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 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 <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>
---
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
