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