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.
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
> > >
>
>
