Patchwork [5/6] qcow2: drop flush in update_cluster_refcount()

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Submitter Stefan Hajnoczi
Date Feb. 19, 2013, 3:45 p.m.
Message ID <>
Download mbox | patch
Permalink /patch/221725/
State New
Headers show


Stefan Hajnoczi - Feb. 19, 2013, 3:45 p.m.
The update_cluster_refcount() function increments/decrements a cluster's
refcount and then returns the new refcount value.

There is no need to flush since both update_cluster_refcount() callers
already take care of this:

1. qcow2_alloc_bytes() calls update_cluster_refcount() when compressed
   sectors will be appended to an existing cluster with enough free
   space.  qcow2_alloc_bytes() already flushes so there is no need to do
   so in update_cluster_refcount().

2. qcow2_update_snapshot_refcount() sets a cache dependency on refcounts
   if it needs to update L2 entries.  It also flushes before completing.

Removing this flush significantly speeds up qcow2 snapshot creation:

  $ qemu-img create -f qcow2 test.qcow2 -o size=50G,preallocation=metadata
  $ time qemu-img snapshot -c new test.qcow2

Time drops from more than 3 minutes to under 1 second.

Signed-off-by: Stefan Hajnoczi <>
 block/qcow2-refcount.c | 2 --
 1 file changed, 2 deletions(-)


diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c
index df29187..12d9065 100644
--- a/block/qcow2-refcount.c
+++ b/block/qcow2-refcount.c
@@ -526,8 +526,6 @@  static int update_cluster_refcount(BlockDriverState *bs,
         return ret;
-    bdrv_flush(bs->file);
     return get_refcount(bs, cluster_index);