From bfe8043e9214d2fc6572cc72b5f2218308747acd Mon Sep 17 00:00:00 2001 From: Stefan Hajnoczi Date: Fri, 27 Jul 2012 09:05:22 +0100 Subject: qcow2: implement lazy refcounts Lazy refcounts is a performance optimization for qcow2 that postpones refcount metadata updates and instead marks the image dirty. In the case of crash or power failure the image will be left in a dirty state and repaired next time it is opened. Reducing metadata I/O is important for cache=writethrough and cache=directsync because these modes guarantee that data is on disk after each write (hence we cannot take advantage of caching updates in RAM). Refcount metadata is not needed for guest->file block address translation and therefore does not need to be on-disk at the time of write completion - this is the motivation behind the lazy refcount optimization. The lazy refcount optimization must be enabled at image creation time: qemu-img create -f qcow2 -o compat=1.1,lazy_refcounts=on a.qcow2 10G qemu-system-x86_64 -drive if=virtio,file=a.qcow2,cache=writethrough Update qemu-iotests 031 and 036 since the extension header size changes when we add feature bit table entries. Signed-off-by: Stefan Hajnoczi Signed-off-by: Kevin Wolf --- block/qcow2-cluster.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-) (limited to 'block/qcow2-cluster.c') diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c index d7e0e19..e179211 100644 --- a/block/qcow2-cluster.c +++ b/block/qcow2-cluster.c @@ -662,7 +662,10 @@ int qcow2_alloc_cluster_link_l2(BlockDriverState *bs, QCowL2Meta *m) qcow2_cache_depends_on_flush(s->l2_table_cache); } - qcow2_cache_set_dependency(bs, s->l2_table_cache, s->refcount_block_cache); + if (qcow2_need_accurate_refcounts(s)) { + qcow2_cache_set_dependency(bs, s->l2_table_cache, + s->refcount_block_cache); + } ret = get_cluster_table(bs, m->offset, &l2_table, &l2_index); if (ret < 0) { goto err; -- cgit v1.1