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How to Configure Multipathing in Linux (RHEL7/CentOS7)

How to Configure Multipathing in Linux (RHEL7/CentOS7)

This article describes you the step by step procedure to configure the Multipathing in Linux (RHEL7/CentOS7) using Device Mapper Multipathing (DM-Multipath) and helps you to understand the multipath configuration..

Device Mapper Multipathing (DM-Multipath) is a native multipathing in Linux, Device Mapper Multipathing (DM-Multipath) can be used for Redundancy and to Improve the Performance. It aggregates or combines the multiple I/O paths between Servers and Storage, so it creates a single device at the OS Level.

For example, Lets say a server with two HBA card attached to a storage controller with single ports on each HBA cards. One lun assigned to the single server via two wwn number of both cards. So OS detects two devices: /dev/sdb and /dev/sdc. Once we installed the Device Mapper Multipathing. DM-Multipath creates a single device with a unique WWID that reroutes I/O to those four underlying devices according to the multipath configuration. So when there is a failure with any of this I/O paths, Data can be accessible using the available I/O Path.


Also watch this demo practically on Youtube!

1. Install the  Device Mapper Multipath package.
Verify the device-mapper-multipath package has been installed or not.
[root@linux1 ~]# rpm -q device-mapper-multipath
If it is not installed, Install the Device Mapper Multipath package using yum to avoid dependencies issue. if yum is not configured, please refer the link Yum Configuration on Linux
[root@linux1 ~]# yum -y install device-mapper-multipath
2. Basic Configuration of Linux Device Mapper Multipathing
Configuration file is /etc/multipath.conf file, take a backup of it. Edit the configuration file to ensure you have the following entries uncommented out.
defaults {
user_friendly_names yes
}
blacklist {
devnode “sda”
}
The blacklist includes the devices which are not to be configured in Multipathing. For example, Lets say our OS installed disk is /dev/sda. So the first entry in the blacklist will exclude them. Same for IDE drives (hd).

3. Start and Enable the multipath daemons.
Start the multipath service if not started bydefault.
[root@linux1 ~]# systemctl start multipathd
Enable the multipath service to start on boot.
[root@linux1 ~]# systemctl enable multipathd
4. Check the multipathing status.
The multipath -ll command prints out multipathed paths that show which devices are multipathed. If the command does not print anything out, ensure that all SAN connections are set up properly and the system is multipathed.
[root@linux1 ~]# multipath -ll
mpathb (360014051f89d2bb3300470fa7d4baa10) dm-2 LIO-ORG ,lun0
size=2.0G features='0' hwhandler='0' wp=rw
|-+- policy='service-time 0' prio=0 status=active
| `- 1:0:0:0 sdb 8:16 active active running
`-+- policy='service-time 0' prio=0 status=enabled
  `- 2:0:0:0 sdc 8:32 active active running
The above output shows 1 LUN (mpathb) with 2 paths (sdb and sdc). Linux Multipathing basic configuration is done. Refer this link to check further for troubleshooting any issue related with multipathing and also understand its terminologies.

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