Editing Multipath

https://en.wikipedia.org/wiki/Linux_DM_Multipath

* https://linux.die.net/man/5/multipath.conf
* https://linux.die.net/man/8/multipath
* https://linux.die.net/man/8/multipathd



* <code>mpathconf</code>
** <code>mpathconf --enable --with_multipathd y</code>
* <code>multipath -l</code> (show the current multipath topology from information fetched in [[sysfs]] and the [[device mapper]])
* <code>[[multipath -ll]]</code>
** <code>multipath -ll DEVICE_NAME</code>

* <code>[[multipath -v2]]</code> or  <code>[[multipath -v3]]</code> 
* <code>multipath -t</code>
:<code>-t</code> Display the currently used [[multipathd]] configuration
* <code>[[multipathd]] show config</code>

* [[Ubuntu]]: <code>multipath-tools</code> package
** Binaries: <code>multipath</code> <code>[[multipathd]]</code> <code>[[mpathpersist]]</code>
* [[RHEL]]: <code>device-mapper-multipath</code> package

Config files: 
* <code>/etc/[[multipath.conf]]</code><ref>https://www.thegeekdiary.com/understanding-the-dm-multipath-configuration-file-etc-multipath-conf/</ref><code>https://manpages.debian.org/unstable/multipath-tools/multipath.conf.5.en.html</code>
* <code>/etc/multipath/[[wwids]]</code>

== Configuration file: /etc/multipath.conf <ref>https://en.wikipedia.org/wiki/Linux_DM_Multipath</ref>==
# System level defaults (''defaults''): User can override system level defaults.
# Blacklisted devices (''blacklist''): User specifies the list of devices that is not to be under the control of DM-MPIO.
# Blacklist exceptions (''blacklist_exceptions''): Specific devices to be treated as multipath devices even if listed in the blacklist.
# Storage controller specific settings (''devices''): User specified configuration settings will be applied to devices with specified "Vendor" and "Product" information.
# Device specific settings (''multipaths''): Fine tune the configuration settings for individual LUNs.



== Routing policies implemented: failover, multibus and group_by_serial ==
* failover : 1 path per priority group. IO thus get routed to one path only.
* multibus : 1 priority group containing all paths to the LU. Brings the maximum spreading, but assumes that all paths are excitable without penalty.
* group_by_serial : 1 priority group per storage controller (serial), paths through one controller are assigned to the associated PG. This policy applies to controllers that impose a latency penalty on LU management hand-over between a pair of redundant controllers.
To modify a policy for device [[major]] 8 and [[minor]]: 
* <code>multipath -p multibus -D 8 0</code>

== Readings ==
* Varoqui, C. (2010.) The Linux multipath implementation. Retrieved from http://christophe.varoqui.free.fr/refbook.html
* Changelog: https://git.opensvc.com/?p=multipath-tools/.git;a=log

== Related terms ==
* <code>[[blkid]]</code> <code>[[multipath -f]]</code> <code>[[multipath -w]]</code> <code>[[pvcreate]]</code>
* <code>[[multipath-tools]]</code> Ubuntu package
* [[Hitachi Dynamic Link Manager (HDLM)]]

== Activities ==
* Review <code>no_path_retry</code> configuration options in [[multipath.conf]] to avoid [[pvs]] to hang : https://www.suse.com/support/kb/doc/?id=7022310
* [[Multipath: remove multipath device unpresented to host]]: <code>multipath -f</code>
* Review [[multipath-tools]] source code: https://git.opensvc.com/gitweb.cgi?p=multipath-tools%2F.git
* Map some [[LUN]] to your system with multipath uninstalled and verify no [[dm]] devices are created. Install multipath and check again.

== See also ==
* [[EMC]] <code>[[powerpath]]</code>
* <code>[[kpartx]]</code>
* {{HBA}}
* {{Linux Fibre Channel}}
* {{Linux block devices}}
* {{lvm commands}}

[[Category:Linux commands]]