Recommendations for the usage of user_friendly_names in multipath configurations

SUSE Linux Enterprise Server 15

Environment:
 - with devicemapper multipathing (DM/MPIO) enabled.
 - System root on a multipathed storage device.

The global multipath option user_friendly_names is used to create custom device names for the multipath devices.
There are, however, certain situations when the use of user_friendly_names is problematic.

1. If the system root is on multipath, the userfriendly settings from /var/lib/multipath/bindings get included into the initrd. If afterwards the storage setup is changed, e.g. by adding or removing devices, there is a mismatch between the bindings setting inside the initrd and the bindings in /var/lib/multipath bindings.                                                                                     
2. The default location of the user_friendly_names configuration file is /var/lib/multipath/bindings. If /var is not located on the system root but mounted from another partition the bindings file is not available when setting up multipathing.

Especially the first problem can lead to a wrong assignment of mountpoints to devices which in turn can result in file system corruption and data loss.

Concerning issue 1. it is not recommended to use user_friendly_names if the system root is on multipath.
It is advised to use the alias configuration option in the multipath.conf file instead, e.g.:

multipaths {
       multipath {
               wwid                 36006048000028350131253594d303030
               alias                   mpatha
       }
       multipath {
               wwid                 36006048000028350131253594d303041
               alias                   mpathb
       }
       multipath {
               wwid                 36006048000028350131253594d303145
               alias                   mpathc
       }
       multipath {
               wwid                 36006048000028350131253594d303334
               alias                   mpathd
       }
}


If root is not on multipath, but multipath is included in the initrd anyway (this can happen if root is on LVM), and user_friendly_names is used, then one should boot with the parameter "multipath=off", to avoid problems.

This will disable multipath in the initrd only. Once the system boots, the boot.multipath and multipathd boot scripts will still be able to activate multipathing.


Concerning issue 2. it needs to be assured that the bindings file is available on the system root and multipath can find it.
This be done for example by moving the bindings file to: /etc/multipath/bindings,
and setting the option "bindings_file" in the defaults section of /etc/multipath.conf, e.g:

defaults {
               user_friendly_names yes
               bindings_file "/etc/multipath/bindings"
}

Note: Since SUSE Linux Enterprise Server 11 Service Pack 3 the bindings are on default already stored in /etc/multipath/bindings.
An extra entry in multipath.conf is not longer needed. (this can be checked with command:  # multipath -t | grep bindings)