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From: Paul Brook <paul@codesourcery.com>
Subject: Re: [Qemu-devel] [PATCH 3/10] Add bdrv_flush_all()
Date: Wed, 10 Sep 2008 17:37:17 +0100
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To: qemu-devel@nongnu.org
Cc: Chris Wright <chrisw@redhat.com>, Glauber Costa <glommer@gmail.com>, Anthony Liguori <aliguori@us.ibm.com>, kvm@vger.kernel.org, Uri Lublin <uri.lublin@qumranet.com>

> I'm actually liking bdrv_flush_all() less and less.  If there are any
> outstanding IO requests, it will increase the down time associated with
> live migration.  I think we definitely need to add a live save handler
> that waits until there are no outstanding IO requests to converge.  I'm
> concerned though that it's not totally unreasonable to expect a guest to
> always have an IO request in flight.  That leads me to think that maybe
> we should be cancelling outstanding requests, and somehow saving their
> state?

That's not possible with the current code because the IO callbacks 
(particularly when you start involving the SCSI layer) are generated 
dynamically.

Paul