SCSI Glossary

 

| A | B | C | D | E | F | G | H | I | L | M | N | O | P | Q | R | S | T | U | W |


-A-

Adapter - SCSI Host Bus Adapter or HBA
ANSI - American National Standards Institute. - The coordinating organization for voluntary standards in the United States.
API (Application Program Interface) -A set of routines, protocols and tools for building software applications.
Array Group - A group of disk drives which appear to the computer as a single LSU. RAID-1 and RAID-5 Array Groups can be composed of any number of Hardware Arrays. RAID-0 Array Groups can be composed of any combination of individual disk drives.
ASPI - Advanced SCSI Programming Interface - A protocol used by some SCSI application programs to communicate with SCSI adapters under DOS, OS/2 and Netware.
Asynchronous - Data transfer protocol which is not synchronized to a set timing interval. Asynchronous SCSI data transmitting devices must wait after each byte for acknowledgement from the receiving device. Either device can take as long as it wishes to send or acknowledge data. Asynchronous SCSI has no defined maximum transfer rate but is typically limited to 1.5 to 3MHz.
Asynchronous Information Protection (AIP) - Although most Ultra320 traffic is sent synchronously and protected by CRC, some information is still sent asynchronously. AIP implements CRC-level error checking on asynchronous traffic. This ensures end-to-end data integrity.

-B-

BIOS - Basic Input Output System - A ROM-based collection of device drivers and system boot-up routines which is provided as an integral part of every PC computer. The BIOS provide enough intelligence to enable the computer to understand some simple keyboard commands and load the operating system from disk upon power-up. In order to do this, the BIOS must contain drivers for the CRT display, keyboard and disk controller.
Block - In data management, a block is a group of records on a storage device. Blocks are manipulated as units. For example, disk drives often read and write data in 512-byte blocks.
Bridge Controller - A SCSI device which appears as a single ID on the SCSI bus, but which bridges to multiple devices. Each of these devices is assigned a Logical Unit Number or LUN. Bridge controllers are typically used to control remote RAID subsystems.
Build - The operation of initializing a Redundant Array by creating consistent redundant information. In the case of a RAID-1 array, one drive must be copied to the other. In the case of RAID-5 arrays, the parity information must be generated.
Burst - A term used to describe data words which are transmitted as a single group across a bus without interruption by another device.
Bus - A collection of wires through which data is transmitted from one part of a computer to another. Think of a bus as a highway on which data travels within a computer.
Bus Mastering - A method of data transfer which allows data to be moved between a peripheral controller and system memory without interaction with the host CPU or a third party DMA controller. This technique allows the peripheral controller to take control of the system bus and move data at up to 10MB/sec., 33MB/sec. or 132MB/sec. for ISA, EISA and PCI systems respectively.

-C-

Cache - A temporary fast storage area for data which would normally be accessed from a slower storage device. A cache management algorithm monitors the data access patterns and selects which data from the slower device is to be kept in the cache for quick access. Caches are normally transparent or hidden from the accessing device.
Channel - Refers to a communications path between two computers or devices. It can refer to the physical medium (the wires) or to a set of properties that distinguishes one channel from another.
Command Overlapping - A feature which allows the simultaneous execution of more than one I/O command by a peripheral controller.
Command Packet - An area of host computer memory which is used to store an I/O command and associated parameters for a peripheral controller which uses EATA protocol.
Command Queuing - A feature which allows multiple I/O commands to be executed by a peripheral controller, in a more efficient order.
Cyclic Redundancy Check (CRC) - verifies that the data sent matches with the received data. CRC is more robust than the simple byte parity checks used in pre-Ultra160 SCSI. Increased data rates and longer cable lengths, among other factors, have resulted in the potential for increased error rates, necessitating the use of CRC.

-D-

Degraded Mode - The mode of operation of a Redundant Array in which it can continue to be accessed after a component drive has failed. For read accesses, data is synthesized from the remaining good drives in the array. For write accesses, data is stored on the remaining good drives in such a way that it can be restored to the failed drive once it is replaced.
Device - According to the SCSI specification, up to fifteen SCSI devices may be connected to a single SCSI bus. SCSI devices include peripherals such as disk and tape drives, optical devices, scanners, printers, and host adapters. Each SCSI device is assigned a SCSI ID number from 0-7 for 8-bit SCSI, or 0-15 for Wide SCSI.
Differential (Now called High Voltage Differential) - An electrical signal protocol which transmits information through a current loop rather than by changes in voltage, thereby reducing the susceptibility to electrical interference. Differential SCSI uses RS-485 transceivers to transfer 10MHz data at distances up to 25 meters (82 feet).
DMA - Direct Memory Access - A method of data transfer which allows data to be moved between a peripheral controller and system memory without interaction with the host CPU. The data may be moved by the peripheral controller itself, or by a separate 3rd-party DMA controller.
Domain Validation - Before sending data, domain validation is performed to verify that the physical connection (cables, connectors, targets, etc.) is capable of handling the negotiated transfer speed. Although a target and initiator may indicate that they are able to sustain Ultra320 transfer rates, during Domain Validation a series of tests are performed to ensure the sustainability and reliability of that connection. If it is determined that Ultra320 speeds are not feasible, a slower speed is enforced.
Double Transition Clocking - SCSI transactions (data transfers) include request signals and data lines. Ultra2 and earlier SCSI implementations used single transition clocking whereby the frequency of the data lines was half of the request signal. This allowed data sampling only on the leading edge of the request signal. Double transition clocking increases the data line frequency to equal that of the request signal, allowing sampling on both the leading and trailing edges of the request signal. Clocking can be settable to ensure compatibility with legacy devices.
Driver - A software routine which receives I/O requests from higher levels within the operating system and converts those requests to the protocol required by a specific hardware device such as a SCSI adapter.
Duty Cycle - The percentage of time in which an operation is active.

-E-

ECC - Error Correcting Code - A method of generating redundant information which can be used to detect and correct errors in stored or transmitted data. ECC is stored on disk drives at the end of every sector to correct errors caused by media flaws. ECC is used in SmartRAID adapters to detect and correct errors caused by cache RAM defects or electrical interference over the SCSI bus.

-F-

Failed - The mode of operation of a drive or array in which the drive or array, because of a malfunction, can no longer be accessed.
Fast SCSI - The original SCSI specification defined synchronous data transmission rates of up to 5MHz. By assuming transceivers which provide tighter timing margins, the SCSI-2 standard allows synchronous transfers of up to 10MHz. This provides a transfer speed of 10MB/sec. for an 8-bit bus, and 20MB/sec. for a 16-bit bus. Devices which utilize these faster timings are called Fast SCSI devices.
Fast-20 SCSI - The SCSI-3 specification defined synchronous data transmission rates of up to 20MHz. The 20MHz transfer rate provides speeds up to 20MB/sec. on an 8-bit bus and 40MB/sec. on a 16-bit bus. Devices that support these timings are called Fast-20 SCSI devices.
Flash ROM - A ROM on the adapter containing firmware that can be reprogrammed via software without removing it from the board.
Forced-Perfect Termination - A type of terminator containing a sophisticated circuit that can compensate for variations in the power supplied by the host adapter, as well as variations in bus impedance of complex SCSI systems.
Flow Control - The target indicates to the initiator when the last packet of a data stream will be transferred so that the initiator can flush FIFOs and terminate pre-fetch sooner than previously possible. Basically, the target is warning the initiator that the transfer is almost complete so that it can prepare for the next transfer while the target completes the current transfer.
Free-Running Clock - Allows use of a higher frequency clock with legacy cables and infrastructure. This is accomplished through the use of a Data Enable signal in addition to the Request signal and Data lines.
Function Button - An object used in a graphical user interface, on which the user can point and click with a mouse, to execute a specific function.

-G-

GUI - Graphical User Interface. A software interface which interacts with the user through color graphics and a mouse.

-H-

Hardware Array - A group of disk drives which are all members of the same RAID-1 or 5 array implemented in host adapter hardware. Multiple Hardware Arrays can be combined into a common Array Group.
HBA - Host Bus Adapter - Refers to the peripheral controller cards which provide host computer access to the SCSI bus.
Hot Plug - The operation of adding or removing a device from a bus while transactions involving other devices are occurring over the bus.
Hot Spare - A spare disk drive which, upon failure of a member of a redundant disk array, will automatically be used to replace the failed disk drive.
Hot Swap - The operation of removing a failed disk drive which is a member of a Redundant Array and replacing it with a good drive, while transactions involving other devices are occurring over the bus.
HVD - High Voltage Differential. Uses two wires, transmitting a signal on one and its inverse on the other. At the receiving end, the difference between the two signals is measured and interpreted. Noise on the bus will affect both the signal and its inverse equally, so the difference between the two lines will remain the same and the noise can not be misread as a signal.

-I-

ID - A number from 0 to 7 (or 0 to 15 for 16-bit WIDE SCSI) which SCSI devices use to address one another. SCSI IDs are typically selected by setting jumpers on the SCSI device.
Icon - A graphical representation of an object or function used by a GUI.
ISA - Industry Standard Architecture - the name given to the original IBM PC/AT 16-bit bus architecture.

-I-

JBOD - Just a Bunch of Drives. A storage subsystems using multiple independent disk drives, as opposed to one form of RAID or another.

-L-

Latency - The time required by a device to access stored data, excluding the data transfer time. Reducing disk latency results in more I/O operations per second being performed on a disk drive.
LVD - Low Voltage Differential - SCSI signaling method that combines the benefits of HVD and Single-Ended technologies, allowing longer cabling configurations (25 meters point-to-point, 12 meters with multiple devices) while consuming less power than HVD technology.
LUN - Logical Unit Number - Each SCSI device may contain up to eight sub-devices or Logical Units, assigned Logical Unit Numbers 0-7. Typically, SCSI devices such as a disk or tape drives contain only LUN 0.

-M-

Menu - An item at the top of a GUI display which, when selected by clicking with a mouse, will cause a list of related functions to be displayed.
Mirroring - A popular term for RAID-1. A method of creating disk-fault tolerance by redundantly storing information on pairs of drives.
MTBF - Mean Time Between Failure. The average time between expected failures of a device.

-N-

Non-Redundant Array - An Array Group with no fault tolerance (RAID-0). If one drive in a Non-Redundant Array fails, the entire Array Group will fail.

-O-

Optimal - The mode of operation of a disk array in which no drive failures have occurred.
OS - Operating System - Software which manages the computer's resources and provides the operating environment for application programs.

-P-

Packetization - Later generations of SCSI are more adversely impacted by an increase in overhead since traffic is negotiated at synchronous and asynchronous speeds. Packetization creates information units (IUs) comprised of commands, data, status information and other things. These IUs are passed as synchronous transfers, reducing overhead and improving overall efficiency.
Parity - A method of generating redundant information which can be used to detect errors in stored or transmitted data. Parity is used in standard RAM SIMMs and over the SCSI bus to detect data errors. Parity is used in RAID-5 disk arrays to reconstruct flawed or missing data sectors.
Peripheral - Any part of a computer other than the CPU or working memory i.e. disks, keyboards, monitors, mice, printers, scanners, tape drives, microphones, speakers, cameras to list just a few.
PCI - Peripheral Component Interconnect - An intelligent computer bus specification which supports 32-bit data paths, 132 Mbyte/sec data transfers from Bus Mastering devices and plug-and-play automatic configuration of peripheral cards.
PIO - Programmed Input/Output - The method of data transfer used whereby data is moved between the peripheral controller and system memory via the CPU. PIO transfers typically occur at about 2.5MB/s.
Plug-and-Play - The ability to install peripheral cards or devices without requiring manual configuration by the user.
Pre-Compensation - Although SCSI transfer speeds have changed dramatically over the past several generations, cable specifications have remained constant. Higher speed (higher frequency, too) signals have a greater potential for reflection and distortion over distance. Pre-compensation techniques slightly modify the SCSI signal to reduce the chance of these types of problems.

-Q-

Quick Arbitration Select (QAS) - Arbitration, the process of devices negotiating for control of the bus, is a critical part of the SCSI specification. This process has built-in “quiet times” so that both fast and legacy devices have an opportunity to take control of the bus. Although fair, this process is somewhat inefficient. QAS speeds up the arbitration process by eliminating the bus free phase. Combined with packetization, this significantly improves bus efficiency.

-R-

RAID - Redundant Array of Independent Disks - A method of combining hard disks into one logical storage unit which offers disk-fault tolerance and can operate at higher throughput levels than a single hard disk.
Read and Write Data Streaming - Minimizes data transfer overhead by allowing a target to send one data stream (LQ) packet followed by multiple data packets.
Rebuild - The operation of recreating data belonging to a failed member of a redundant disk array and writing that data onto a replacement disk drive.
Redundant Array - A fault tolerant Array Group. (RAID-1 or RAID-5).

-S-

Scatter/Gather - A feature which allows data to be transferred to or from multiple discontiguous areas of host computer memory with a single I/O command.
SCSI - Small Computer Systems Interface - an ANSI standard parallel interface designed to communicate with intelligent peripheral devices. The SCSI definition, created by ANSI is defined in document number X3.131-1986. Copies of this document can be obtained from the ANSI X3 Secretariat.
SIMM - Single In-line Memory Module - A standard way of packaging RAM on a small circuit board with a defined edge connector. SmartRAID adapters can use any combination of 1MB, 4MB or 16MB 36-bit single-sided low-profile RAM SIMMs. DPT also offers 4MB and 16MB ECC SIMMs. When used with SmartRAID Storage Cabinets, DPT ECC SIMMs are required for ECC protection over the SCSI bus.
Single-Ended - An electrical signal protocol which transmits information through changes in voltage. Single-Ended SCSI uses standard TTL signal-and-ground pairs to transmit information over the SCSI bus.
Striping - Also called RAID-0. A method of distributing data evenly across all drives in an array by concatenating interleaved stripes from each drive.
Synchronous - Data transmission protocol which is synchronized to a defined time interval. Synchronous SCSI can transmit data faster than asynchronous SCSI because the transmitting device does not wait for acknowledgement of each byte from the receiving device. Instead, it continues to transmit data at the rate negotiated by both devices (up to 20MHz).

-T-

Target - A SCSI device that executes a command from a SCSI initiator to perform some task. Typically the target is a SCSI peripheral device but the host adapter can also be a target.
Tagged Command Queuing - A feature of SCSI-2 and SCSI-3 protocols that allows SCSI commands to be executed out of order.
Termination - A method of matching the transmission impedance of a electrical bus so as to eliminate signal reflections from the physical ends of the bus.
TERMPWR - A signal on the SCSI bus which provides power for SCSI bus terminators on remote devices.
Throughput - A term used to describe the amount of data which can be processed by a system in a given amount of time.
Training Pattern - SCSI is a parallel bus technology. Therefore, it is dependent on signals being transmitted on parallel wires simultaneously. At higher speeds, minute differences in wire lengths and transmission characteristics could cause problems. Training pattern testing measures these minute differences and compensates for them.

-U-

Ultra SCSI - Uses an 8-bit bus, and supports data rates of 20 MB/sec.
An extension of SCSI 2 proposed by a group of manufacturers which doubles the transfer speed of Fast-SCSI to give 20MB/sec. on an 8-bit connection and 40MB/sec. on a 16-bit connection
Ultra2 SCSI - Uses an 8-bit bus and supports data rates of 40 MB/sec.using Fast-40 signaling. below for more on this type of SCSI.
Ultra3 SCSI - Uses a (16-bit) only The units can support a data rates of up to 160 MB/sec. using Fast-80 signaling with double transition clocking.
Ultra320 SCSI - The next generation of SCSI/IO technology. Supports data transfer speeds of up to 320MB/sec.

-W-

Write-Back - A method postponing data to be written to a slow device such as a disk drive by temporarily saving the data in a cache. The data can then be written at a time when the device would otherwise be idle. In addition, the data can be processed through techniques such as elevator sorting, so that the write operations will go faster.
Write-Through - The opposite of write-back. In write-through mode, data must be written to the final destination before a write operation is completed.
Wide SCSI - A SCSI protocol and signal definition which provides a greater than 8-bit wide data path. Wide SCSI devices may support either 8 and 16-bit, or 8, 16 and 32-bit data transfers.