Compaq Infortrend manual Operation Theory, 2.3.1 I/O Channel, SCSI ID, and LUN, 3 SCSI ID/LUNs

2.3Operation Theory

2.3.1I/O Channel, SCSI ID, and LUN

Depending on the interface used by a RAID system, a SCSI drive channel (SCSI bus) can connect up to 15 drives (excluding the RAID controller itself). A Fibre channel 125 drives in a loop. Each device occupies one unique ID.

Figure 2 - 3 SCSI ID/LUNs

The figure on the left illustrates the idea of mapping a system drive to host ID/LUN combinations. The host ID is like a cabinet, and the drawers are the LUNs (LUN is short for Logical Unit Number). Each cabinet (host ID) can have up to 32 drawers (LUNs). Data can be made available through one of the LUNs of a host ID. Most host adapters treat a LUN like another device.

2.3.2Grouping Drives into an Array

Figure 2 - 4 Connecting Drives

The physical connection of a RAID controller should be similar to the one shown above. Drives are connected through I/O paths that have been designated as drive channels.

The next diagram shows two logical configurations of drives and the physical locations of its members. Using drives from different channels can lower the risk of fatal failure if one of the drive channels should fail. There is no limitation on the locations of spares.

RAID Planning 2-7