Multi-path settings on SCSI devices

First of all there can be differences between the various versions of Linux, so defining the version of the Linux would be appropriate.

[root@tslnx01 ~]# uname -a
Linux tslnx01 3.8.13-44.1.1.el6uek.x86_64 #2 SMP Wed Sep 10 06:10:25 PDT 2014 x86_64 x86_64 x86_64 GNU/Linux

[root@tslnx01 ~]# cat /etc/oracle-release 
Oracle Linux Server release 6.6

On linux, every SCSI disk is defined by a unique WWN number. By using scsi_id command, all the WWN numbers of the SCSI disks can be listed as follows;

[root@tslnx01 ~]# scsi_id -g /dev/sdi
360080e50004316c8000001d655b8432e

If you are using a shared storage, then WWN number identifies a LUN on the storage. If the LUN on the storage attached via multiple SAN cables (paths), then the same WWN number can be used for more than one SCSI disks.

For instance, the following output shows that sdi and sdq SCSI devices are identified by the same WWN number. The reason for this is, host and FC storage are attached with two different Fibre channels (check Direct host connection with netapp EF560).

[root@tslnx01 ~]# scsi_id -g /dev/sdq
360080e50004316c8000001d655b8432e
[root@tslnx01 ~]# scsi_id -g /dev/sdi
360080e50004316c8000001d655b8432e
[root@tslnx01 ~]#

For this kind of configuration, a multipath software should be used on the operating system, so two different SCSI devices for the same LUN can be configured on one multipath disk. By using multipath, if any of the two paths malfunctions then the access to the storage LUN will not be effected which brings high availability. As we are using Linux 6.6 for this post, we used dm-multipath software for this high available configuration.

Before starting multipath configuration, we should first identify the multipath pointer for this two SCSI disks.

[root@tslnx01 ~]#  multipath -ll /dev/mapper/mpathv
mpathv (360080e50004316c8000001d655b8432e) dm-7 NETAPP,INF-01-00
size=30G features='3 queue_if_no_path pg_init_retries 50' hwhandler='1 rdac' wp=rw
|-+- policy='round-robin 0' prio=14 status=active
| `- 1:0:0:8 sdi 8:128 active ready running
`-+- policy='round-robin 0' prio=9 status=enabled
  `- 2:0:0:8 sdq 65:0  active ready running
[root@tslnx01 ~]#

As can be seen from the command output, mpathv points to dm-7, and dm-7 points to sdi and sdq SCSI disks respectively. Also can be seen that, the WWN identifiers are same so pointing to the same LUN on the storage. In summary, whichever SCSI disk is active (sdi or sdq), access to the LUN will be available.

Up to this point, we have seen the detailed configuration of the multipath access to a LUN defined on a shared storage.