Red Hat DEVICE-MAPPER MULTIPATH 4.6 Configuration And Administration Manual

Contents

Multipath DevicesWithout DM-Multipath, each path from a server node to a storage controller is treated by thesystem as a separate device, even when the I/O path connects the same server node to thesame storage controller. DM-Multipath provides a way of organizing the I/O paths logically, bycreating a single multipath device on top of the underlying devices.1. Multipath Device IdentifiersEach multipath device has a World Wide Identifier (WWID), which is guaranteed to be globallyunique 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 singleunzoned 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 fourunderlying 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 mpathn.When new devices are brought under the control of DM-Multipath, the new devices may beseen 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 toaccess the multipathed devices, for example when creating logical volumes.• The devices in /dev/mpath are provided as a convenience so that all multipathed devicescan be seen in one directory. These devices are created by the udev device manager andmay not be available on startup when the system needs to access them. Do not use thesedevices 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_namesconfiguration option, see see Section 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 multipaths section of the multipath configuration file. For information on themultipaths section of the multipath configuration file, see see Section 4, “Multipaths DeviceConfiguration Attributes”.2. Consistent Multipath Device Names in a ClusterWhen the user_friendly_names configuration option is set to yes, the name of the multipathdevice is unique to a node, but it is not guaranteed to be the same on all nodes using theChapter 2.7