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From: Paul Brook <paul@codesourcery.com>
Subject: Re: [Qemu-devel] [PATCH v2] make windows notice media change
Date: Wed, 29 Jul 2009 21:05:18 +0100
References: <20090729120719.GN30449@redhat.com>
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To: Filip Navara <filip.navara@gmail.com>
Cc: qemu-devel@nongnu.org, Gleb Natapov <gleb@redhat.com>

> >> 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.

Paul