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
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/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
When new devices are brought under the control of DM-Multipath, the new devices may be seen in three different places under the
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
n are for internal use only and should never be used.
You can also set the name of a multipath device to a name of your choosing by using the
option in the
section of the multipath configuration file. For information on the
section of the multipath configuration file, see Section 4.4, “Multipaths Device Configuration Attributes”