DotHill – Linux Device Mapper Multipath “How To” for Storage Revision 4
DotHill – Copyright 2009 7
block devices:
/dev/sda, /dev/sdb, /dev/sdc, and /dev/sdd. All of these devices refer to the
same underlying storage resource.
DM-MP creates a single virtual block device, e.g., /dev/mapper/mpath1, that can be used by
applications. DM-MP routes I/O for that virtual device through the four underlying physical
devices. If one path fails, DM-MP routes uses the remaining paths to maintain connectivity
between the host and the storage.
4.1 Terms and Concepts
Path – The connection from an HBA to a storage controller. Each path appears as a separate
block device. A path can be in one of two states: ready or faulty depending up on whether or not
it is able to handle I/O requests.
Path Group – A collection of paths. For Dot Hill storage arrays a path group can be in one of
two states: active (the path group is currently receiving I/O requests) or enabled (the path group
to try if the active path group has no paths in the ready state). Only one path group, the active
path group, receives I/O at any time. Within an active path group, DM-MP selects the path to be
used for I/O in a round-robin fashion.
Multipath Device – A multipath device is the virtual device created by DM-MP. A multipath
device can be identified by either its WWID or its alias. A multipath device has one or more path
groups.
Blacklisted Device – The user may wish to prevent some storage resources from coming under
the control of DM-MP. These are called blacklisted devices, and they can be specified in the
configuration file.
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 ready paths in that group. By default, DM-MP 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, DM-MP tries to shift I/O operations to an enabled path
group. If there are no enabled path groups, SCSI error return codes of 10000 will be displayed
stating no device is found. I/Os may eventually timeout until a valid path is enabled.
Failback – At regular intervals the multipath daemon, multipathd, checks the priority of all
path groups. If the active path group is not the highest priority path group, multipathd acts
according to the failback mode. By default it immediately switches to the highest priority path
group. Other options for failback are to (a) wait a user-defined length of time and then switch, or
(b) do nothing and wait for manual intervention. Failback can be forced manually at any time by
running the multipath command.
WWID – The WWID (World Wide Identifier) is an identifier for a multipath device that is guar-
anteed to be globally unique and unchanging. It is obtained from the storage array by the getuid
(48 strony)
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