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diff --git a/Documentation/device-mapper/dm-queue-length.rst b/Documentation/device-mapper/dm-queue-length.rst new file mode 100644 index 000000000000..d8e381c1cb02 --- /dev/null +++ b/Documentation/device-mapper/dm-queue-length.rst @@ -0,0 +1,48 @@ +=============== +dm-queue-length +=============== + +dm-queue-length is a path selector module for device-mapper targets, +which selects a path with the least number of in-flight I/Os. +The path selector name is 'queue-length'. + +Table parameters for each path: [<repeat_count>] + +:: + + <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. + +Status for each path: <status> <fail-count> <in-flight> + +:: + + <status>: 'A' if the path is active, 'F' if the path is failed. + <fail-count>: The number of path failures. + <in-flight>: The number of in-flight I/Os on the path. + + +Algorithm +========= + +dm-queue-length increments/decrements 'in-flight' when an I/O is +dispatched/completed respectively. +dm-queue-length selects a path with the minimum 'in-flight'. + + +Examples +======== +In case that 2 paths (sda and sdb) are used with repeat_count == 128. + +:: + + # echo "0 10 multipath 0 0 1 1 queue-length 0 2 1 8:0 128 8:16 128" \ + dmsetup create test + # + # dmsetup table + test: 0 10 multipath 0 0 1 1 queue-length 0 2 1 8:0 128 8:16 128 + # + # dmsetup status + test: 0 10 multipath 2 0 0 0 1 1 E 0 2 1 8:0 A 0 0 8:16 A 0 0 |