From: Stefan Hajnoczi Date: Mon, 4 Mar 2013 14:02:34 +0000 (+0100) Subject: qcow2: drop flush in update_cluster_refcount() X-Git-Tag: qemu-xen-4.4.0-rc1~6^2~906^2~15 X-Git-Url: http://xenbits.xensource.com/gitweb?a=commitdiff_plain;h=f9cb2860bd12e223036b61a1f5d1444e57b7efce;p=qemu-xen.git 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: Stefan Hajnoczi Signed-off-by: Kevin Wolf --- diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c index 3d29d30f03..92519ea62a 100644 --- a/block/qcow2-refcount.c +++ b/block/qcow2-refcount.c @@ -532,8 +532,6 @@ static int update_cluster_refcount(BlockDriverState *bs, return ret; } - bdrv_flush(bs->file); - return get_refcount(bs, cluster_index); }