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.
> + }
> + 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
>
> int copy_on_read; /* if nonzero, copy read backing sectors into image.
> note this is a reference count */
> --
> 2.1.4
>
