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Date: Sun, 22 Nov 2009 20:17:46 -0600
From: Anthony Liguori <anthony@codemonkey.ws>
MIME-Version: 1.0
Subject: Re: [Qemu-devel] Re: Live migration protocol, device features, ABIs
	and   other beasts
References: <4B0952C9.9010803@redhat.com> <4B095D86.700@codemonkey.ws>
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: qemu-devel@nongnu.org

Paolo Bonzini wrote:
>
>> I don't see how this fixes anything. If you used feature bits, how do
>> you migrate from a version that has a feature bit that an older version
>> doesn't know about? Do you just ignore it?
>
> I'd go with chunk instead of feature bits, specifying them like in the 
> PNG specification:

You mean, each device would have multiple sections?  We already use 
chunks for each device state.

>> Migration needs to be conservative. There should be only two possible
>> outcomes: 1) a successful live migration or 2) graceful failure with the
>> source VM still running correctly. Silently ignoring things that could
>> affect the guests behavior means that it's possible that after failure,
>> the guest will fail in an unexpected way.
>
> It's up to the source to decide what information is extra.  For 
> example, the state of a RNG emulation is nice-to-have, but as long as 
> it is initialized from another random source on the destination you 
> shouldn't care.

We only migrate things that are guest visible.  Everything else is left 
to the user to configure.  We wouldn't migrate the state of a RNG 
emulation provided that it doesn't have an impact on the guest.

By definition, anything that is guest visible is important because it 
affects the guest's behavior.

Regards,

Anthony Liguori