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Re: [Qemu-devel] Re: Live migration protocol, device features, ABIs and

From: Anthony Liguori
Subject: Re: [Qemu-devel] Re: Live migration protocol, device features, ABIs and other beasts
Date: Sun, 22 Nov 2009 20:17:46 -0600
User-agent: Thunderbird (X11/20090825)

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.


Anthony Liguori

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