Figure 1-1 : Controllers and Disk Drives Each disk drive is managed by a controller. Each type of controller can support a fixed number of drives. Your workstation can support a fixed number of controllers. (For the number and type of controllers supported by your model of workstation, see your hardware owner's guide.) SCSI controllers support up to seven disks per controller or up to 15 disks per controller (depending upon the SCSI controller type), and VME controllers support up to 14 disks per controller.
Each disk is assigned a drive address (called the unit number in output from the hinv command and also known as a SCSI ID). This address is set by a switch, a dial, or jumpers on the disk, or by the physical location of the disk. See the hardware owner's guide for the system for information on setting the drive address of a disk.
Some SCSI devices, such as RAIDs (an array of disks with built-in redundancy), have an additional identifying number called a logical unit number or lun. It is used to address disks within the device.