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dm-service-time
===============

dm-service-time is a path selector module for device-mapper targets,
which selects a path with the shortest estimated service time for
the incoming I/O.

The service time for each path is estimated by dividing the total size
of in-flight I/Os on a path with the performance value of the path.
The performance value is a relative throughput value among all paths
in a path-group, and it can be specified as a table argument.

The path selector name is 'service-time'.

Table parameters for each path: [<repeat_count> [<relative_throughput>]]
	<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.
	<relative_throughput>: The relative throughput value of the path
			among all paths in the path-group.
			The valid range is 0-100.
			If not given, minimum value '1' is used.
			If '0' is given, the path isn't selected while
			other paths having a positive value are available.

Status for each path: <status> <fail-count> <in-flight-size> \
		      <relative_throughput>
	<status>: 'A' if the path is active, 'F' if the path is failed.
	<fail-count>: The number of path failures.
	<in-flight-size>: The size of in-flight I/Os on the path.
	<relative_throughput>: The relative throughput value of the path
			among all paths in the path-group.


Algorithm
=========

dm-service-time adds the I/O size to 'in-flight-size' when the I/O is
dispatched and subtracts when completed.
Basically, dm-service-time selects a path having minimum service time
which is calculated by:

	('in-flight-size' + 'size-of-incoming-io') / 'relative_throughput'

However, some optimizations below are used to reduce the calculation
as much as possible.

	1. If the paths have the same 'relative_throughput', skip
	   the division and just compare the 'in-flight-size'.

	2. If the paths have the same 'in-flight-size', skip the division
	   and just compare the 'relative_throughput'.

	3. If some paths have non-zero 'relative_throughput' and others
	   have zero 'relative_throughput', ignore those paths with zero
	   'relative_throughput'.

If such optimizations can't be applied, calculate service time, and
compare service time.
If calculated service time is equal, the path having maximum
'relative_throughput' may be better.  So compare 'relative_throughput'
then.


Examples
========
In case that 2 paths (sda and sdb) are used with repeat_count == 128
and sda has an average throughput 1GB/s and sdb has 4GB/s,
'relative_throughput' value may be '1' for sda and '4' for sdb.

# echo "0 10 multipath 0 0 1 1 service-time 0 2 2 8:0 128 1 8:16 128 4" \
  dmsetup create test
#
# dmsetup table
test: 0 10 multipath 0 0 1 1 service-time 0 2 2 8:0 128 1 8:16 128 4
#
# dmsetup status
test: 0 10 multipath 2 0 0 0 1 1 E 0 2 2 8:0 A 0 0 1 8:16 A 0 0 4


Or '2' for sda and '8' for sdb would be also true.

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