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Re: [Qemu-block] [PATCH v7 3/8] mirror: Do zero write on target if secto
From: |
Fam Zheng |
Subject: |
Re: [Qemu-block] [PATCH v7 3/8] mirror: Do zero write on target if sectors not allocated |
Date: |
Thu, 5 Nov 2015 13:42:19 +0800 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
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 <address@hidden>
> > ---
> > 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