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