linux/Documentation/device-mapper/delay.txt
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   1dm-delay
   2========
   3
   4Device-Mapper's "delay" target delays reads and/or writes
   5and maps them to different devices.
   6
   7Parameters:
   8    <device> <offset> <delay> [<write_device> <write_offset> <write_delay>]
   9
  10With separate write parameters, the first set is only used for reads.
  11Delays are specified in milliseconds.
  12
  13Example scripts
  14===============
  15[[
  16#!/bin/sh
  17# Create device delaying rw operation for 500ms
  18echo "0 `blockdev --getsize $1` delay $1 0 500" | dmsetup create delayed
  19]]
  20
  21[[
  22#!/bin/sh
  23# Create device delaying only write operation for 500ms and
  24# splitting reads and writes to different devices $1 $2
  25echo "0 `blockdev --getsize $1` delay $1 0 0 $2 0 500" | dmsetup create delayed
  26]]
  27
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