Multipath

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

• multipath -l
• multipath -t

-t Display the currently used multipathd configuration

• multipathd show config

• Ubuntu: multipath-tools package
  • Binaries: multipath multipathd mpathpersist
• RHEL: device-mapper-multipath package

Config files:

• /etc/multipath.confhttps://manpages.debian.org/unstable/multipath-tools/multipath.conf.5.en.html
• /etc/multipath/wwids

Configuration file: /etc/multipath.conf

1. System level defaults (defaults): User can override system level defaults.
2. Blacklisted devices (blacklist): User specifies the list of devices that is not to be under the control of DM-MPIO.
3. Blacklist exceptions (blacklist_exceptions): Specific devices to be treated as multipath devices even if listed in the blacklist.
4. Storage controller specific settings (devices): User specified configuration settings will be applied to devices with specified "Vendor" and "Product" information.
5. Device specific settings (multipaths): Fine tune the configuration settings for individual LUNs.

Default multipath.conf configuration

RHEL
cat /etc/multipath.conf | grep -v "#" | grep .
defaults {
    polling_interval            5
    no_path_retry               4
    user_friendly_names         no
    flush_on_last_del           yes
    fast_io_fail_tmo            5
    dev_loss_tmo                30
    max_fds                     4096
}
blacklist {
        protocol "(scsi:adt|scsi:sbp)"
}
devices {
    device {
        all_devs                yes
        no_path_retry           4
    }
}

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.

Readings

• Varoqui, C. (2010.) The Linux multipath implementation. Retrieved from http://christophe.varoqui.free.fr/refbook.html

See also

• EMC powerpath
• Fibre Channel: SAN, HBA, LUN, WWN, WWPN, LIF, rescan-scsi-bus.sh

https://www.thegeekdiary.com/understanding-the-dm-multipath-configuration-file-etc-multipath-conf/

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