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, 14 insertions, 12 deletions

diff --git a/block_int.h b/block_int.h
index d72317fbe3..6c1d9cafb1 100644
--- a/block_int.h
+++ b/block_int.h
@@ -31,8 +31,9 @@
 #include "qemu-timer.h"
 #include "qapi-types.h"
 
-#define BLOCK_FLAG_ENCRYPT	1
-#define BLOCK_FLAG_COMPAT6	4
+#define BLOCK_FLAG_ENCRYPT          1
+#define BLOCK_FLAG_COMPAT6          4
+#define BLOCK_FLAG_LAZY_REFCOUNTS   8
 
 #define BLOCK_IO_LIMIT_READ     0
 #define BLOCK_IO_LIMIT_WRITE    1
@@ -41,16 +42,17 @@
 #define BLOCK_IO_SLICE_TIME     100000000
 #define NANOSECONDS_PER_SECOND  1000000000.0
 
-#define BLOCK_OPT_SIZE          "size"
-#define BLOCK_OPT_ENCRYPT       "encryption"
-#define BLOCK_OPT_COMPAT6       "compat6"
-#define BLOCK_OPT_BACKING_FILE  "backing_file"
-#define BLOCK_OPT_BACKING_FMT   "backing_fmt"
-#define BLOCK_OPT_CLUSTER_SIZE  "cluster_size"
-#define BLOCK_OPT_TABLE_SIZE    "table_size"
-#define BLOCK_OPT_PREALLOC      "preallocation"
-#define BLOCK_OPT_SUBFMT        "subformat"
-#define BLOCK_OPT_COMPAT_LEVEL  "compat"
+#define BLOCK_OPT_SIZE              "size"
+#define BLOCK_OPT_ENCRYPT           "encryption"
+#define BLOCK_OPT_COMPAT6           "compat6"
+#define BLOCK_OPT_BACKING_FILE      "backing_file"
+#define BLOCK_OPT_BACKING_FMT       "backing_fmt"
+#define BLOCK_OPT_CLUSTER_SIZE      "cluster_size"
+#define BLOCK_OPT_TABLE_SIZE        "table_size"
+#define BLOCK_OPT_PREALLOC          "preallocation"
+#define BLOCK_OPT_SUBFMT            "subformat"
+#define BLOCK_OPT_COMPAT_LEVEL      "compat"
+#define BLOCK_OPT_LAZY_REFCOUNTS    "lazy_refcounts"
 
 typedef struct BdrvTrackedRequest BdrvTrackedRequest;