aboutsummaryrefslogtreecommitdiff
path: root/Documentation/device-mapper/dm-service-time.txt
diff options
context:
space:
mode:
Diffstat (limited to 'Documentation/device-mapper/dm-service-time.txt')
-rw-r--r--Documentation/device-mapper/dm-service-time.txt91
1 files changed, 91 insertions, 0 deletions
diff --git a/Documentation/device-mapper/dm-service-time.txt b/Documentation/device-mapper/dm-service-time.txt
new file mode 100644
index 00000000..fb1d4a0c
--- /dev/null
+++ b/Documentation/device-mapper/dm-service-time.txt
@@ -0,0 +1,91 @@
+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: [<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.
+
+
+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