dm-service-time.rst (3306B)
1=============== 2dm-service-time 3=============== 4 5dm-service-time is a path selector module for device-mapper targets, 6which selects a path with the shortest estimated service time for 7the incoming I/O. 8 9The service time for each path is estimated by dividing the total size 10of in-flight I/Os on a path with the performance value of the path. 11The performance value is a relative throughput value among all paths 12in a path-group, and it can be specified as a table argument. 13 14The path selector name is 'service-time'. 15 16Table parameters for each path: 17 18 [<repeat_count> [<relative_throughput>]] 19 <repeat_count>: 20 The number of I/Os to dispatch using the selected 21 path before switching to the next path. 22 If not given, internal default is used. To check 23 the default value, see the activated table. 24 <relative_throughput>: 25 The relative throughput value of the path 26 among all paths in the path-group. 27 The valid range is 0-100. 28 If not given, minimum value '1' is used. 29 If '0' is given, the path isn't selected while 30 other paths having a positive value are available. 31 32Status for each path: 33 34 <status> <fail-count> <in-flight-size> <relative_throughput> 35 <status>: 36 'A' if the path is active, 'F' if the path is failed. 37 <fail-count>: 38 The number of path failures. 39 <in-flight-size>: 40 The size of in-flight I/Os on the path. 41 <relative_throughput>: 42 The relative throughput value of the path 43 among all paths in the path-group. 44 45 46Algorithm 47========= 48 49dm-service-time adds the I/O size to 'in-flight-size' when the I/O is 50dispatched and subtracts when completed. 51Basically, dm-service-time selects a path having minimum service time 52which is calculated by:: 53 54 ('in-flight-size' + 'size-of-incoming-io') / 'relative_throughput' 55 56However, some optimizations below are used to reduce the calculation 57as much as possible. 58 59 1. If the paths have the same 'relative_throughput', skip 60 the division and just compare the 'in-flight-size'. 61 62 2. If the paths have the same 'in-flight-size', skip the division 63 and just compare the 'relative_throughput'. 64 65 3. If some paths have non-zero 'relative_throughput' and others 66 have zero 'relative_throughput', ignore those paths with zero 67 'relative_throughput'. 68 69If such optimizations can't be applied, calculate service time, and 70compare service time. 71If calculated service time is equal, the path having maximum 72'relative_throughput' may be better. So compare 'relative_throughput' 73then. 74 75 76Examples 77======== 78In case that 2 paths (sda and sdb) are used with repeat_count == 128 79and sda has an average throughput 1GB/s and sdb has 4GB/s, 80'relative_throughput' value may be '1' for sda and '4' for sdb:: 81 82 # echo "0 10 multipath 0 0 1 1 service-time 0 2 2 8:0 128 1 8:16 128 4" \ 83 dmsetup create test 84 # 85 # dmsetup table 86 test: 0 10 multipath 0 0 1 1 service-time 0 2 2 8:0 128 1 8:16 128 4 87 # 88 # dmsetup status 89 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 90 91 92Or '2' for sda and '8' for sdb would be also true:: 93 94 # echo "0 10 multipath 0 0 1 1 service-time 0 2 2 8:0 128 2 8:16 128 8" \ 95 dmsetup create test 96 # 97 # dmsetup table 98 test: 0 10 multipath 0 0 1 1 service-time 0 2 2 8:0 128 2 8:16 128 8 99 # 100 # dmsetup status 101 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