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From: "Denis V. Lunev" <den-lists@virtuozzo.com>
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Date: Wed, 15 Jun 2016 13:37:18 +0300
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Subject: Re: [Qemu-devel] [PATCH 5/9] mirror: improve performance of
 mirroring of empty disk
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To: Stefan Hajnoczi <stefanha@redhat.com>, Eric Blake <eblake@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>, vsementsov@virtuozzo.com, Fam Zheng <famz@redhat.com>, qemu-block@nongnu.org, Jeff Cody <jcody@redhat.com>, qemu-devel@nongnu.org, Max Reitz <mreitz@redhat.com>, "Denis V. Lunev" <den@openvz.org>

On 06/15/2016 12:19 PM, Stefan Hajnoczi wrote:
> On Tue, Jun 14, 2016 at 09:20:47PM -0600, Eric Blake wrote:
>> On 06/14/2016 09:25 AM, Denis V. Lunev wrote:
>>> We should not take into account zero blocks for delay calculations.
>>> They are not read and thus IO throttling is not required. In the
>>> other case VM migration with 16 Tb QCOW2 disk with 4 Gb of data takes
>>> days.
>>>
>>> Signed-off-by: Denis V. Lunev <den@openvz.org>
>>> Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
>>> CC: Stefan Hajnoczi <stefanha@redhat.com>
>>> CC: Fam Zheng <famz@redhat.com>
>>> CC: Kevin Wolf <kwolf@redhat.com>
>>> CC: Max Reitz <mreitz@redhat.com>
>>> CC: Jeff Cody <jcody@redhat.com>
>>> CC: Eric Blake <eblake@redhat.com>
>>> ---
>>>   block/mirror.c | 7 +++++--
>>>   1 file changed, 5 insertions(+), 2 deletions(-)
>> Seems reasonable, but I'll let others more familiar with throttling give
>> the final say.
> There is a bounce buffer fallback when !drv->bdrv_co_pwrite_zeroes.  In
> that case we need to account for the bytes transferred.  I don't see
> where the patch takes this into account.
Interesting point.

I think we are charging for IO performed. Thus with the
absence of the callback we should charge for io_sectors/2
The write will be full scale, there is no reading.

Correct?