linux-libre/Documentation/device-mapper/dm-queue-length.txt

dm-queue-length
===============

dm-queue-length is a path selector module for device-mapper targets,
which selects a path with the least number of in-flight I/Os.
The path selector name is 'queue-length'.

Table parameters for each path: [<repeat_count>]
	<repeat_count>: The number of I/Os to dispatch using the selected
			path before switching to the next path.
			If not given, internal default is used. To check
			the default value, see the activated table.

Status for each path: <status> <fail-count> <in-flight>
	<status>: 'A' if the path is active, 'F' if the path is failed.
	<fail-count>: The number of path failures.
	<in-flight>: The number of in-flight I/Os on the path.


Algorithm
=========

dm-queue-length increments/decrements 'in-flight' when an I/O is
dispatched/completed respectively.
dm-queue-length selects a path with the minimum 'in-flight'.


Examples
========
In case that 2 paths (sda and sdb) are used with repeat_count == 128.

# echo "0 10 multipath 0 0 1 1 queue-length 0 2 1 8:0 128 8:16 128" \
  dmsetup create test
#
# dmsetup table
test: 0 10 multipath 0 0 1 1 queue-length 0 2 1 8:0 128 8:16 128
#
# dmsetup status
test: 0 10 multipath 2 0 0 0 1 1 E 0 2 1 8:0 A 0 0 8:16 A 0 0