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

From: Eric Blake
Subject: Re: [Qemu-discuss] [Qemu-devel] Supporting unsafe create when backing file is not accessible
Date: Mon, 17 Jul 2017 06:36:56 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.1

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.

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.

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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