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
102