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Multipathing is used to detect multiple paths to devices for fail-over or performance reasons and coalesces them.

Prerequisites:

We are configuring multipath through iscsi, before we move ahead you have to setup iscsi first, if you don’t know how to set up it please refer How to setup iSCSI in linux. After you setup iscsi targets we will move ahead and install multipath package. You must set up multiple ip’s to the server and client, you can use different ip subnet. in our scenario we used same ip subnet.

On the initiator you need to install below packages :

# yum install device-mapper-multipath

# yum install iscsi-initiator-utils

Once you install above packages start iscsi service:

# service iscsi start
# service iscsid start

Configure it at startup :

# chkconfig iscsid on
# chkconfig iscsi on

Discover the exported luns from the server  10.192.8.170 and 10.192.8.171 via  iscsiadmin :

# iscsiadm -m discovery -t sendtargets -p 10.192.8.170
# iscsiadm -m discovery -t sendtargets -p 10.192.8.171

Now you have to login to same lun using different IP:

# iscsiadm -m node -T iqn.2016-02.testlab.example:node1.target1 -p 10.192.8.170 -l

# iscsiadm -m node -T iqn.2016-02.testlab.example:node1.target1 -p 10.192.8.171 -l

Now run fdisk -l command which will show you newly added iscsi devices as your local drives ie. /dev/sdc and /dev/sdd.

Edit the /etc/iscsi/iscsid.conf file and configure iscsi for automatic mounting after reboot.

# vi /etc/iscsi/iscsid.conf

Add or un-comment below line

node.startup = automatic

By default there is no file multipath.conf file you have to copy it from the example documents. Copy file from below location and place it to /etc/

cp /usr/share/doc/device-mapper-multipath-0.4.9/multipath.conf /etc/

If you don’t have /etc/multipath.conf file. Just   Edit /etc/multipath.conf  file add below lines to it.

# vi /etc/multipath.conf

defaults  {
 udev_dir                   /dev
 polling_interval           10
 path_selector              "round-robin 0"
 path_grouping_policy       multibus
 getuid_callout             "/lib/udev/scsi_id --whitelisted --replace-whitespace --device=/dev/%n"
 prio                      const
 path_checker              directio
 rr_min_io                 100
 flush_on_last_del         no
 max_fds                   8192
 rr_weight                 priorities
 failback                  immediate
 no_path_retry             fail
 queue_without_daemon      no
 user_friendly_names       no
 mode                      644
 uid                       0
 gid                       disk
}
blacklist {
 wwid 26353900f02796769
 devnode "^(ram|raw|loop|fd|md|dm-|sr|scd|st)[0-9]*"
 devnode "^hd[a-z][[0-9]*]"
 devnode "^dcssblk[0-9]*"

}

Now start the multipathd service

# service multipathd start

Start up the service after next reboot.

# chkconfig multipathd on

Now check whether  you are able to see VIRTUAL-DISKS or not.

multipath -ll

1IET_00010001 dm-3 IET,VIRTUAL-DISK
 size=100M features='0' hwhandler='0' wp=rw
 `-+- policy='round-robin 0' prio=1 status=active
 |- 8:0:0:1 sdb 8:16 active ready running
 `- 9:0:0:1 sdc 8:32 active ready running

You will be able to see  the new virtual disk as 1IET_00010001 (in your case it might display different name) in /dev directory   :

ls /dev/mapper/1IET_00010001

Now create new partition on virtual disk using below command:(if you don’t know how to create partition please refer How to create partition  )

fdisk /dev/mapper/1IET_00010001

Now you will be able to see new partition as /dev/mapper/1IET_00010001p1  in the partition table of /dev/mapper/1IET_00010001. Now run partprobe command :  partprobe is a program that informs the operating system kernel of partition table changes, by requesting that the operating system re-read the partition table.

# partprobe /dev/mapper/1IET_00010001p1

Now create file system on the new partition in this case we are creating ext4:

# mkfs.ext4 /dev/mapper/1IET_00010001p1

fdisk -l will show you the new disk as

Disk /dev/mapper/1IET_00010001: 104 MB, 104857600 bytes
4 heads, 29 sectors/track, 1765 cylinders
Units = cylinders of 116 * 512 = 59392 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x13099dd6

Device Boot Start End Blocks Id System
/dev/mapper/1IET_00010001p1 1 100 5785+ 83 Linux

Disk /dev/mapper/1IET_00010001p1: 100 MB, 5924352 bytes
255 heads, 63 sectors/track, 0 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00000000

Create new directory to mount the new virtual partition :

# mkdir /share

Mount the file system using below command :

mount -t ext4 /dev/mapper/1IET_00010001p1 /share

If you want to add it to fstab: edit /etc/fstab file and add below line to end of the fstab file.

/dev/mapper/1IET_00010001p1 /share ext4 defaults,_netdev 0 0