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

["Denis V. Lunev" <den@openvz.org>]

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 unnessesary 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>
---
 block.c                   | 1 +
 block/dirty-bitmap.c      | 3 +++
 block/io.c                | 6 ++++++
 include/block/block_int.h | 1 +
 4 files changed, 11 insertions(+)

diff --git a/block.c b/block.c
index b331eb9..1a4e154 100644
--- a/block.c
+++ b/block.c
@@ -2591,6 +2591,7 @@ int bdrv_truncate(BlockDriverState *bs, int64_t offset)
         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..ad208ad 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 */
+    bs->dirty = true;
 }
 
 /**
diff --git a/block/io.c b/block/io.c
index ebdb9d8..ef372d2 100644
--- a/block/io.c
+++ b/block/io.c
@@ -2234,6 +2234,12 @@ int coroutine_fn bdrv_co_flush(BlockDriverState *bs)
         }
     }
 
+    /* Check if storage is actually dirty before flushing to disk */
+    if (!bs->dirty) {
+        goto flush_parent;
+    }
+    bs->dirty = false;
+
     /* But don't actually force it to the disk with cache=unsafe */
     if (bs->open_flags & BDRV_O_NO_FLUSH) {
         goto flush_parent;
diff --git a/include/block/block_int.h b/include/block/block_int.h
index 688c6be..a62943b 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -417,6 +417,7 @@ 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 */
-- 
2.1.4