[Qemu-devel] [PATCH] qcow2: Remove unnecessary flush after L2 write

[Kevin Wolf <kwolf@redhat.com>]

When a new cluster was allocated, we only need a flush after the write to the
L2 table if it was a COW and we need to decrease the refcounts of the old
clusters.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
 block/qcow2-cluster.c |   16 ++++++++++++----
 1 files changed, 12 insertions(+), 4 deletions(-)

diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
index b023148..0ce24d6 100644
--- a/block/qcow2-cluster.c
+++ b/block/qcow2-cluster.c
@@ -656,7 +656,7 @@ static int write_l2_entries(BlockDriverState *bs, uint64_t *l2_table,
     int ret;
 
     BLKDBG_EVENT(bs->file, BLKDBG_L2_UPDATE);
-    ret = bdrv_pwrite_sync(bs->file, l2_offset + start_offset,
+    ret = bdrv_pwrite(bs->file, l2_offset + start_offset,
         &l2_table[l2_start_index], len);
     if (ret < 0) {
         return ret;
@@ -719,9 +719,17 @@ int qcow2_alloc_cluster_link_l2(BlockDriverState *bs, QCowL2Meta *m)
         goto err;
     }
 
-    for (i = 0; i < j; i++)
-        qcow2_free_any_clusters(bs,
-            be64_to_cpu(old_cluster[i]) & ~QCOW_OFLAG_COPIED, 1);
+    /*
+     * If this was a COW, we need to decrease the refcount of the old cluster.
+     * Also flush bs->file to get the right order for L2 and refcount update.
+     */
+    if (j != 0) {
+        bdrv_flush(bs->file);
+        for (i = 0; i < j; i++) {
+            qcow2_free_any_clusters(bs,
+                be64_to_cpu(old_cluster[i]) & ~QCOW_OFLAG_COPIED, 1);
+        }
+    }
 
     ret = 0;
 err:
-- 
1.7.2.2