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

[Paolo Bonzini <pbonzini@redhat.com>]

On 24/06/2016 17:06, 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 unnesessary 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 |  2 ++
>  4 files changed, 25 insertions(+)
> 
> diff --git a/block.c b/block.c
> index f4648e9..e36f148 100644
> --- a/block.c
> +++ b/block.c
> @@ -2582,6 +2582,7 @@ 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 7cf3645..8078af2 100644
> --- a/block/io.c
> +++ b/block/io.c
> @@ -2239,6 +2239,25 @@ 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;

Can you just have a CoQueue specific to flushes, where a completing
flush does a restart_all on the CoQueue?

Flushes are never serialising, so there's no reason for them to be in
tracked_requests (I posted patches a while ago that instead use a simple
atomic counter, but they will only be in 2.8).

Paolo

> +    }
> +    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 2057156..616058b 100644
> --- a/include/block/block_int.h
> +++ b/include/block/block_int.h
> @@ -418,6 +418,8 @@ struct BlockDriverState {
>      int sg;        /* if true, the device is a /dev/sg* */
>      int copy_on_read; /* if true, copy read backing sectors into image
>                           note this is a reference count */
> +
> +    bool dirty;
>      bool probed;
>  
>      BlockDriver *drv; /* NULL means no media */
>