dm-service-time

dm-service-time is a path selector module for device-mapper targets,which selects a path with the shortest estimated service time forthe incoming I/O.

The service time for each path is estimated by dividing the total sizeof in-flight I/Os on a path with the performance value of the path.The performance value is a relative throughput value among all pathsin 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 selectedpath before switching to the next path.If not given, internal default is used. To checkthe default value, see the activated table.

<relative_throughput>:

The relative throughput value of the pathamong 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 whileother 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 pathamong all paths in the path-group.

Algorithm

dm-service-time adds the I/O size to ‘in-flight-size’ when the I/O isdispatched and subtracts when completed.Basically, dm-service-time selects a path having minimum service timewhich is calculated by:

('in-flight-size' + 'size-of-incoming-io') / 'relative_throughput'

However, some optimizations below are used to reduce the calculationas much as possible.

  1. If the paths have the same ‘relative_throughput’, skipthe division and just compare the ‘in-flight-size’.

  2. If the paths have the same ‘in-flight-size’, skip the divisionand just compare the ‘relative_throughput’.

  3. If some paths have non-zero ‘relative_throughput’ and othershave zero ‘relative_throughput’, ignore those paths with zero‘relative_throughput’.

If such optimizations can’t be applied, calculate service time, andcompare service time.If calculated service time is equal, the path having maximum‘relative_throughput’ may be better. So compare ‘relative_throughput’then.

Examples

In case that 2 paths (sda and sdb) are used with repeat_count == 128and sda has an average throughput 1GB/s and sdb has 4GB/s,‘relative_throughput’ value may be ‘1’ for sda and ‘4’ for sdb:

# echo "0 10 multipath 0 0 1 1 service-time 0 2 2 8:0 128 1 8:16 128 4" \  dmsetup create test## dmsetup tabletest: 0 10 multipath 0 0 1 1 service-time 0 2 2 8:0 128 1 8:16 128 4## dmsetup statustest: 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:

# echo "0 10 multipath 0 0 1 1 service-time 0 2 2 8:0 128 2 8:16 128 8" \  dmsetup create test## dmsetup tabletest: 0 10 multipath 0 0 1 1 service-time 0 2 2 8:0 128 2 8:16 128 8## dmsetup statustest: 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