rbd: define some new format constants

Define constant symbols related to the rbd format 2 object names.
This begins to bring this version of the "rbd_types.h" header
more in line with the current user-space version of that file.
Complete reconciliation of differences will be done at some
point later, as a separate task.
Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>

drivers/block/rbd_types.h

@@ -15,15 +15,30 @@
 
 #include <linux/types.h>
 
+/* For format version 2, rbd image 'foo' consists of objects
+ *   rbd_id.foo		- id of image
+ *   rbd_header.<id>	- image metadata
+ *   rbd_data.<id>.0000000000000000
+ *   rbd_data.<id>.0000000000000001
+ *   ...		- data
+ * Clients do not access header data directly in rbd format 2.
+ */
+
+#define RBD_HEADER_PREFIX      "rbd_header."
+#define RBD_DATA_PREFIX        "rbd_data."
+#define RBD_ID_PREFIX          "rbd_id."
+
 /*
- * rbd image 'foo' consists of objects
+ * For format version 1, rbd image 'foo' consists of objects
  *   foo.rbd		- image metadata
- *   foo.00000000
- *   foo.00000001
+ *   rb.<idhi>.<idlo>.00000000
+ *   rb.<idhi>.<idlo>.00000001
  *   ...		- data
+ * There is no notion of a persistent image id in rbd format 1.
  */
 
 #define RBD_SUFFIX		".rbd"
+
 #define RBD_DIRECTORY           "rbd_directory"
 #define RBD_INFO                "rbd_info"