91 lines
3.2 KiB
Text
91 lines
3.2 KiB
Text
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
|