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Re: [Qemu-block] [PATCH 1/3] block: ignore flush requests when storage i
From: |
Paolo Bonzini |
Subject: |
Re: [Qemu-block] [PATCH 1/3] block: ignore flush requests when storage is clean |
Date: |
Tue, 28 Jun 2016 23:01:40 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.1.1 |
On 24/06/2016 17:06, Denis V. Lunev wrote:
> From: Evgeny Yakovlev <address@hidden>
>
> 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 <address@hidden>
> Signed-off-by: Denis V. Lunev <address@hidden>
> CC: Kevin Wolf <address@hidden>
> CC: Max Reitz <address@hidden>
> CC: Stefan Hajnoczi <address@hidden>
> CC: Fam Zheng <address@hidden>
> CC: John Snow <address@hidden>
> ---
> 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 */
>