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

From: Anthony Liguori
Subject: [Qemu-devel] Re: Live migration protocol, device features, ABIs and other beasts
Date: Mon, 23 Nov 2009 12:44:14 -0600
User-agent: Thunderbird (X11/20090825)

Juan Quintela wrote:
The problem here isn't migration, it's what you've decided to backport
into your stable branch.

No. the problem is that I made a mistake in the past.  And didn't add a
field to the state that I should.  It just happens to work without that
field in several use cases.  But in others, it fails spectacularly.
What to do here?

Blacklist the broken version of the format. If it's known to fail under normal usage, that's the only right thing to do given the mechanisms we possess today.

And from an upstream position, I would oppose implementing the pvclock
change in the stable branch exactly because of the problems it would
create with live migration.

Let's say that the problem is when half of the users told you that you
can't change it in the stable branch, and the other half told you that
without the change, migration don't work in a reliable way :(

If one user needs one behavior and another user absolute needs a different behavior, you provide a mechanism for a user to choose which policy they want.

See my proposal about configurable device blacklists.

It does'nt matter what you do, you lose :(

If we stick strictly to providing mechanisms and default policies, we can't lose because we always pass the blame higher in the stack :-)


Anthony Liguroi

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