dm delay: enhance kernel documentation

This commit improves documentation of the dm-delay target.

Signed-off-by: Heinz Mauelshagen <[email protected]>
Signed-off-by: Mikulas Patocka <[email protected]>

Author: mauelsha

Documentation/admin-guide/device-mapper/delay.rst

@@ -3,29 +3,52 @@ dm-delay
 ========
 
 Device-Mapper's "delay" target delays reads and/or writes
-and maps them to different devices.
+and/or flushs and optionally maps them to different devices.
 
-Parameters::
+Arguments::
 
     <device> <offset> <delay> [<write_device> <write_offset> <write_delay>
 			       [<flush_device> <flush_offset> <flush_delay>]]
 
-With separate write parameters, the first set is only used for reads.
+Table line has to either have 3, 6 or 9 arguments:
+
+3: apply offset and delay to read, write and flush operations on device
+
+6: apply offset and delay to device, also apply write_offset and write_delay
+   to write and flush operations on optionally different write_device with
+   optionally different sector offset
+
+9: same as 6 arguments plus define flush_offset and flush_delay explicitely
+   on/with optionally different flush_device/flush_offset.
+
 Offsets are specified in sectors.
+
 Delays are specified in milliseconds.
 
+
 Example scripts
 ===============
 
 ::
-
 	#!/bin/sh
-	# Create device delaying rw operation for 500ms
-	echo "0 `blockdev --getsz $1` delay $1 0 500" | dmsetup create delayed
+	#
+	# Create mapped device named "delayed" delaying read, write and flush operations for 500ms.
+	#
+	dmsetup create delayed --table  "0 `blockdev --getsz $1` delay $1 0 500"
 
 ::
+	#!/bin/sh
+	#
+	# Create mapped device delaying write and flush operations for 400ms and
+	# splitting reads to device $1 but writes and flushs to different device $2
+	# to different offsets of 2048 and 4096 sectors respectively.
+	#
+	dmsetup create delayed --table "0 `blockdev --getsz $1` delay $1 2048 0 $2 4096 400"
 
+::
 	#!/bin/sh
-	# Create device delaying only write operation for 500ms and
-	# splitting reads and writes to different devices $1 $2
-	echo "0 `blockdev --getsz $1` delay $1 0 0 $2 0 500" | dmsetup create delayed
+	#
+	# Create mapped device delaying reads for 50ms, writes for 100ms and flushs for 333ms
+	# onto the same backing device at offset 0 sectors.
+	#
+	dmsetup create delayed --table "0 `blockdev --getsz $1` delay $1 0 50 $2 0 100 $1 0 333"