if someone is
> backporting -u, presumably they will also backport whatever
> introspection mechanisms you would also use against upstream qemu to
> learn if -u is present.  If the introspection mechanism is not present
> or gives the answer that -u is not present, then you can safely assume
> the old behavior (because the new behavior is only going to be present
> intentionally).  But that is STILL not something that we are going to
> explicitly document, because it makes more sense to implement the
> working feature than to document the workaround to a missing feature.

While I think adding -u today is reasonably realistic, I'm doubtful that
we can get an introspection mechanism in place today. Perhaps we can
declare it a bug fix, but I'd rather not rush something like that.

How does libvirt detect qemu-img features today? Just try and then check
the error message?


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