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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: Mon, 23 Nov 2009 08:53:54 -0600
User-agent: Thunderbird (X11/20090825)

Eduardo Habkost wrote:
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.

Right, but I wouldn't be surprised if a user complains that "I know that
my guest don't use that VM feature, so I want to be able to migrate to
an older version anyway".

This could be addressed with a "force" migration feature. That said, I don't believe that the overwhelming majority of users are in a position to determine whether they can safely migrate to an older version.


Anthony Liguori

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