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How to Setup DM-Multipath on RHEL / CentOS / Fedora

 

Overview

DM-Multipath  stands for Device mapper multipathing. It allows us to configure multiple I/O paths between server  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 and remove the Single Point of Failure Problem.

 

Why To Use Multipathing :

 

Below are the two reasons why we are using DM-Multipath on Linux Servers :


Redundancy : In Case of active/passive multipathing we will be using two paths. One path will be used for I/O operations and other will be in stand by. If any element of an I/O path (like cable, switch, or controller) fails, DM-Multipath switches to an alternate path or stand by path.


High Performance : In case of active/active multipathing , I/O operations are shared over available paths in round-robin fashion and provide improved performance.

 

Installation of DM-Multipath on RHEL / CentOS / Fedora :


‘device-mapper-multipath’ is the multipath rpm package

 

# rpm -q device-mapper-multipath

 

if it is not installed then use the below yum command :

 

# yum install device-mapper-multipath

 

Setting up multipathing :

 

‘/etc/multipath.conf’ is the default configuration file for multipathing , if this file doesn’t exists , copy the sample file :

 

# cp /usr/share/doc/device-mapper-multipath-<version>/multipath.conf /etc


Now Edit the file /etc/multipath.conf and make sure following line exists and commented out:

 

devnode_blacklist {
devnode "*"
}

 

Above Section of multipath.conf file blacklists all the devices by default. We must comment it out to enable multipathing.

 

 

Starting the multipath service

 

Multipath is controlled by the multipathd service

 

# service multipathd start ; chkconfig multipathd on

# multipath -v2

 

multipath -v2”  command scans, identifies, and configures multipath maps for devices that could have multiple paths to storage, as configured in /etc/multipath.conf

 

To display the multipath :

 

# multipath –ll

 

Note : In Case Sample file doesn’t exist on above path, then we can use mpathconf utility which will create /etc/multipath.conf file from scratch.

 

To Enable Basic multipathing failover :

 

# mpathconf --enable --with_multipathd y


Now  enable multipath, allow it to use user_friendly_names, as well as to find_multipaths

 

# mpathconf  --enable  --user_friendly_names  y  --find_multipaths  y

 

Restart the Service

 

# service multipathd restart

 

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