What is the purpose of creating LUNs, and in what situation would I create only one LUN and not several?
The purpose of creating a logical unit number (LUN) is to carve out a portion of the SCSI target (a disk system, for instance) to allocate a certain amount of storage to the host. Each logical unit is given a specific number (LUN) to identify it to the host. In SCSI terminology, the SCSI target is identified by a bus/target/LUN convention. This goes back to the days of parallel SCSI (actually still the majority of shipments in the market) where there was a SCSI bus and several disk drives were hanging off of it. Sometimes there were several busses available to the host. So a bus/target/LUN triad identified the bus, the specific disk drive on that bus (target) and a LUN on that target disk. Fibre Channel and iSCSI have retained a variation of this terminology, i.e. LUNs are still used. Therefore, I will stay with the standard SCSI terminology to explain the concept.
In more complex storage systems, for example a RAID system, either modular or monolithic, there can be many RAID controllers and hundreds of disk drives. In these environments, a LUN can be made up of several drives (for example, a RAID 5 set created with six disk drives) but the concept remains the same. LUNs are a way to identify a specific type and amount of storage to the host. A large monolithic storage system, such as a DMX system from EMC, could have hundreds of LUNs defined by the storage administrator and allocated to the hosts. A lot of a storage administrator's time is spent in allocating, reallocating, expanding and adding to LUNs.
It is unrealistic to imagine a single LUN scenario in today's enterprise environment. The only example I can think of, in which a single LUN would be adequate is perhaps a very specific, dedicated storage system that caters to a single application. Even then more than one LUN is often needed.
Related Q&A from Arun Taneja
Hyper-converged systems and software-defined storage provide similar storage management features, but the technologies are architected very ...continue reading
Object storage is a popular choice for moving storage to the cloud, but it can present a number of challenges. This tip presents three alternatives.continue reading
Storage expert Arun Taneja discusses the differences between a two-switch fabric and a full fabric.continue reading
Have a question for an expert?
Please add a title for your question
Get answers from a TechTarget expert on whatever's puzzling you.