===============
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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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