btrfs-progs: mkfs: enable space_cache=v2 (free-space-tree) by default

The free space tree is a better way to track the free space and has been
tested in the wild for a long time. The backward compatibility is
sufficient, several long term kernels. On-line conversion from v1 to v2
can be done by mount, switching from v2 to v1 can be done by 'btrfs
check'.

Issue: #295
Signed-off-by: David Sterba <dsterba@suse.com>

Documentation/mkfs.btrfs.asciidoc

@@ -269,7 +269,7 @@ Enable quota support (qgroups). The qgroup accounting will be consistent,
 can be used together with '--rootdir'.  See also `btrfs-quota`(8).
 
 *free-space-tree*::
-(kernel support since 4.5)
+(default since btrfs-progs 5.15, kernel support since 4.5)
 +
 Enable the free space tree (mount option space_cache=v2) for persisting the
 free space cache.

common/fsfeatures.c

@@ -159,7 +159,7 @@ static const struct btrfs_feature runtime_features[] = {
 		.sysfs_name = "free_space_tree",
 		VERSION_TO_STRING2(compat, 4,5),
 		VERSION_TO_STRING2(safe, 4,9),
-		VERSION_NULL(default),
+		VERSION_TO_STRING2(default, 5,15),
 		.desc		= "free space tree (space_cache=v2)"
 	},
 	/* Keep this one last */

common/fsfeatures.h

@@ -25,6 +25,9 @@
 		| BTRFS_FEATURE_INCOMPAT_NO_HOLES		\
 		| BTRFS_FEATURE_INCOMPAT_SKINNY_METADATA)
 
+#define BTRFS_MKFS_DEFAULT_RUNTIME_FEATURES			\
+	(BTRFS_RUNTIME_FEATURE_FREE_SPACE_TREE)
+
 /*
  * Avoid multi-device features (RAID56), mixed block groups, and zoned mode
  */

mkfs/main.c

@@ -903,7 +903,7 @@ int BOX_MAIN(mkfs)(int argc, char **argv)
 	int saved_optind;
 	char fs_uuid[BTRFS_UUID_UNPARSED_SIZE] = { 0 };
 	u64 features = BTRFS_MKFS_DEFAULT_FEATURES;
-	u64 runtime_features = 0;
+	u64 runtime_features = BTRFS_MKFS_DEFAULT_RUNTIME_FEATURES;
 	struct mkfs_allocation allocation = { 0 };
 	struct btrfs_mkfs_config mkfs_cfg;
 	enum btrfs_csum_type csum_type = BTRFS_CSUM_TYPE_CRC32;