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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_namesoption in the multipath configuration file, which sets the alias to a node-unique name of the form
For example, a node with two HBAs attached to a storage controller with two ports by means of a single unzoned FC switch sees four devices:
/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_namesconfiguration option is set to
yes, the name of the multipath device is set to
When new devices are brought under the control of DM-Multipath, the new devices may be seen in two different places under the
- The devices in
/dev/mapperare created early in the boot process. Use these devices to access the multipathed devices, for example when creating logical volumes.
- Any devices of the form
nare for internal use only and should never be used.
For information on the multipath configuration defaults, including the
user_friendly_namesconfiguration option, 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
aliasoption in the
multipathssection of the multipath configuration file. For information on the
multipathssection of the multipath configuration file, see Section 4.4, “Multipaths Device Configuration Attributes”.