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Date: Tue, 02 Jul 2013 12:51:14 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] [PATCHv2 08/11] block-migration: efficiently
 encode zero blocks
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To: Stefan Hajnoczi <stefanha@gmail.com>
Cc: kwolf@redhat.com, Stefan Hajnoczi <stefanha@redhat.com>, Juan Quintela <quintela@redhat.com>, Peter Lieven <pl@kamp.de>, qemu-devel@nongnu.org, ronniesahlberg@gmail.com

Il 02/07/2013 09:40, Stefan Hajnoczi ha scritto:
> 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.

Yes, a capability makes sense.

Paolo