Commit f9cb2860 authored by Stefan Hajnoczi's avatar Stefan Hajnoczi
Browse files

qcow2: drop flush in update_cluster_refcount() 



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: default avatarStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: default avatarKevin Wolf <kwolf@redhat.com>
parent 2154f24e

block/qcow2-refcount.c

+0 −2
Original line number Diff line number Diff line
@@ -532,8 +532,6 @@ static int update_cluster_refcount(BlockDriverState *bs, 
        return ret;

    }



    bdrv_flush(bs->file);



    return get_refcount(bs, cluster_index);

}