What Is A LUN? Logical Unit Number Explained

Aaron Paul updated on Sep 16, 2022 to Knowledge Center

Logical Unit Number or LUN is a paramount element of storage devices. But what exactly is it?

LUN or Logical Unit Number is one of the key things about storage devices. Every now and then, you might hear people talk about it or read about it on the internet. This number used in the SCSI protocol recognizes logical units within a storage device.

LUNs array

But what exactly is it? How does it work? And what are the types of Logical Unit Numbers? In this article, we'll discuss them and analyze each thoroughly. So, let us dive right into it.

What Is A Logical Unit Number?

Logical units in a storage device determine the area upon which data is written. That's why the SCSI protocol requires a series of numbers to address each logical unit. Therefore, when storage is in use, it employs Storage Area Network or SAN protocols to encapsulate the SCSI.

Thus, a number is used to address the logical unit in use, and SCSI employs them by their numbers, coining the term LUN or logical unit number. This numbered device can be used in various scenarios, including the obsolete tape drive.

Or anything that basically writes data on a physical disk. So, it's a number used to refer to a logical disk on Storage Area Network (SAN). Sometimes, the logical disk itself is also addressed as LUN. However, most experts believe it to be technically wrong.

The storage device is a sum of various units, and each unit is addressed by a granted serial number, which we call a logical unit number.

How Does Logical Unit Number Works?

For some, the working of LUN in storage devices is a bit complicated to understand. However, a hard disk drive uses multiple physical disks to write data. However, each unit (physical disk/memory chip) is granted a serial number.

This serial number is targeted by SCSI when required. So, when the user formats the disk array or creates a partition, it moves the storage into separate volumes. Then, to discern each volume, SCSI configures a logical unit and gives it a number.

Therefore, when a simple task occurs on a PC, something as simple as opening the drive in your PC, the computer goes to the LUN provided by SCSI. So, when operations like that are commenced by a computer administrator, the CDB (Command Descriptor Block) is delivered to a physical storage unit.

Then, a 3-bit logical number unit identifies the logical unit within the specific target. This allows the device to commence operation according to the number and continue writing on that specific storage area.

Once that area is filled or used, SCSI sends CDB to another physical storage unit. Then, this logical number unit becomes primary for the time being until it runs out too.

Types of Logical Unit Number

While it may sound like one singular type, LUN or Logical Unit Number comes in many types. Each type is targeted toward a specific operation of a storage device. Therefore, when various logical unit types are required, the number types change too. Here are four main such types:

Mirrored LUN

Mirrored LUN is a fault-tolerant LUN type, making two data copies on separate physical drives. It allows for better control of redundancy and backup. This is also the type employed by programs that identify duplicate or redundant files.

Concatenated LUN

Concatenated LUN's primary job is to condense various logical number units into a single unit or volume. It's the primary type of logical number unit used in RAID groups, especially when two logical number units are barred together.

Striped LUN

Striped LUN writes data on various physical drives using a single Logical Unit Number. This allows the computer to achieve better performance and enhanced durability of various physical drives. Simply because it distributes the Input/Output across different disks.

Striped LUN with parity

Striped LUN with parity is more or less the same as striped LUN. However, the job of this Logical Unit Number type is parity information across physical drives—preferably three or more. This ensures that if one drive fails or doesn't work, it can be reused from information on the remaining drives.

Logical Unit Number Use Cases

The primary purpose of LUN is to identify storage devices. While this can surely differ depending on the LUN type, the principles remain the same. For instance, if you create a partition on your drive, then a simple Logical Unit Number will be used to identify that one portion on the physical drive.

Moreover, LUNs are used to zone and mask SANs so they can map multiple physical LUNS by virtualizing each physical sector. And as mentioned before, the term itself is used to identify the number, but it's also commonly used to discern a logical unit itself.

Conclusion

These are some of the key things you need to know about the logical unit number. It is an essential identification of physical drives and a key component of usual computing operations. So, it's imperative to understand LUNs to comprehend the storage process.

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