SCSI Storage Technology Quiz

SCSI-2 introduced 'Wide SCSI' which doubled the data path from 8 bits to 16 bits. The standard SCSI-1 and baseline SCSI-2 used an 8-bit (narrow) data bus. Wide SCSI's 16-bit bus effectively doubled the theoretical throughput compared to narrow SCSI. Wider implementations (32-bit or 64-bit) were not part of the standard SCSI-2 Wide specification.

SCSI defaults to asynchronous data transfer mode, where each byte/word transfer requires handshaking signals (REQ/ACK) before the next can begin. Synchronous transfer is an optional negotiated mode that allows multiple data transfers per handshake cycle, improving throughput. 'Semi-synchronous' is not a standard SCSI transfer mode. The default is asynchronous for maximum compatibility.

In the SCSI Architecture Model (SAM-2 and later), the entire I/O system is represented by the 'SCSI Domain' class. A SCSI Domain encompasses one or more SCSI devices interconnected via a Service Delivery Subsystem. SCSI Device represents individual devices, and Service Delivery Subsystem represents the interconnect (cables, buses, etc.). The Domain is the highest-level abstraction.

The Service Delivery Subsystem resides at the Interconnect Layer, which is the lowest layer in the SCSI Architecture Model. This layer handles the physical and logical interconnection between SCSI devices, including cables, connectors, signaling protocols, and bus arbitration. The Application Layer handles command processing, and the Transport Layer manages data delivery.

The SCSI I_T_L_Q nexus notation identifies: I=Initiator, T=Target, L=Logical Unit, Q=Command (or task queue). The Q field represents the command queue tag used to tag command queuing operations, allowing multiple outstanding commands from an initiator to be tracked. It's not 'Quality', 'Quota', or a general 'Queue' - it specifically represents the queued command identifier.

In SCSI architecture, a Target is the device that responds to commands from an Initiator. The Initiator (usually the host computer) starts the SCSI session and sends commands to the Target (usually a peripheral device like a disk drive). The Target passively waits for commands and executes them. A LUN (Logical Unit Number) is an addressable entity within a Target, not the device role itself.

LUN stands for Logical Unit Number. While it contains 'Number', in SCSI standards a LUN is officially defined as a 64-bit identifier used to address a specific logical unit.

When a previously disconnected Target device needs to regain control of the SCSI bus to complete an I/O operation, it enters the Reselection phase. The device reconnects to the Initiator to continue the pending transaction. Bus Free is when no device owns the bus, Selection is when an Initiator chooses a Target, and Message In is a phase for status/message transfer - none of these apply to a device reconnecting after disconnection.

CDB stands for Command Descriptor Block, which is the standard format for commands sent to SCSI devices. The other options are plausible-sounding but incorrect terms.

SCSI command architecture defines CDB groups by length. A 12-byte CDB belongs to Group 5 (Group 0=6-byte, Group 1=10-byte, Group 4=16-byte, Group 5=12-byte).

In a SCSI Command Descriptor Block (CDB), the operation code (opcode) is always located in the first byte (byte 0) of the command block, making it exactly 1-byte long.

Status Code is the immediate indicator returned by a SCSI device to show whether a command succeeded or failed. Sense Data provides additional error details only when requested.