Static load balancing in Device-Mapper Multipathing (DM-MPIO)

This document (3858277) is provided subject to the disclaimer at the end of this document.

Environment

SUSE Linux Enterprise Server 10

Situation

You would like to to have configurable path priority among multiple paths.

This path priority settings should be persistent even after a reboot.

These path priority settings should be restored once the preferred paths are
available again. After a path failure for example.

Resolution

The solution for this issue is to use a script for

prio_callout

in /etc/multipath.conf

This script would be a shell script which gets a selector defined by you and the variable %n passed. This script has to be invoked explicitly with /bin/sh, so the line in/etc/multipath.confwould read:

devices {
device {
vendor "EMC"
product "SYMMETRIX"
path_grouping_policy group_by_prio
path_checker tur
prio_callout "/bin/sh /usr/sbin/path_priority %n SELECTOR"
}
}

assuming that the script is /usr/sbin/path_priority. Please keep in mind that you will have to replace SELECTOR with Information to be used to find the path.

/usr/sbin/path_prioritycould be:

#!/bin/sh

PATH=/bin:/usr/bin
selector=$2
prio=1
if [ -e /sys/block/$1/device ]; then
target=$(ls -l /sys/block/$1/device | tr / "\n" | grep .:.:.:)
if [ $(echo $target | grep $selector) ]; then
prio=4
fi
fi
echo $prio

$1 is %n which is the block device passed from the multipathd to the script, which would read something like sdaj for example.

$2 is SELECTOR which is not set yet. The script parses /sys/block/$1/device and looks for an entry with .:.:.: which would be Host : Bus : Target : Lun of the device.

Based on this you write your SELECTOR, for example 3:0, which would mean Host scsi3 Channel 0 as preferred path.

You can deduce your own selector by checking cat /proc/scsi/scsi

If this script is called it will set a default priority of 1 and then get the Host / Bus / Target / Lun from the block device passed from %n and check whether SELECTOR matches with this information. If selector matches, then the priority will be set to 4 and then returned via the final echo $prio.

Test system with SELECTOR set to 3:0

prio_callout "/bin/sh /usr/sbin/path_priority %n 3:0"

snippet from multipath -v3 output:

getprio = /bin/sh /usr/sbin/path_priority %n 3:0 (controler setting)
reformated callout = /bin/sh /usr/sbin/path_priority sdct 3:0
prio = 4
uid = 36006048000028350131253594d303338 (cache)
===== path info sdcu (mask 0x3f) =====
device sdcu is on bus scsi
bus = 1
dev_t = 70:32
size = 17677440
vendor = EMC
product = SYMMETRIX
rev = 5267
h:b:t:l = 3:0:0:25

multipath -ll(different partition)

36006048000028350131253594d303433
[size=8 GB][features="0"][hwhandler="0"]
\_ round-robin 0 [prio=8][active]
\_ 3:0:0:35 sdde 70:192 [active][ready]
\_ 3:0:1:35 sdes 129:64 [active][ready]
\_ round-robin 0 [prio=2][enabled]
\_ 2:0:0:35 sdac 65:192 [active][ready]
\_ 2:0:1:35 sdbq 68:64 [active][ready]

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