How to Setup DM-Multipath on RHEL & CentOS


Overview:

DM-Multipath stands for Device mapper multipathing. It allows us to configure multiple I/O paths between server  and storage arrays into asingle 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

‘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 if 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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