linux/Documentation/device-mapper/dm-queue-length.txt
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   1dm-queue-length
   2===============
   3
   4dm-queue-length is a path selector module for device-mapper targets,
   5which selects a path with the least number of in-flight I/Os.
   6The path selector name is 'queue-length'.
   7
   8Table parameters for each path: [<repeat_count>]
   9        <repeat_count>: The number of I/Os to dispatch using the selected
  10                        path before switching to the next path.
  11                        If not given, internal default is used. To check
  12                        the default value, see the activated table.
  13
  14Status for each path: <status> <fail-count> <in-flight>
  15        <status>: 'A' if the path is active, 'F' if the path is failed.
  16        <fail-count>: The number of path failures.
  17        <in-flight>: The number of in-flight I/Os on the path.
  18
  19
  20Algorithm
  21=========
  22
  23dm-queue-length increments/decrements 'in-flight' when an I/O is
  24dispatched/completed respectively.
  25dm-queue-length selects a path with the minimum 'in-flight'.
  26
  27
  28Examples
  29========
  30In case that 2 paths (sda and sdb) are used with repeat_count == 128.
  31
  32# echo "0 10 multipath 0 0 1 1 queue-length 0 2 1 8:0 128 8:16 128" \
  33  dmsetup create test
  34#
  35# dmsetup table
  36test: 0 10 multipath 0 0 1 1 queue-length 0 2 1 8:0 128 8:16 128
  37#
  38# dmsetup status
  39test: 0 10 multipath 2 0 0 0 1 1 E 0 2 1 8:0 A 0 0 8:16 A 0 0
  40
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