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# 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. This chapter provides a summary of the features of DM-Multipath that are new for the initial release of Ubuntu Server 12.04. Following that, this chapter provides a high-level overview of DM Multipath and its components, as well as an overview of DM-Multipath setup.

- New and Changed Features for Ubuntu Server 12.04
- Panoramica
- Storage Array Overview
- DM-Multipath components
- DM-Multipath Setup Overview

## New and Changed Features for Ubuntu Server 12.04

Migrated from multipath-0.4.8 to multipath-0.4.9

### Migration from 0.4.8

The priority checkers are no longer run as standalone binaries, but as shared libraries. The key value name for this feature has also slightly changed. Copy the attribute named prio_callout to prio, also modify the argument the name of the priority checker, a system path is no longer necessary. Example conversion:

    device {
            vendor "NEC"
            product "DISK ARRAY"
            prio_callout mpath_prio_alua /dev/%n
            prio    alua
    }

See Table Priority Checker Conversion for a complete listing

#### Priority Checker Conversion

v0.4.8 | v0.4.9
--- | ---
prio_callout mpath_prio_emc /dev/%n | prio emc
prio_callout mpath_prio_alua /dev/%n | prio alua
prio_callout mpath_prio_netapp /dev/%n | prio netapp
prio_callout mpath_prio_rdac /dev/%n | prio rdac
prio_callout mpath_prio_hp_sw /dev/%n | prio hp_sw
prio_callout mpath_prio_hds_modular %b | prio hds

Since the multipath config file parser essentially parses all key/value pairs it finds and then makes use of them, it is safe for both prio_callout and prio to coexist and is recommended that the prio attribute be inserted before beginning migration. After which you can safely delete the legacy prio_calliout attribute without interrupting service.

## Panoramica

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.

## DM-Multipath components

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

## 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-tools and multipath-tools-boot packages

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

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

4. Start the multipath daemon

5. Update initial ramdisk

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