Commit 5d3691a8 authored by Heinz Mauelshagen's avatar Heinz Mauelshagen Committed by Mikulas Patocka
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dm delay: enhance kernel documentation 



This commit improves documentation of the dm-delay target.

Signed-off-by: default avatarHeinz Mauelshagen <heinzm@redhat.com>
Signed-off-by: default avatarMikulas Patocka <mpatocka@redhat.com>
parent 013f510d

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

+32 −9
Original line number Diff line number Diff line
@@ -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"