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Date: Tue, 2 Jul 2013 09:40:38 +0200
From: Stefan Hajnoczi <stefanha@gmail.com>
Message-ID: <20130702074038.GD6800@stefanha-thinkpad.redhat.com>
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Subject: Re: [Qemu-devel] [PATCHv2 08/11] block-migration: efficiently
 encode zero blocks
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To: Peter Lieven <pl@kamp.de>
Cc: kwolf@redhat.com, Stefan Hajnoczi <stefanha@redhat.com>, Juan Quintela <quintela@redhat.com>, qemu-devel@nongnu.org, ronniesahlberg@gmail.com, pbonzini@redhat.com

On Mon, Jul 01, 2013 at 05:55:15PM +0200, Peter Lieven wrote:
> 
> Am 01.07.2013 um 16:13 schrieb Stefan Hajnoczi <stefanha@redhat.com>:
> 
> > On Thu, Jun 27, 2013 at 03:11:32PM +0200, Peter Lieven wrote:
> > 
> > This patch breaks cross-version blog migration.  We need to control
> > whether or not to use the new BLK_MIG_FLAG_ZERO_BLOCK flag.
> 
> you are right the upgrade way works, but downgrade not. what is the
> proposed way to fix this?

The safest way is to add a block migration flag that toggles the zero
block.  By default it would be off.

Maybe you would also add a MigrationCapability in qapi-schema.json to
indicated that zero blocks are supported in block migration.  That would
allow management tools to ask QEMU on both sides if they support zero
blocks.

Use migrate-set-capabilities to enable the zero block feature.

CCed Juan Quintela for advice on migration.

Stefan