DM-Multipath introduction

DM-Multipathing (DM-MPIO) provides input-output (I/O) fail-over and load-balancing by using multipath I/O within Linux for block devices. By utilizing device-mapper, the multipathd daemon provides the host-side logic to use multiple paths of a redundant network to provide continuous availability and higher-bandwidth connectivity between the host server and the block-level device.

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 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.

Storage Array Overview

By default, DM-Multipath includes support for the most common storage arrays that support DM-Multipath. The supported devices can be found in the multipath.conf.defaults file. If your storage array supports DM-Multipath and is not configured by default in this file, you may need to add them to the DM-Multipath configuration file, multipath.conf. For information on the DM-Multipath configuration file, see Section, The DM-Multipath Configuration File. Some storage arrays require special handling of I/O errors and path switching. These require separate hardware handler kernel modules.

The figure below 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.

Active/Passive Multipath Configuration with One RAID Device

DM-Multipath components

Table DM-Multipath Components describes the components of the DM-Multipath package.

DM-Multipath Components

Component Description
dm_multipath kernel module Reroutes I/O and supports failover for paths and path groups.
multipath command Lists and configures multipath devices. Normally started up with /etc/rc.sysinit, it can also be started up by a udev program whenever a block device is added or it can be run by the initramfs file system.
multipathd daemon Monitors paths; as paths fail and come back, it may initiate path group switches. Provides for interactive changes to multipath devices. This daemon must be restarted for any changes to the /etc/multipath.conf file to take effect.
kpartx command Creates device mapper devices for the partitions on a device It is necessary to use this command for DOS-based partitions with DM-Multipath. The kpartx is provided in its own package, but the multipath-tools package depends on it.

DM-Multipath Setup Overview

DM-Multipath includes compiled-in default settings that are suitable for common multipath configurations. Setting up DM-multipath is often a simple procedure. The basic procedure for configuring your system with DM-Multipath is as follows:

  1. Install the multipath packages

  2. Create an empty config file, /etc/multipath.conf, that re-defines the configuration

  3. If necessary, edit the multipath.conf configuration file to modify default values and save the updated file.

  4. Start the multipath daemon

For detailed setup instructions for multipath configuration see Section, Setting Up DM-Multipath.

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