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f392ba88 KU |
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 | |
25985edc | 40 | dispatched and subtracts when completed. |
f392ba88 KU |
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 |