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101 lines
3.2 KiB
101 lines
3.2 KiB
=============== |
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dm-service-time |
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=============== |
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dm-service-time is a path selector module for device-mapper targets, |
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which selects a path with the shortest estimated service time for |
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the incoming I/O. |
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The service time for each path is estimated by dividing the total size |
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of in-flight I/Os on a path with the performance value of the path. |
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The performance value is a relative throughput value among all paths |
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in a path-group, and it can be specified as a table argument. |
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The path selector name is 'service-time'. |
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Table parameters for each path: |
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[<repeat_count> [<relative_throughput>]] |
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<repeat_count>: |
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The number of I/Os to dispatch using the selected |
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path before switching to the next path. |
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If not given, internal default is used. To check |
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the default value, see the activated table. |
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<relative_throughput>: |
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The relative throughput value of the path |
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among all paths in the path-group. |
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The valid range is 0-100. |
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If not given, minimum value '1' is used. |
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If '0' is given, the path isn't selected while |
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other paths having a positive value are available. |
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Status for each path: |
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<status> <fail-count> <in-flight-size> <relative_throughput> |
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<status>: |
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'A' if the path is active, 'F' if the path is failed. |
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<fail-count>: |
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The number of path failures. |
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<in-flight-size>: |
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The size of in-flight I/Os on the path. |
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<relative_throughput>: |
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The relative throughput value of the path |
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among all paths in the path-group. |
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Algorithm |
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========= |
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dm-service-time adds the I/O size to 'in-flight-size' when the I/O is |
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dispatched and subtracts when completed. |
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Basically, dm-service-time selects a path having minimum service time |
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which is calculated by:: |
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('in-flight-size' + 'size-of-incoming-io') / 'relative_throughput' |
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However, some optimizations below are used to reduce the calculation |
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as much as possible. |
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1. If the paths have the same 'relative_throughput', skip |
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the division and just compare the 'in-flight-size'. |
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2. If the paths have the same 'in-flight-size', skip the division |
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and just compare the 'relative_throughput'. |
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3. If some paths have non-zero 'relative_throughput' and others |
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have zero 'relative_throughput', ignore those paths with zero |
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'relative_throughput'. |
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If such optimizations can't be applied, calculate service time, and |
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compare service time. |
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If calculated service time is equal, the path having maximum |
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'relative_throughput' may be better. So compare 'relative_throughput' |
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then. |
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Examples |
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======== |
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In case that 2 paths (sda and sdb) are used with repeat_count == 128 |
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and sda has an average throughput 1GB/s and sdb has 4GB/s, |
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'relative_throughput' value may be '1' for sda and '4' for sdb:: |
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# echo "0 10 multipath 0 0 1 1 service-time 0 2 2 8:0 128 1 8:16 128 4" \ |
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dmsetup create test |
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# |
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# dmsetup table |
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test: 0 10 multipath 0 0 1 1 service-time 0 2 2 8:0 128 1 8:16 128 4 |
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# |
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# dmsetup status |
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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 |
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Or '2' for sda and '8' for sdb would be also true:: |
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# echo "0 10 multipath 0 0 1 1 service-time 0 2 2 8:0 128 2 8:16 128 8" \ |
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dmsetup create test |
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# |
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# dmsetup table |
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test: 0 10 multipath 0 0 1 1 service-time 0 2 2 8:0 128 2 8:16 128 8 |
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# |
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# dmsetup status |
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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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