dm-service-time.rst

  1===============
  2dm-service-time
  3===============
  4
  5dm-service-time is a path selector module for device-mapper targets,
  6which selects a path with the shortest estimated service time for
  7the incoming I/O.
  8
  9The service time for each path is estimated by dividing the total size
 10of in-flight I/Os on a path with the performance value of the path.
 11The performance value is a relative throughput value among all paths
 12in a path-group, and it can be specified as a table argument.
 13
 14The path selector name is 'service-time'.
 15
 16Table parameters for each path:
 17
 18    [<repeat_count> [<relative_throughput>]]
 19	<repeat_count>:
 20			The number of I/Os to dispatch using the selected
 21			path before switching to the next path.
 22			If not given, internal default is used.  To check
 23			the default value, see the activated table.
 24	<relative_throughput>:
 25			The relative throughput value of the path
 26			among all paths in the path-group.
 27			The valid range is 0-100.
 28			If not given, minimum value '1' is used.
 29			If '0' is given, the path isn't selected while
 30			other paths having a positive value are available.
 31
 32Status for each path:
 33
 34    <status> <fail-count> <in-flight-size> <relative_throughput>
 35	<status>:
 36		'A' if the path is active, 'F' if the path is failed.
 37	<fail-count>:
 38		The number of path failures.
 39	<in-flight-size>:
 40		The size of in-flight I/Os on the path.
 41	<relative_throughput>:
 42		The relative throughput value of the path
 43		among all paths in the path-group.
 44
 45
 46Algorithm
 47=========
 48
 49dm-service-time adds the I/O size to 'in-flight-size' when the I/O is
 50dispatched and subtracts when completed.
 51Basically, dm-service-time selects a path having minimum service time
 52which is calculated by::
 53
 54	('in-flight-size' + 'size-of-incoming-io') / 'relative_throughput'
 55
 56However, some optimizations below are used to reduce the calculation
 57as much as possible.
 58
 59	1. If the paths have the same 'relative_throughput', skip
 60	   the division and just compare the 'in-flight-size'.
 61
 62	2. If the paths have the same 'in-flight-size', skip the division
 63	   and just compare the 'relative_throughput'.
 64
 65	3. If some paths have non-zero 'relative_throughput' and others
 66	   have zero 'relative_throughput', ignore those paths with zero
 67	   'relative_throughput'.
 68
 69If such optimizations can't be applied, calculate service time, and
 70compare service time.
 71If calculated service time is equal, the path having maximum
 72'relative_throughput' may be better.  So compare 'relative_throughput'
 73then.
 74
 75
 76Examples
 77========
 78In case that 2 paths (sda and sdb) are used with repeat_count == 128
 79and sda has an average throughput 1GB/s and sdb has 4GB/s,
 80'relative_throughput' value may be '1' for sda and '4' for sdb::
 81
 82  # echo "0 10 multipath 0 0 1 1 service-time 0 2 2 8:0 128 1 8:16 128 4" \
 83    dmsetup create test
 84  #
 85  # dmsetup table
 86  test: 0 10 multipath 0 0 1 1 service-time 0 2 2 8:0 128 1 8:16 128 4
 87  #
 88  # dmsetup status
 89  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
 90
 91
 92Or '2' for sda and '8' for sdb would be also true::
 93
 94  # echo "0 10 multipath 0 0 1 1 service-time 0 2 2 8:0 128 2 8:16 128 8" \
 95    dmsetup create test
 96  #
 97  # dmsetup table
 98  test: 0 10 multipath 0 0 1 1 service-time 0 2 2 8:0 128 2 8:16 128 8
 99  #
100  # dmsetup status
101  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