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From: Paul Brook <paul@codesourcery.com>
Date: Tue, 24 Nov 2009 13:59:49 +0000
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Subject: [Qemu-devel] Re: Live migration protocol, device features,
	ABIs and other beasts
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To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>, qemu-devel@nongnu.org

> > Reading in old state files is a whole lot easier (to write
> > maintain, and stay sane) than producing state that is bug-compatible with
> > previous versions.
> 
> It seems to me that old->new and new->old migrations are
> of about the same level of difficulty.
> Supporting one of these but not the other is of course
> easier than supporting both, but I don't see where
> "a whole lot" comes from.

Migrating from old version requires the restore routine be version aware. 
Migrating to old versions requires the the save routine also be version aware, 
which I'd expect to be about double the amount of work.

Paul