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 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: [ []] : 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. : 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: \ : 'A' if the path is active, 'F' if the path is failed. : The number of path failures. : The size of in-flight I/Os on the path. : The relative throughput value of the path among all paths in the path-group. Algorithm ========= 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. Examples ======== 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. # 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 table test: 0 10 multipath 0 0 1 1 service-time 0 2 2 8:0 128 1 8:16 128 4 # # dmsetup status 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. # 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 table test: 0 10 multipath 0 0 1 1 service-time 0 2 2 8:0 128 2 8:16 128 8 # # dmsetup status 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