On Wed, Jun 29, 2016 at 09:12:41AM +0800, Fam Zheng wrote:
> 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 <eyakov...@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 <eyakov...@virtuozzo.com>
> > > > Signed-off-by: Denis V. Lunev <d...@openvz.org>
> > > > CC: Kevin Wolf <kw...@redhat.com>
> > > > CC: Max Reitz <mre...@redhat.com>
> > > > CC: Stefan Hajnoczi <stefa...@redhat.com>
> > > > CC: Fam Zheng <f...@redhat.com>
> > > > CC: John Snow <js...@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.Flush only guarantees that already completed writes are persistent. It is not a barrier operation. It does not wait for in-flight writes and makes no guarantee regarding them. Stefan
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