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Date: Wed, 29 Jul 2009 23:14:44 +0300
From: Gleb Natapov <gleb@redhat.com>
Subject: Re: [Qemu-devel] [PATCH v2] make windows notice media change
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To: Paul Brook <paul@codesourcery.com>
Cc: Filip Navara <filip.navara@gmail.com>, qemu-devel@nongnu.org

On Wed, Jul 29, 2009 at 09:05:18PM +0100, Paul Brook wrote:
> > >> BTW, why would there be the version parameter in the first place if it
> > >> wasn't supposed to load older versions?!
> > >
> > > Like I already said: it's there to prevent an old version being loaded
> > > accidentally. Without this an incompatible change will  result in
> > > anything from a crash to corrupt/inconsistent guest state. Versioning
> > > allows us to reject the snapshot and fail safely.
> >
> > If that was the case then the if (version != x) return -EINVAL check could
> > have been in the generic code and there would be no need for the version
> > parameter in the load function.
> 
> Preventing loading bad snapshots is the primary goal.
> 
> Allowing loading old snapshots is a secondary feature. Personally I think it's 
> not worth the effort, and in practice is unlikely to be feasible for whole 
> machines over any significant length of time. However I don't feel strongly 
> enough to actually rip the code out.
> 
Agree. All this "migrate from older version" just produce code that is
never tested in practice.

--
			Gleb.