Table of Contents
Advertisement
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_names option in the multipath configuration file, which sets the alias to a node-
unique name of the form mpathn.
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_names configuration
option is set to yes, the name of the multipath device is set to mpathn.
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/mpathn, and
/dev/dm-n.
• The devices in /dev/mapper 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/mpath are provided as a convenience so that all multipathed devices can
be seen in one directory. These 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_names
configuration option, see see
You can also set the name of a multipath device to a name of your choosing by using the alias
option in the multipaths section of the multipath configuration file. For information on the
multipaths section of the multipath configuration file, see see
Configuration
Attributes".
2.2. Consistent Multipath Device Names in a Cluster
When the user_friendly_names 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. This 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 multipaths section of the multipath configuration file to set the name
of the multipath device. The alias for the multipath device is consistent across all the nodes in
Section 4.3, "Configuration File
Defaults".
Section 4.4, "Multipaths Device
5
Advertisement
Table of Contents
Troubleshooting
