[Qemu-devel] [PATCH 1/1] qcow2: avoid extra flushes in qcow2

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

qcow2_cache_flush() calls bdrv_flush() unconditionally after writing
cache entries of a particular cache. This can lead to as many as
2 additional fdatasyncs inside bdrv_flush.

We can simply skip all fdatasync calls inside qcow2_co_flush_to_os
as bdrv_flush for sure will do the job. This seriously affects the
performance of database operations inside the guest.

Signed-off-by: Denis V. Lunev <den@openvz.org>
CC: Pavel Borzenkov <pborzenkov@virtuozzo.com>
CC: Kevin Wolf <kwolf@redhat.com>
CC: Max Reitz <mreitz@redhat.com>
---
 block/qcow2-cache.c | 11 +++++++++--
 block/qcow2.c       |  4 ++--
 block/qcow2.h       |  1 +
 3 files changed, 12 insertions(+), 4 deletions(-)

diff --git a/block/qcow2-cache.c b/block/qcow2-cache.c
index 0fe8eda..6079c4a 100644
--- a/block/qcow2-cache.c
+++ b/block/qcow2-cache.c
@@ -226,7 +226,7 @@ static int qcow2_cache_entry_flush(BlockDriverState *bs, Qcow2Cache *c, int i)
     return 0;
 }
 
-int qcow2_cache_flush(BlockDriverState *bs, Qcow2Cache *c)
+int qcow2_cache_flush_nosync(BlockDriverState *bs, Qcow2Cache *c)
 {
     BDRVQcow2State *s = bs->opaque;
     int result = 0;
@@ -242,8 +242,15 @@ int qcow2_cache_flush(BlockDriverState *bs, Qcow2Cache *c)
         }
     }
 
+    return result;
+}
+
+int qcow2_cache_flush(BlockDriverState *bs, Qcow2Cache *c)
+{
+    int result = qcow2_cache_flush_nosync(bs, c);
+
     if (result == 0) {
-        ret = bdrv_flush(bs->file->bs);
+        int ret = bdrv_flush(bs->file->bs);
         if (ret < 0) {
             result = ret;
         }
diff --git a/block/qcow2.c b/block/qcow2.c
index 38caa66..bb6b788 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -2837,14 +2837,14 @@ static coroutine_fn int qcow2_co_flush_to_os(BlockDriverState *bs)
     int ret;
 
     qemu_co_mutex_lock(&s->lock);
-    ret = qcow2_cache_flush(bs, s->l2_table_cache);
+    ret = qcow2_cache_flush_nosync(bs, s->l2_table_cache);
     if (ret < 0) {
         qemu_co_mutex_unlock(&s->lock);
         return ret;
     }
 
     if (qcow2_need_accurate_refcounts(s)) {
-        ret = qcow2_cache_flush(bs, s->refcount_block_cache);
+        ret = qcow2_cache_flush_nosync(bs, s->refcount_block_cache);
         if (ret < 0) {
             qemu_co_mutex_unlock(&s->lock);
             return ret;
diff --git a/block/qcow2.h b/block/qcow2.h
index a063a3c..0751225 100644
--- a/block/qcow2.h
+++ b/block/qcow2.h
@@ -583,6 +583,7 @@ int qcow2_cache_destroy(BlockDriverState* bs, Qcow2Cache *c);
 void qcow2_cache_entry_mark_dirty(BlockDriverState *bs, Qcow2Cache *c,
      void *table);
 int qcow2_cache_flush(BlockDriverState *bs, Qcow2Cache *c);
+int qcow2_cache_flush_nosync(BlockDriverState *bs, Qcow2Cache *c);
 int qcow2_cache_set_dependency(BlockDriverState *bs, Qcow2Cache *c,
     Qcow2Cache *dependency);
 void qcow2_cache_depends_on_flush(Qcow2Cache *c);
-- 
2.1.4