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Device Mapper Multipath Module

Device Mapper is an infrastructure in the Linux kernel. It provides a generic way to create virtual layers of block devices. It supports striping, mirroring, snapshots, concatenation, and multipathing. The multipath feature is provided with combination of DM Multipath kernel modules and multipath-tools user-space package.

The Device Mapper Multipath (DM-MP) module provides the multipathing capability for Linux. DM-MP is the only multipathing option shipped with SLES.

DM-MP features automatic configuration of the multipathing subsystem for a large variety of setups. Configurations of up to 8 paths to each device are supported. Configurations are supported for active/passive (one path active, others passive) or active/active (all paths active with round-robin load balancing).

The DM-MP framework is extensible in two ways:

Using specific hardware handlers. For information, see Storage Arrays that Require Specific Hardware Handlers.
Using more sophisticated load-balancing algorithms than round-robin

The user-space component of DM-MP takes care of automatic path discovery and grouping, as well as automated path retesting, so that a previously failed path is automatically reinstated when it becomes healthy again. This minimizes the need for administrator attention in a production environment.

DM-MP protects against failures in the paths to the device, and not failures in the device itself. If one of the active paths is lost (for example, a network adapter breaks or a fiber-optic cable is removed), I/O is redirected to the remaining paths. If the configuration is active/passive, then the path fails over to one of the passive paths. If you are using the round-robin load-balancing configuration, the traffic is balanced across the remaining healthy paths. If all active paths fail, inactive secondary paths must be waked up, so failover occurs with a delay of approximately 30 seconds.

If a disk array has more than one storage processor, make sure that the SAN switch has a connection to the storage processor that owns the LUNs you want to access. On most disk arrays, all LUNs belong to both storage processors, so both connections are active.

NOTE: On some disk arrays, the storage array manages the traffic through storage processors so that it presents only one storage processor at a time. One processor is active and the other one is passive until there is a failure. If you are connected to the wrong storage processor (the one with the passive path) you might not see the expected LUNs, or you might see the LUNs but get errors when trying to access them.

Features of Storage Arrays

Active/passive controllers One controller is active and serves all LUNs. The second controller acts as a standby. The second controller also presents the LUNs to the multipath component so that the operating system knows about redundant paths. If the primary controller fails, the second controller takes over, and it serves all LUNs. In some arrays, the LUNs can be assigned to different controllers. A given LUN is assigned to one controller to be its active controller. One controller does the disk I/O for any given LUN at a time, and the second controller is the standby for that LUN. The second controller also presents the paths, but disk I/O is not possible. Servers that use that LUN are connected to the LUN’s assigned controller. If the primary controller for a set of LUNs fails, the second controller takes over, and it serves all LUNs.
Active/active controllers. Both controllers share the load for all LUNs, and can process disk I/O for any given LUN. If one controller fails, the second controller automatically handles all traffic.
Load balancing. The Device Mapper Multipath driver automatically load balances traffic across all active paths.
Controller failover. When the active controller fails over to the passive, or standby, controller, the Device Mapper Multipath driver automatically activates the paths between the host and the standby, making them the primary paths.
Boot/Root device support. Multipathing is supported for the root (/) device in SUSE Linux Enterprise Server 10 and later. The host server must be connected to the currently active controller and storage processor for the boot device. Device Mapper Multipath detects every path for a multipathed device as a separate SCSI device. The SCSI device names take the form /dev/sdN, where N is an autogenerated letter for the device, beginning with a and issued sequentially as the devices are created, such as /dev/sda, /dev/sdb, and so on. If the number of devices exceeds 26, the letters are duplicated so that the next device after /dev/sdz will be named /dev/sdaa, /dev/sdab, and so on. If multiple paths are not automatically detected, you can configure them manually in the /etc/multipath.conf file. The multipath.conf file does not exist until you create and configure it.
For information, see Section 5.4.5, Creating and Configuring the /etc/multipath.conf File.

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Last modified: March, 12, 2019