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 <stefa...@redhat.com>
---
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);
}
--
1.8.1.2
