Cloud Vedas

In this post we will discuss how to scan new LUNs allocated by storage team to a Redhat Linux system.
There are two ways of scanning the LUNs
Method 1:-
Find how many SCSI bus controllers you have

Go to directory /sys/class/scsi_host/ and list it’s contents.

cd /sys/class/scsi_host/

[root@scsi_host]# ls

host0 host1 host2

[root@scsi_host]#

Here we can see we have three SCSI bus controllers. So in below command replace hostX with these directory names.

Run the Command ,

echo "- - -" > /sys/class/scsi_host/hostX/scan

[root@cloudvedas]# echo "- - -" > /sys/class/scsi_host/host0/scan

[root@cloudvedas]# echo "- - -" > /sys/class/scsi_host/host1/scan

[root@cloudvedas]# echo "- - -" > /sys/class/scsi_host/host2/scan

[root@cloudvedas]# echo "- - -" > /sys/class/scsi_host/host3/scan

TIP:- Here the “- – -” denotes CxTxDx i.e. Channel(controller) , Target ID and Disk or LUN number. This is asked in Linux Admin Interviews also.

Repeat the above step for all three directories.

If you have FC HBA in the system you can follow the steps as below:-

First check number of FC controllers in your system

#ls /sys/class/fc_hosthost0 host1 host2

To scan FC LUNs execute commands as

echo "1" > /sys/class/fc_host/host0/issue_lip

echo "1" > /sys/class/fc_host/host1/issue_lip

echo "1" > /sys/class/fc_host/host2/issue_lip

Tip :- Here echo “1” operation performs a Loop Initialization Protocol (LIP) and then scans the interconnect and causes the SCSI layer to be updated to reflect the devices currently on the bus. A LIP is, essentially, a bus reset, and will cause device addition and removal. This procedure is necessary to configure a new SCSI target on a Fibre Channel interconnect. Bear in mind that issue_lip is an asynchronous operation.

Verify if the new disk is visible now

fdisk -l |egrep '^Disk' |egrep -v 'dm-'

Method 2 :-

Next method is to scan using SG3 utility. You can install it using

yum install sg3_utils

Once installed run the command

/usr/bin/rescan-scsi-bus.sh

Solved: How to scan new LUNs in Redhat Linux