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Date: Wed, 29 Jul 2009 14:34:49 -0500
From: Anthony Liguori <anthony@codemonkey.ws>
MIME-Version: 1.0
Subject: Re: [Qemu-devel] [PATCH v2] make windows notice media change
References: <20090729120719.GN30449@redhat.com>	<20090729135057.GY30449@redhat.com>	<5b31733c0907290735u5dd86d86i293f34df4ce09d10@mail.gmail.com>	<200907292011.38879.paul@codesourcery.com>	<5b31733c0907291218k624f97bbx62d8d6d49b8f0759@mail.gmail.com>
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To: Filip Navara <filip.navara@gmail.com>
Cc: Paul Brook <paul@codesourcery.com>, Gleb Natapov <gleb@redhat.com>, qemu-devel@nongnu.org

Filip Navara wrote:
> On Wed, Jul 29, 2009 at 9:18 PM, Filip Navara<filip.navara@gmail.com> wrote:
>   
>> On Wed, Jul 29, 2009 at 9:11 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?!
>   

You can sometimes load older versions.  When a new version fixes a bug 
in an older version, you cannot migrate.  OTOH, if the new version adds 
a feature that was not exposed to the guest in the previous version, as 
long as you continue to not expose that feature to the guest, you can 
safely migrate.

Regards,

Anthony Liguori