From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([208.118.235.92]:59626) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1UFQOa-0001Ad-Gz for qemu-devel@nongnu.org; Tue, 12 Mar 2013 10:41:49 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1UFQOY-0002PV-Dw for qemu-devel@nongnu.org; Tue, 12 Mar 2013 10:41:40 -0400 Received: from mx1.redhat.com ([209.132.183.28]:12846) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1UFQOY-0002PN-5N for qemu-devel@nongnu.org; Tue, 12 Mar 2013 10:41:38 -0400 From: Kevin Wolf Date: Tue, 12 Mar 2013 15:41:18 +0100 Message-Id: <1363099279-403-13-git-send-email-kwolf@redhat.com> In-Reply-To: <1363099279-403-1-git-send-email-kwolf@redhat.com> References: <1363099279-403-1-git-send-email-kwolf@redhat.com> Subject: [Qemu-devel] [PATCH 12/13] qcow2: drop flush in update_cluster_refcount() List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: anthony@codemonkey.ws Cc: kwolf@redhat.com, qemu-devel@nongnu.org From: Stefan Hajnoczi 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 --- block/qcow2-refcount.c | 2 -- 1 file changed, 2 deletions(-) diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c index 3d29d30..92519ea 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); } -- 1.8.1.4