Q

Creating LUNs to simplify system management

This expert answer explains the purpose of creating LUNs and details reasons for creating multiple LUNs.

What is the purpose of creating LUNs, and in what situation would I create only one LUN and not several?

A LUN, or logical unit number, is a logical arrangement of physical disks that appear to the operating system (and users) to be a single disk. Since the appearance is not physical, it's said to be logical, thus the name logical unit.

You can embed RAID or other disk management functions within a logical unit. For example, a LUN might handle RAID 1 functions, and automatically handle the mirroring of data between multiple physical disks, or automatically include a replication function that copies data to a remote site. Meanwhile, the LUN looks, for all practical purposes, like a standard single disk.

The purpose of creating a LUN is to simplify system management by hiding these functions from the system. In many cases, they also provide a means to ensure that these functions are set up as a routine part of designing a system. A simpler application of a LUN is to create a virtual disk that is larger than any single disk, simply by concatenating the physical disks. (Note that this arrangement will decrease availability because the failure of any disk in a concatenated LUN will cause the whole LUN to fail. If you concatenate and mirror physical disks in a style called RAID 1+0, you will achieve much better reliability and availability.)

As for how many you might need on a particular system, that's a very difficult question to answer. A smaller system might need no more than one or two, while a larger system that is shared by different groups might require more LUNs. More LUNs mean more work in administering a system, but they allow that system to be shared by more groups of users.

 

This was first published in October 2005

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