Linux -- rescan scsi device or rescan scsi host --sysfs

/sys directory is like a virtual filesystem which contains information and stats about the devices in the system.. Furthermore; It gives us the opportunity in a way to control the devices connected to our system.
It is also known as sysfs, and provides a set of virtual files by exporting information about various kernel subsystems, hardware devices and associated device drivers from the kernel's device model to user space. In addition to providing information about various devices and kernel subsystems, exported virtual files are also used for their configuring. 

You may ask, what is the /dev filesystem then? 

/dev contains the real device files, on the other hand; /sysfs contains info and control mechanisms which are exported by the kernel during the runtime.

Okay, I will not go in details because these are deeper issue..
After giving the general information about the /sys directory , lets jump to the scsi_host and scsi_device directories located in this sysfs..  

There are directories to hold the files related with the devices of all kinds in the /sys/class directory.

scsi_host and scsi_device directories are one of these directories, as well.. They both reside the /sys/class directory. 

The files in the scsi_host directory are like the interfaces to the scsi bus and the files in the scsi_device directory can be used to control the scsi devices connected to the system.

In this article , I will focus on the files named rescan  , located in these directories.. 

Both scsi_device and scsi_host directories contains rescan files in their subdirectories.. 

As you expected, rescan file in the scsi_device is there for rescanning the associated scsi device, and the rescan file located in scsi_host directory is used for rescanning the scsi bus .. 

I 'll keep in short and explain scsi scan activities by giving 2 example scenarios.. When to use?  and what to use? according to the scenarios..

• Suppose you have added a disk to your VM .. Then you have to rescan the scsi bus.. The driver should scan for the newly added device and notify the SCSI midlayer.. 

So, you must issue the following command;

echo "- - -" > /sys/class/scsi_host/host0/scan   (host may be changed according to the environment)

- - - signs are wildcard for the channel, SCSI target ID, and LUN..  It is like described in the /proc/scsi/scsi file..

With "- - -" ; we say -> scan everyting..

After this operation use fdisk -l .. You will see your new device listed in the output..

• Suppose you have extended the size of a disk which was already connected to your VM .. 

Then you have to rescan the associated scsi device, as follows;

echo 1 > /sys/class/scsi_device/2\:0\:2\:0/device/rescan

After this operation use fdisk -l .. You will see your device listed with its new size..

As you see above, you can control the devices using /sys filesystem. You can reach the exported options and modify them while the system is running.. Thus, you can manage the devices and make them actually do something you want..
In addition, as explained in the above scenarios ; by rescanning the scsi devices or scsi hosts, you ll not have to reboot your server everytime when a new disk is added or a when a disk is modified.. 

That is ; your system will recognize the change without a need for reboot and these scenarios are valid for both Physical and Virtual servers..