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Appendix C.
Multipath-usage.txt
File for Red Hat Enterprise Linux 4
Update 3 159
Path States:
ready - Path is able to handle I/O requests.
shaky - Path is up, but temporarily not available for normal
operations.
faulty - Path is unable to handle I/O requests.
ghost
- Path is a passive path, on an active/passive
controller.
NOTE: The shaky and ghost states only exist for certain
storage arrays.
Path Group:
A grouping of paths. With DM-MP, only one path group--the
active path group--receives I/O at any time. Within a path
group, DM-MP selects which ready path should receive I/O in
a round robin fashion. Path groups can be in various states
(refer to "Path Group States").
Path Group States:
active - Path group currently receiving I/O requests.
enabled - Path groups to try if the active path group has no paths
in the ready state.
disabled - Path groups to try if the active path group and all
enabled path groups have no paths in the active state.
NOTE: The disabled state only exists for certain storage arrays.
Path Priority:
Each path can have a priority assigned to it by a callout program.
Path priorities can be used to group paths by priority and change
their relative weights for the round robin path selector.
Path Group Priority:
Each path group has a priority that is equal to the sum of the
priorities of all the non-faulty paths in the group. By default, the
multipathd daemon tries to ensure that the path group with the
highest priority is always in the active state.
Failover:
When I/O to a path fails, the dm-multipath module tries to switch to
an enabled path group. If there are no enabled path groups with
any paths in the ready state, dm-multipath tries to switch to a
disabled path group. If necessary, dm-multipath runs the hardware
handler for the multipath device.
Failback:
At regular intervals, multipathd checks the current priority of
all path groups. If the current path group is not the highest
priority path group, multipathd reacts according to the failback
mode. By default, multipathd immediately switches to the highest