Installation guide

Chapter 2. Multipath Devices
Without DM-Multipath, each path from a server node to a storage controller is treated by the system as a
separate device, even when the I/O path connects the same server node to the same storage controller.
DM-Multipath provides a way of organizing the I/O paths logically, by creating a single multipath device
on top of the underlying devices.
2.1. Multipath Device Identifiers
Each multipath device has a World Wide Identifier (WWID), which is guaranteed to be globally unique and
unchanging. By default, the name of a multipath device is set to its WWID. Alternately, you can set the
user_friendly_nam es option in the multipath configuration file, which sets the alias to a node-unique
name of the form m pathn.
For example, a node with two HBAs attached to a storage controller with two ports via a single unzoned
FC switch sees four devices: /dev/sda, /dev/sdb, dev/sdc, and /dev/sdd. DM-Multipath creates a
single device with a unique WWID that reroutes I/O to those four underlying devices according to the
multipath configuration. When the user_friendly_nam es configuration option is set to yes, the name
of the multipath device is set to m pathn.
When new devices are brought under the control of DM-Multipath, the new devices may be seen in three
different places under the /dev directory: /dev/mapper/mpathn, /dev/mpath/m pathn, and
/dev/dm-n.
The devices in /dev/m apper are created early in the boot process. Use these devices to access
the multipathed devices, for example when creating logical volumes.
The devices in /dev/m path are provided as a convenience so that all multipathed devices can be
seen in one directory. T hese devices are created by the udev device manager and may not be
available on startup when the system needs to access them. Do not use these devices for creating
logical volumes or filesystems.
Any devices of the form /dev/dm -n are for internal use only and should never be used.
For information on the multipath configuration defaults, including the user_friendly_nam es
configuration option, see see Section 4.3, “Configuration File Defaults”.
You can also set the name of a multipath device to a name of your choosing by using the alias option
in the m ultipaths section of the multipath configuration file. For information on the m ultipaths
section of the multipath configuration file, see see Section 4.4, “Multipaths Device Configuration
Attributes”.
2.2. Consistent Multipath Device Names in a Cluster
When the user_friendly_nam es configuration option is set to yes, the name of the multipath device
is unique to a node, but it is not guaranteed to be the same on all nodes using the multipath device. T his
should not cause any difficulties if you use LVM to create logical devices from the multipath device, but if
you require that your multipath device names be consistent in every node in the cluster you perform one
of the following procedures:
Use the alias option in the m ultipaths section of the multipath configuration file to set the name
of the multipath device. T he alias for the multipath device is consistent across all the nodes in a
cluster. For information on the m ultipaths section of the multipath configuration file, see see
Section 4.4, “Multipaths Device Configuration Attributes”.
If you want the system-defined user-friendly names to be consistent across all nodes in the cluster,
Red Hat Enterprise Linux 4 DM Multipath
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