Multipath

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https://en.wikipedia.org/wiki/Linux_DM_Multipath https://linux.die.net/man/8/multipath

  • multipath -l
  • multipath -ll
  • 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:

multipath -W successfully reset wwids

Configuration file: /etc/multipath.conf [2]

  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.

To modify a policy for device major 8 and minor:

  • multipath -p multibus -D 8 0

Readings


See also

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