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 <stefanha@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 4 insertions, 1 deletions

diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
index d7e0e19d9c..e179211c57 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;