What does linux lun mean?

linux lun means logical unit number, and its full name is "Logical Unit Number"; the number of devices that can be attached to the SCSI bus is limited, usually 6 or 15, but in fact we need The objects used to describe far exceed this number, so the concept of LUN was introduced, which means that the function of LUN ID is to expand the Target ID.

#The operating environment of this tutorial: linux5.9.8 system, Dell G3 computer.

What does linux lun mean?

The full name of LUN is Logical Unit Number, which is the logical unit number. We know that the number of devices that can be attached to the SCSI bus is limited, usually 6 or 15. We can use Target ID (also called SCSI ID) to describe these devices. As soon as a device is added to the system, there is a Code name, when we distinguish the equipment, we only need to say the number and it will be ok.

In fact, the objects we need to describe far exceed this number, so we introduced the concept of LUN, which means that the function of LUN ID is to expand the Target ID. There can be multiple LUN Devices under each Target. We usually refer to LUN Device as LUN. In this way, we can say that the description of each device has changed from the original Target x to Target x LUN y. So obviously, we describe the device capabilities are enhanced.

Just like there is a physical hard disk in our computer, we need to partition it into logical disks: such as C drive, D drive, E drive...

So we can To sum up, LUN is just a method we introduced in order to use and describe more devices and objects. There is nothing special about it at all.

LUN ID is not equal to a certain device, it is just a number and does not represent any physical attribute. In our actual environment, the LUN we encounter may be disk space, a tape drive, or media changer and so on.

What exactly is a LUN?

The mystery of LUN (relative to some novices) is that it is often not a visible entity, but some virtual objects. For example, an array cabinet is regarded as a Target Device by the host. For some special needs, we have to divide the disk space of the disk array cabinet into several small units for the host to use, so some logic is generated. In terms of drives, they are logical objects with a lower level than Target Device. We are used to calling these smaller disk resources LUN0, LUN1, LUN2... and so on. Due to the mechanism of the operating system, the minimum storage object level recognized by the operating system is LUN Device. This is a logical object, so it is often called Logical Device.

Some people say that in my Windows, I only recognize a disk, but I don’t see anything about LUN. Does it mean LUN=Physical Disk? The answer is no. As long as you pay attention, you can see a LUN value in the disk's properties. This is just because your Disk is not divided into multiple storage resource objects, but the entire disk is used as a LUN. LUN The ID defaults to zero, that's all.

We have encountered such a problem. For example, someone asked, we have a disk array connected to two hosts. We divided a LUN for the two hosts to recognize, and then we thought, first The operating system divides the disk into two partitions, allowing the two hosts to use two partitions respectively. Then, if a host goes down, use cluster software to switch the partition to another host. Is this feasible? The answer is also no. The disk unit operated by the cluster software is a LUN, not a partition, so this operation is not feasible. Of course, in some environments, generally those with relatively low requirements, different disk partitions can be mounted on multiple hosts. However, in this case, there is actually no disk switching involved, so in some high-demand environments In the environment, this situation is simply not allowed to exist.

It should also be noted that in the concepts of some manufacturers and some products, the LUN ID is bound to a specific Device. For example, some of IBM's tape libraries have only one Target ID for the entire tape library, and then The changer and tape drive are assigned to LUN0, LUN1, LUN2..., but we should note that this is only a special design of the product, and it is also a rare case.

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