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Documentation / device-mapper / dm-service-time.txt




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Based on kernel version 3.16. Page generated on 2014-08-06 21:36 EST.

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