Re: [Qemu-devel] [PATCH v4 1/3] block: ignore flush requests when storage is clean

[Fam Zheng <famz@redhat.com>]

On Tue, 06/28 12:10, Denis V. Lunev wrote:
> On 06/28/2016 04:27 AM, Fam Zheng wrote:
> > On Mon, 06/27 17:47, Denis V. Lunev wrote:
> > > From: Evgeny Yakovlev <eyakovlev@virtuozzo.com>
> > > 
> > > Some guests (win2008 server for example) do a lot of unnecessary
> > > flushing when underlying media has not changed. This adds additional
> > > overhead on host when calling fsync/fdatasync.
> > > 
> > > This change introduces a dirty flag in BlockDriverState which is set
> > > in bdrv_set_dirty and is checked in bdrv_co_flush. This allows us to
> > > avoid unnecessary flushing when storage is clean.
> > > 
> > > The problem with excessive flushing was found by a performance test
> > > which does parallel directory tree creation (from 2 processes).
> > > Results improved from 0.424 loops/sec to 0.432 loops/sec.
> > > Each loop creates 10^3 directories with 10 files in each.
> > > 
> > > Signed-off-by: Evgeny Yakovlev <eyakovlev@virtuozzo.com>
> > > Signed-off-by: Denis V. Lunev <den@openvz.org>
> > > CC: Kevin Wolf <kwolf@redhat.com>
> > > CC: Max Reitz <mreitz@redhat.com>
> > > CC: Stefan Hajnoczi <stefanha@redhat.com>
> > > CC: Fam Zheng <famz@redhat.com>
> > > CC: John Snow <jsnow@redhat.com>
> > > ---
> > >   block.c                   |  1 +
> > >   block/dirty-bitmap.c      |  3 +++
> > >   block/io.c                | 19 +++++++++++++++++++
> > >   include/block/block_int.h |  1 +
> > >   4 files changed, 24 insertions(+)
> > > 
> > > diff --git a/block.c b/block.c
> > > index 947df29..68ae3a0 100644
> > > --- a/block.c
> > > +++ b/block.c
> > > @@ -2581,6 +2581,7 @@ int bdrv_truncate(BlockDriverState *bs, int64_t offset)
> > >           ret = refresh_total_sectors(bs, offset >> BDRV_SECTOR_BITS);
> > >           bdrv_dirty_bitmap_truncate(bs);
> > >           bdrv_parent_cb_resize(bs);
> > > +        bs->dirty = true; /* file node sync is needed after truncate */
> > >       }
> > >       return ret;
> > >   }
> > > diff --git a/block/dirty-bitmap.c b/block/dirty-bitmap.c
> > > index 4902ca5..54e0413 100644
> > > --- a/block/dirty-bitmap.c
> > > +++ b/block/dirty-bitmap.c
> > > @@ -370,6 +370,9 @@ void bdrv_set_dirty(BlockDriverState *bs, int64_t cur_sector,
> > >           }
> > >           hbitmap_set(bitmap->bitmap, cur_sector, nr_sectors);
> > >       }
> > > +
> > > +    /* Set global block driver dirty flag even if bitmap is disabled */
> > > +    bs->dirty = true;
> > >   }
> > >   /**
> > > diff --git a/block/io.c b/block/io.c
> > > index b9e53e3..152f5a9 100644
> > > --- a/block/io.c
> > > +++ b/block/io.c
> > > @@ -2247,6 +2247,25 @@ int coroutine_fn bdrv_co_flush(BlockDriverState *bs)
> > >           goto flush_parent;
> > >       }
> > > +    /* Check if storage is actually dirty before flushing to disk */
> > > +    if (!bs->dirty) {
> > > +        /* Flush requests are appended to tracked request list in order so that
> > > +         * most recent request is at the head of the list. Following code uses
> > > +         * this ordering to wait for the most recent flush request to complete
> > > +         * to ensure that requests return in order */
> > > +        BdrvTrackedRequest *prev_req;
> > > +        QLIST_FOREACH(prev_req, &bs->tracked_requests, list) {
> > > +            if (prev_req == &req || prev_req->type != BDRV_TRACKED_FLUSH) {
> > > +                continue;
> > > +            }
> > > +
> > > +            qemu_co_queue_wait(&prev_req->wait_queue);
> > > +            break;
> > > +        }
> > > +        goto flush_parent;
> > Should we check bs->dirty again after qemu_co_queue_wait()? I think another
> > write request could sneak in while this coroutine yields.
> no, we do not care. Any subsequent to FLUSH write does not guaranteed to
> be flushed. We have the warranty only that all write requests completed
> prior to this flush are really flushed.

I'm not worried about subsequent requests.

A prior request can be already in progress or be waiting when we check
bs->dirty, though it would be false there, but it will become true soon --
bdrv_set_dirty is only called when a request is completing.

Fam

> 
> 
> 
> > > +    }
> > > +    bs->dirty = false;
> > > +
> > >       BLKDBG_EVENT(bs->file, BLKDBG_FLUSH_TO_DISK);
> > >       if (bs->drv->bdrv_co_flush_to_disk) {
> > >           ret = bs->drv->bdrv_co_flush_to_disk(bs);
> > > diff --git a/include/block/block_int.h b/include/block/block_int.h
> > > index 0432ba5..59a7def 100644
> > > --- a/include/block/block_int.h
> > > +++ b/include/block/block_int.h
> > > @@ -435,6 +435,7 @@ struct BlockDriverState {
> > >       bool valid_key; /* if true, a valid encryption key has been set */
> > >       bool sg;        /* if true, the device is a /dev/sg* */
> > >       bool probed;    /* if true, format was probed rather than specified */
> > > +    bool dirty;     /* if true, media is dirty and should be flushed */
> > How about renaming this to "need_flush"? The one "dirty" we had is set by
> > bdrv_set_dirty, and cleared by bdrv_reset_dirty_bitmap. I'd avoid the
> > confusion between the two concepts.
> > 
> > Fam
> 
> can be
> 
> > >       int copy_on_read; /* if nonzero, copy read backing sectors into image.
> > >                            note this is a reference count */
> > > -- 
> > > 2.1.4
> > > 
> 
>