SCSI stands for Small Computer System Interface and is a set of standards for transferring data and connecting devices and computers. Sometimes you make changes to disk devices on your Linux box while online which is even more often nowadays using virtual machines. Sometimes you also add a new disk and do not want to restart your Linux system. Wether you need to add a new disk or increase an existing disk size of your virtual machine, the changes will not be instantly visible from the Linux operating system because you need to rescan SCSI bus on Linux to see the change. It is quite easy to rescan SCSI bus on Linux, you can do it with a simple one line command (depending on the number of SCSI devices you have connected).

Follow the steps described below and you will see the new size of your disk.

Rescan SCSI bus on Linux

Please note that the rescan SCSI bus on Linux does not interrupt operating system operation nor remove any of the disks. Rescanning SCSI bus on Linux with the below procedure will add/freshen devices but not remove them.

When Adding a New Disk

When adding a new disk to your Linux system you need to rescan SCSI host.

You can do this with the following command:

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

..where X is the number of SCSI host to scan.

You probably have more than one SCSI host available so to make it right you should repeat the above command for each SCSI host available.

Below is a practical example on how to rescan SCSI bus on Linux and and see the newly added disk:

[root@node-1 ~]# echo "- - -" > /sys/class/scsi_host/host0/scan
[root@node-1 ~]# echo "- - -" > /sys/class/scsi_host/host1/scan
[root@node-1 ~]# echo "- - -" > /sys/class/scsi_host/host2/scan

The “- – -” part is where you tell the SCSI host what exactly to rescan and the hyphens are wildcards which tell SCSI host to rescan all controllers, channels and LUNs.

When Increasing Existing Disk Size

If you changed the size of an existing disk you might notice that the operating system can not see the new disk size until you rescan SCSI bus on Linux operating system.

The easiest way i’ve found is to rescan the specific device with the following command:

echo "1" > /sys/class/block/sdX/device/rescan

..where X is the device you increased and want to rescan.

Below is a practical example on how to rescan SCSI bus on Linux and and see the newly added disk:

[root@node-1 ~]# echo "1" > /sys/class/block/sdb/device/rescan

The “1” is a flag which causes the SCSI host to rescan the “sdb” block device and therefore refresh the data about the disk size. Please note that i choose this command due to the human readable “sdb” naming which is really easy to remember.

Otherwise the command could also be triggered as follows:

[root@node-1 ~]# echo "1" > /sys/class/scsi_device/2:0:1:0/device/rescan

..but in the last case you should know which device you want to rescan by the SCSI bus ID.