Re: [Qemu-devel] [PATCH v2] make windows notice media change

[Filip Navara <filip.navara@gmail.com>]

On Wed, Jul 29, 2009 at 9:52 PM, Paul Brook<paul@codesourcery.com> wrote:
>> >>> Why shouldn't we support loading version 2 snapshots? Afterall that's
>> >>> why we had the versioning in the first place.
>> >>
>> >> No it's not. Versioning was introduced to *prevent* loading old
>> >> snapshots and crashing or ending up with inconsistent guest state. I'm
>> >> still unconvinced that anything other than very short term backward
>> >> compatibility is worthwhile or even viable.
>> >
>> > I see it as a way to migrate a running guest to newer QEMU version,
>> > possibly even with live migration. In fact I used it quite often back in
>> > the day when snapshots were not part of qcow2 yet and when kqemu was
>> > still in its heydays.
>>
>> 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.

- F.