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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 |
92 |
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