What is a LUN, and why do we need one?
In simple terms, a logical unit number (LUN) is a slice or portion of a configured set of disks that is presentable to a host and mounted as a volume within the OS.
Internal architecture and front end connections aside, the primary reason for purchasing a disk array is to provide data storage capacity through presenting a number of physical disks, and making them available and configurable to the IT administrator for the purposes of distributing portions of that capacity (e.g. volumes) to specific applications.
The disks in an array are usually configured into smaller sets (Raid groups) to provide protection against failure. These RAID groups define the manner in which the group of physical disks handle data when it is written to the disks within that group (and as such, the way in which that data is protected). However, a RAID group (being the underlying structure of that group of physical disks), is not presentable to the host. In order to do this, an administrator must create a LUN which can be presented to and mounted on a host operating system.
In terms of LUN management, the LUN can either be configured as a small ‘slice’ of the total usable capacity of the RAID group, or configured to use the total space available within that RAID group, (dependant on the requirements of the application that it wishes to use it). The logical unit number (LUN), when presented to the host, shows as a mountable volume of the same capacity as the ‘slice’ configured by the administrator, hiding any remaining capacity on that RAID group. Any remaining capacity can then be 'sliced' into additional logical unit number (LUNs) as required.
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