If suppose we got a large storage array, and requirement is to not allow one server to use all storage spaces, so it need to divided into logical units as LUN(Logical Unit Number). So LUN allow us slice storage array into usable storage chunks and present same to server. LUN basically refer to either a entire physical volume or subset of larger physical disk or volume. LUN represent logical abstraction or you can say virtual layer between physical disk and application. A LUN is scsi concept.
