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;