From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:36950) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1ZuDJe-0001e7-55 for qemu-devel@nongnu.org; Thu, 05 Nov 2015 00:42:31 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1ZuDJc-0006Ho-Vd for qemu-devel@nongnu.org; Thu, 05 Nov 2015 00:42:30 -0500 Date: Thu, 5 Nov 2015 13:42:19 +0800 From: Fam Zheng Message-ID: <20151105054219.GG24893@ad.usersys.redhat.com> References: <1433742974-20128-1-git-send-email-famz@redhat.com> <1433742974-20128-4-git-send-email-famz@redhat.com> <20151104183526.GA8620@noname.redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20151104183526.GA8620@noname.redhat.com> Subject: Re: [Qemu-devel] [PATCH v7 3/8] mirror: Do zero write on target if sectors not allocated List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Kevin Wolf Cc: qemu-block@nongnu.org, rjones@redhat.com, Jeff Cody , qemu-devel@nongnu.org, qemu-stable@nongnu.org, Stefan Hajnoczi , pbonzini@redhat.com, jsnow@redhat.com, wangxiaolong@web.ucloud.cn On Wed, 11/04 19:35, Kevin Wolf wrote: > Am 08.06.2015 um 07:56 hat Fam Zheng geschrieben: > > If guest discards a source cluster, mirroring with bdrv_aio_readv is overkill. > > Some protocols do zero upon discard, where it's best to use > > bdrv_aio_write_zeroes, otherwise, bdrv_aio_discard will be enough. > > > > Signed-off-by: Fam Zheng > > --- > > block/mirror.c | 20 ++++++++++++++++++-- > > 1 file changed, 18 insertions(+), 2 deletions(-) > > > > diff --git a/block/mirror.c b/block/mirror.c > > index d2515c7..3c38695 100644 > > --- a/block/mirror.c > > +++ b/block/mirror.c > > @@ -164,6 +164,8 @@ static uint64_t coroutine_fn mirror_iteration(MirrorBlockJob *s) > > int64_t end, sector_num, next_chunk, next_sector, hbitmap_next_sector; > > uint64_t delay_ns = 0; > > MirrorOp *op; > > + int pnum; > > + int64_t ret; > > > > s->sector_num = hbitmap_iter_next(&s->hbi); > > if (s->sector_num < 0) { > > @@ -290,8 +292,22 @@ static uint64_t coroutine_fn mirror_iteration(MirrorBlockJob *s) > > s->in_flight++; > > s->sectors_in_flight += nb_sectors; > > trace_mirror_one_iteration(s, sector_num, nb_sectors); > > - bdrv_aio_readv(source, sector_num, &op->qiov, nb_sectors, > > - mirror_read_complete, op); > > + > > + ret = bdrv_get_block_status_above(source, NULL, sector_num, > > + nb_sectors, &pnum); > > + if (ret < 0 || pnum < nb_sectors || > > Earlier today I told Richard Jones that qemu-img commit should really > be using zero cluster support in the backing file since 2.4 because I > remembered this commit. Turns out it doesn't actually use it but writes > explicit zeros instead. > > The reason is the condition 'pnum < nb_sectors' here, which makes mirror > fall back to explicit writes if bdrv_get_block_status_above() doesn't > return enough sectors (enough being relatively large here, I think in > qemu-img commit it's always the full 10 MB buffer). > > In other words, we are ignoring any zero areas smaller than 10 MB! > > (What made this worse is that qcow2 had a bug that reports only a single > zero cluster at a time, so it would never report more than 10 MB, even > if the image was completely zeroed. I've sent a fix for that one.) > > In order to fix this, we'll probably need to move the call to > bdrv_get_block_status_above() before actually allocating memory and > all that for the full nb_chunks. We should detect zeros on the usual > block job granularity (64k by default, I think). > > > + (ret & BDRV_BLOCK_DATA && !(ret & BDRV_BLOCK_ZERO))) { > > + bdrv_aio_readv(source, sector_num, &op->qiov, nb_sectors, > > + mirror_read_complete, op); > > + } else if (ret & BDRV_BLOCK_ZERO) { > > + bdrv_aio_write_zeroes(s->target, sector_num, op->nb_sectors, > > + s->unmap ? BDRV_REQ_MAY_UNMAP : 0, > > + mirror_write_complete, op); > > + } else { > > + assert(!(ret & BDRV_BLOCK_DATA)); > > + bdrv_aio_discard(s->target, sector_num, op->nb_sectors, > > + mirror_write_complete, op); > > + } > > return delay_ns; > > } > > Paolo also noticed that there's no reason at all to allocate buffers > and a qiov for the write_zeroes and discard cases. I'll write a patch to address these. Thanks! Fam