lun与volume的关系（摘自wiki）

To provide a practical example, a typical disk array has multiple physical SCSI ports, each with one SCSI target address assigned. Then the disk array is formatted as a RAID and then this RAID is partitionedinto several separated storage volumes. To represent each volume, aSCSI target is configured to provide a logical unit. Each SCSI targetmay provide multiple logical units and thus represent multiple volumes,but this does not mean that those volumes are concatenated.The computer that accesses a volume on the disk array identifies whichvolume to read or write with the LUN of the associated logical unit.

Another example is a single disk drive with one physical SCSI port.It usually provides just a single target, which in turn usuallyprovides just a single logical unit whose LUN is zero. This logicalunit represents the entire storage of the disk drive.