--- /dev/null
+Transaction Number
+~~~~~~~~~~~~~~~~~~
+[[transno]]
+
+For each target there is a sequence of values (a strictly increasing
+series of numbers) where each operation that can modify the file
+system is assigned the next number in the series. This is the
+transaction number, and it imposes a strict serial ordering for all
+file system modifying operations. For file system modifying
+requests the server assigns the next value in the sequence and
+informs the client of the value in the 'pb_transno' field of the
+'ptlrpc_body' of its reply to the client's request. For replys to
+requests that do not modify the file system the 'pb_transno' field in
+the 'ptlrpc_body' is just set to 0.
+