Chapter 1.
1
Device Mapper Multipathing
Device Mapper Multipathing (DM-Multipath) allows you to configure multiple I/O paths between server
nodes and storage arrays into a single device. These I/O paths are physical SAN connections that can
include separate cables, switches, and controllers. Multipathing aggregates the I/O paths, creating a
new device that consists of the aggregated paths.
1.1. Overview of DM-Multipath
DM-Multipath can be used to provide:
• Redundancy
DM-Multipath can provide failover in an active/passive configuration. In an active/passive
configuration, only half the paths are used at any time for I/O. If any element of an I/O path (the
cable, switch, or controller) fails, DM-Multipath switches to an alternate path.
• Improved Performance
DM-Multipath can be configured in active/active mode, where I/O is spread over the paths in a
round-robin fashion. In some configurations, DM-Multipath can detect loading on the I/O paths and
dynamically re-balance the load.
Figure 1.1, “Active/Passive Multipath Configuration with One RAID Device”
shows an active/passive
configuration with two I/O paths from the server to a RAID device. There are 2 HBAs on the server, 2
SAN switches, and 2 RAID controllers.
Figure 1.1. Active/Passive Multipath Configuration with One RAID Device
In this configuration, there is one I/O path that goes through hba1, SAN1, and controller 1 and a
second I/O path that goes through hba2, SAN2, and controller2. There are many points of possible
failure in this configuration:
