Using the "find_multipaths yes" option in multipath.conf

This document (7024049) is provided subject to the disclaimer at the end of this document.

Environment

SUSE Linux Enterprise Server 12 Service Pack 3 (SLES 12 SP3)

SUSE Linux Enterprise Server 12 Service Pack 4 (SLES 12 SP4)
SUSE Linux Enterprise Server 15 GA

Situation

The system has the following option configured in multipath.conf :

find_multipaths yes

But it appears as the system is not creating a multipath map when there are at least two non-blacklisted paths with the same WWID according to man pages. (man multipath.conf)

Resolution

Manually running multipath command is needed to create the new LUN.

Example given :

s12sp4:~ # multipath -ll
s12sp4:~ # multipath
create: mpatha (36001405b7a08dd303bc4efda76fa90fa) undef LIO-ORG,s12sp4-mpio
size=900M features='0' hwhandler='0' wp=undef
|-+- policy='service-time 0' prio=50 status=undef
| `- 2:0:0:0 sda 8:0 undef ready running
`-+- policy='service-time 0' prio=50 status=undef
`- 3:0:0:0 sdb 8:16 undef ready running
s12sp4:~ # multipath -ll
mpatha (36001405b7a08dd303bc4efda76fa90fa) dm-0 LIO-ORG,s12sp4-mpio
size=900M features='0' hwhandler='1 alua' wp=rw
|-+- policy='service-time 0' prio=50 status=active
| `- 2:0:0:0 sda 8:0 active ready running
`-+- policy='service-time 0' prio=50 status=enabled
`- 3:0:0:0 sdb 8:16 active ready running
s12sp4:~ # cd /etc/multipath/
s12sp4:/etc/multipath # ll
total 8
-rw------- 1 root root 241 Aug 4 10:00 bindings
-rw------- 1 root root 226 Aug 4 10:00 wwids

As seen in the above example, the LUN was already presented to the system, however multipath did not create any new map for this LUN due to the existance of the "find_multipath yes" setting

Executing the 'multipath' command as root created the needed map and added it to wwids file.

On subsequent (re-)boots here after, there is no further need to run the multipath command again since the map was already written to wwids file.

Cause

When configuring multipath with "find_multipath yes", the system will start multipathd using '-n' (to 'ignore new devices').

As such, multipathd will not create a multipath device unless the WWID for the device is already listed in the WWIDs file.)

This can be seen in the logs:

Aug 04 09:58:25 s12sp4 multipathd[6134]: --------start up--------
Aug 04 09:58:25 s12sp4 multipathd[6134]: read /etc/multipath.conf
Aug 04 09:58:25 s12sp4 systemd[1]: Started Device-Mapper Multipath Device Controller.
Aug 04 09:58:25 s12sp4 multipathd[6134]: find_multipaths is set: -n is implied

From the wording used in the manual pages, an administrator may belief that multipathd should automatically create the new map once two devices with same WWID appeared.

This understanding is not correct, as a manually executing the multipath command is needed.

Additional Information

man multipath.conf :

find_multipaths
If set to yes, instead of trying to create a multipath device for every non-blacklisted path, multipath will only create a device if one of three conditions are met.
1 There are at least two non-blacklisted paths with the same WWID,
2 the user manually forces the creation, by specifying a device with the multipath command, or
3 a path has the same WWID as a multipath device that was previously created while find_multipaths was set (even if that multipath device doesn't currently exist).

Whenever a multipath device is created with find_multipaths set, multipath will remember the WWID of the device, so that it will automatically create the device again, as soon as it sees a path with that WWID. This should allow most users to have multipath automatically choose the correct paths to make into multipath devices, without having to edit the blacklist.

The default is: no

More about SUSE recommended/default settings can be found in SUSE Storage Admin Guide

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