User guide

7. INTRODUCTION TO DM-MULTIPATH

DDN’s multipath ddn_mpath_RHEL5_SLES10-0.4-7.x86_64.rpm is an extension to the linux

multipath tools package: device mapper multipathing, dm-multipath. dm-multipath provides a

means for accessing a device with multiple paths to that device in Linux. The device mapper

kernel module creates a single SCSI block device for every LUN probed by Linux at boot time (or

manually – see later in this document). This device(s) for each LUN can be found in /dev/mpath

as well as /dev/mapper directories in Linux.

7.1 HOW IT WORKS

Linux dm-multipath queries each Linux SCSI disk devices and determines which disk devices are

duplicate paths to disk targets on a Linux host computer.

Each path is a physical connection (Fibre Channel or Infiniband for DDN storage) between the

initiator (the server) and a specific LUN on the target (data storage) device. Paths to the same

target are assembled into priority groups. Only one of these priority groups will be used at a

time for I/O to the device. The priority group that is being utilized for IO is labeled “active”.

A component to dm-multipath is used to determine which path to use for the next IO.

This component is called the Path Selector.

If an I/O fails on the selected active path, that path will be disabled and the I/O is retried down

a different path within the same group of paths called a Priority Group. There can be more than

one path in this priority group, and each path is weighted for a priority level called a Path Group

Priority. The highest priority level in the Group determines the primary path to use to access the

device. If every path in the path group fails, then a different priority group will chosen and

enabled to continue IO to the target device.