Kernel-3.10.0-957.el7_dm-service-time

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