Home | Contents | Help   Raidhelp     Interfaces: The Physical Interface SCSI: Small Computer System Interface LUN: Logical Unit Number Each device on a SCSI chain has a separate SCSI ID number. Simultaneously, each separate SCSI ID number can have up to 8 LUNs . For example, a single SCSI hard drive can be set at ID 0-15 depending on whether it is a Wide device on a Wide bus or a narrow device. Once the drive is formatted it can appear as a single drive that allows the system complete access to its entire capacity. If the drive is first partitioned (say 3 partitions) then each partition can be assigned a separate LUN number on the same SCSI ID. Assume our hard drive is a filing cabinet. Its name is SCSI ID 5. It contains 3 drawers (our partitions). Each drawer can be given a name (LUN) Partition 1 can be SCSI ID 5 LUN 1 Partition 2 can be SCSI ID 5 LUN 2 Partition 3 can be SCSI ID 5 LUN 3 This is the SCSI equivalent of assigning drive letters to partitions - creating logical drives rather than physical ones. Providing your host adapter supports multiple LUNs, the number of separate partitions you can create from multiple logical drives is limited only be the physical restrictions on the number of drives you can attach to the I/O bus.  If your RAID controller supports 8 Logical Drives for example, each of these may then be partitioned via your controller and assigned a separate SCSI ID LUN. Each partition may have a separate filing system installed and hold different data, and even be used by different systems. A hardware RAID controller offers the option of supporting various flavours of operating systems simultaneously. Related topics: The SCSI Interface SCSI Cabling & Connections     Raidhelp   RAIDhelp© Copyright 1999-2004 Antony Kershaw