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Date: Wed, 15 Jun 2016 10:19:43 +0100
From: Stefan Hajnoczi <stefanha@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH 5/9] mirror: improve performance of
 mirroring of empty disk
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To: Eric Blake <eblake@redhat.com>
Cc: "Denis V. Lunev" <den@openvz.org>, qemu-devel@nongnu.org, qemu-block@nongnu.org, vsementsov@virtuozzo.com, Fam Zheng <famz@redhat.com>, Kevin Wolf <kwolf@redhat.com>, Max Reitz <mreitz@redhat.com>, Jeff Cody <jcody@redhat.com>


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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.
> >=20
> > 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(-)
>=20
> 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.

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