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SCSI Hard Drive Glossary

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Software Glossary

Access Read, write, or update information on some storage medium, such as a disk. (n) The operation of reading, writing, or updating stored information.

Access time The interval between the time a request for data is made by the system and the time the data is available from the drive. Access time includes the actual seek time, rotational latency, and command processing overhead time. See also average seek time, rotational latency, and overhead.

Actuator The internal mechanism that moves the read/write head to the proper track. The Quantum actuator consists of a rotary voice coil and the head mounting arms. One end of each head mounting arm attaches to the rotor with the read/write heads attached at the opposite end of each arm. As current is applied to the rotor, it rotates, positioning the heads over the desired cylinder on the media. Also known as the rotary actuator or positioner.

Allocation The process of assigning particular areas of the disk to particular data or instructions. See also allocation unit.

Allocation unit An allocation unit is a group of sectors on the disk that can be reserved for the use of a particular file. On hard disks for small computer systems, the allocation unit is usually in the form of a sector, block, or cluster.

Average seek time The average time it takes for the read/write head to move to a specific location. To compute the average seek time, divide the time it takes to complete a large number of random seeks by the number of seeks performed.

Backup A copy of a file, directory, or volume on a separate storage device from the original, for the purpose of retrieval in case the original is accidentally erased, damaged, or destroyed.

Bad block A block (usually the size of a sector) that cannot reliably hold data because of a media flaw or damaged format markings.

Bad track table A label affixed to the casing of a hard disk drive that tells which tracks are flawed and cannot hold data. The list is typed into the low-level formatting program when the drive is being installed. Quantum's built-in defect-management protections compensate for these flaws automatically.

Bezel A plastic panel that extends the face of a drive so that it covers a computer's drive bay opening. The bezel usually contains a drive-activity LED. Also known as the faceplate.

Bit Abbreviation for binary digit. A binary digit may have one of two values - 1 or 0. This contrasts with a decimal digit, which may have a value from 0 to 9. A bit is one of the logic 1 or logic 0 binary settings that make up a byte of data. See also byte.

Block A sector or group of sectors. By default, a sector of data consists of 512 bytes.

BPI Acronym for bits per inch. A measure of how densely information is packed on a storage medium. See flux density.

Buffer An area of RAM reserved for temporary storage of data that is waiting to be sent to a device. The data is usually on its way to or from the hard disk drive or some other peripheral device.

Bus The part of a chip, circuit board, or interface designed to send and receive data. Byte The basic unit of computer memory, large enough to hold one character of alphanumeric data. Comprised of eight bits. See also bit.

Cache Specialized RAM used to optimize data transfers between system elements with different performance characteristics, e.g., disk to main memory or main memory to CPU.

Capacity The amount of information that can be stored on a hard disk drive. The data is stored in bytes, and capacity is usually expressed in megabytes.

Clean room An environmentally controlled dust-free assembly or repair facility in which hard disk drives are assembled or can be opened for internal servicing.

Cluster A group of sectors on a hard disk drive that is addressed as one logical unit by the operating system. Controller The chip or circuit that translates computer data and commands into a form suitable for use by the hard drive. Also known as the disk controller.

Controller card An adapter containing the control electronics for one or more hard disks. Usually installed in a slot in the computer.

CPU Acronym for central precessing unit. The microprocessor chip that performs the bulk of data processing in a computer.

CRC Acronym for cyclic redundancy check. An error detection code that is recorded within each sector and is used to see whether parts of a string of data are missing or erroneous.

Cylinder When disks are placed directly above one another along the shaft, the circular, vertical "slice" consisting of all the tracks located in a particular position.

Data separator On a hard disk drive that stores data and timing information in an encoded form, the circuit that extracts the data from the combined data and clock signal.

Dedicated servo A positioning mechanism using a dedicated surface of the disk that contains timing and positioning information only, as compared to surfaces that also are used for data.

Defect management A technique ensuring long-term data integrity. Defect management consists of scanning disk drives both at the factory and during regular use, deallocating defective sectors before purchase and compensating for new defective sectors afterward.

Disk In general, any circular-shaped data-storage medium that stores data on the flat surface of the platter. The most common type of disk is the magnetic disk, which stores data as magnetic patterns in a metal coating. Magnetic disks come in two forms: floppy and hard. Optical recording is a newer disk technology that gives higher capacity storage but at slower access times.

DMA Acronym for direct memory access. A process by which data moves directly between a disk drive (or other device) and system memory without passing through the CPU, thus allowing the system to continue processing other tasks while the new data is being retrieved.

Drive geometry The functional dimensions of a drive in terms of the number of heads, cylinders, and sectors per track.

ECC Acronym for Error Correction Code. The incorporation of extra parity bits in transmitted data in order to detect errors that can be corrected by the controller.

Embedded servo A timing or positioning signal included on the disk's surface that also stores data, allowing the actuator to fine tune the position of the read/write heads.

Encoding The protocol by which particular data patterns are changed prior to being written on the disk surface as a pattern of On and Off or 1 and 0 signals.

EPROM Acronym for erasable programmable read only memory. An integrated circuit memory chip that can store programs and data in a non-volatile state. These de-vices can be erased by ultraviolet light and reprogrammed with new data. External drive - A drive mounted in an enclosure, separate from the computer system enclosure, with its own power supply and fan, and connected to the system by a cable.

Faceplate See bezel.

FCI Acronym for flux changes per inch. See also BPI.

File server A computer that provides network stations with controlled access to shareable resources. The network operating system is loaded on the file server, and most shareable devices (disk subsystems, printers) are attached to it. The file server controls system security and monitors station-to-station communications. A dedicated file server can be used only as a file server while it is on the network. A non-dedicated file server can be used simultaneously as a file server and a workstation.

Firmware Permanent instructions and data programmed directly into the circuitry of read-only memory for controlling the operation of the computer or disk drive. Distinct from software, which is stored in read/write memory and can be altered.

Flux density The number of magnetic field patterns that can be stored on a given area of disk surface, used as a measure of data density. The number is usually stated as flux changes per inch (FCI), with typical values in the tens of thousands.

Flying height The distance between the read/write head and the disk surface, made up of a cushion of air that keeps the head from contacting the media. Smaller flying heights permit denser data storage but require more precise mechanical designs.

Format A magnetic track pattern that specifies the locations of the tracks and sectors. This information must exist on a disk before it can store any user data. Formatting erases any previously stored data.

Formatted capacity The amount of room left to store data on a disk after writing the sector headers, boundary definitions, and timing information during a format operation. The size of a Quantum drive always is expressed in formatted capacity, accurately reflecting the usable space available.

Form factor The industry standard that defines the physical, external dimensions of a particular device. Quantum hard disk drives use 3.5-inch or 2.5-inch form factors.

Gigabyte (GB) A unit of measure consisting of one billion bytes (one thousand megabytes).

Guide Rails Plastic or metal strips attached to the sides of a hard disk drive mounted in an IBM AT and compatible computers so that the drive easily slides into place.

Half height Standard drive size equivalent to half the vertical space of a 5.25-inch drive.

Hard disk A type of storage medium that retains data as magnetic patterns on a rigid disk, usually made of a magnetic thin film deposited on an aluminum platter. Because hard disks spin more rapidly than floppy disks, and the head flies closer to the disk, hard disks can transfer data faster and store more in the same volume.

Hard error A data error that persists when the disk is reread, usually caused by defects in the physical surface.

HDA Acronym for head disk assembly, typically a sealed unit.

Head The tiny electromagnetic coil and metal pole used to create and read back the magnetic pat-terns on the disk. Also known as the read/write head.

Head crash Damage to a read/ write head and magnetic media, usually caused by sudden contact of the heads with the disk surface. Head crash also can be caused by dust and other contamination inside the HDA.

High-capacity drive By industry conventions, typically a drive of 500 megabytes or more.

High-level formatting Formatting performed by the operating system's format program (for example, the DOS FORMAT pro-gram). Among other things, the formatting program creates the root directory, file allocation tables, and other basic configurations. See also low-level formatting.

Home Reference position track for recalibration of the actuator, usually the outer track (Track 0).

Host adapter A plug-in board or circuitry on the motherboard that acts as the interface between a computer system bus and the disk drive. Typically used with embedded controller drives such as SCSI and IDE.

Initialize See low-level formatting.

Interface A hardware or software protocol, contained in the electronics of the disk controller and disk drive, that manages the exchange of data between the drive and computer. The most common interfaces for small computer systems are AT (IDE) and SCSI.

Interleave To intersperse the sectors on a track. See also inter-leave factor.

Interleave factor The number of sectors that pass beneath the read/ write heads before the next sector arrives. For example, a 3:1 interleave factor means that the heads read a sector, then let two pass by before reading another, requiring three full revolutions of the disk to access the complete data track. Quantum drives have an interleave of 1:1, allowing the system to access a full track of data in a single revolution. See also interleave.

Internal drive A drive mounted inside one of a computer's drive bays (or a hard disk on a card, which is installed in one of the computer's slots).

Jumper A tiny connector box that slips over two pins that protrude from a circuit board. The jumper can be moved to change electrical connectors. When in place, the jumper connects the pins electrically. Some board manufacturers use dual in-line package (DIP) switches instead of jumpers.

Kilobyte (KB) A unit of measure consisting of 1,024 bytes.

Landing zone A non-data area on the disk's inner cylinder where the heads can rest when the power is off.

Latency The time during which the read/write heads wait for data to rotate into position after the controller starts looking for a particular data track. If a disk rotates at 3,600 rpm, the maximum latency time is 16.4 milliseconds, and the average latency time is 8.2 milliseconds. Also known as rotational latency.

Look ahead The technique of buffering data into cache RAM by reading subsequent blocks in advance to anticipate the next request for data. The look ahead technique speeds up disk access of sequential blocks of data.

Low-level formatting The process of creating sectors on the disk surface so that the operating system can access the required areas for generating the file structure. Quantum drives are shipped with the low-level formatting already completed. Also known as initialization.

Low profile Describes drives built to the 3.5-inch form factor, which are only 1-inch high. The standard form factor drives are 1.625-inches high.

MB See megabyte.

Media The magnetic material deposited or coated on an aluminium substrate that is very flat and in the shape of a disk. The media is over-coated with a lubricant to prevent damage to the heads or media during head take off and landing. The media is where the data is stored inside the disk in the form of magnetic flux or polarity changes.

Megabyte (MB) A unit of measurement equal to 1 million bytes (measurement used by Quantum and most OEMs) or 1,024 kilobytes or 1,048,576 bytes (measurement infrequently used in the disk drive industry).

Megahertz (MHz) A measurement of frequency in millions of cycles per second.

MHz See megahertz.

Microprocessor The integrated circuit chip that performs the bulk of data processing and controls the operation of all of the parts of the system. A disk drive also contains a microprocessor to handle all of the internal functions of the drive and to support the embedded controller.

Microsecond(s) One millionth of a second (.000001 sec.).

Millisecond (ms) One thousandth of a second (.001 sec.). MTBF Acronym for mean time between failure. Reliability rating indicating the expected failure rate of a product in power on hours (POH). Since manufacturers differ in the ways they determine the MTBF, comparisons of products should always take into account the MTBF calculation method.

MTTR Acronym for mean time to repair. The average time it takes to repair a drive that has failed for some reason. This only takes into consideration the changing of the major subassemblies such as the circuit board or sealed housing. Component-level repair is not included in this number as this type of repair cannot be performed in the field.

Overhead Command overhead refers to the processing time required by the controller, host adapter, or drive prior to the execution of a command. Lower command overhead yields higher drive performance. Disk overhead refers to the space required for non- data information such as location and timing. Disk overhead often accounts for about ten percent of drive capacity. Lower disk overhead yields greater disk capacity.

Overwrite To write data on top of existing data thus erasing it.

Partition A portion of a hard disk dedicated to a particular operating system or application and accessed as a single logical volume.

Performance A measure of the speed of the drive during normal operation. Factors affecting performance are seek times, transfer rate, and command overhead.

Peripheral A device added to a system as a complement to the basic CPU, such as a disk drive, tape drive, or printer.

Physical format The actual physical layout of cylinders, tracks, and sectors on a disk drive.

Plated media Disks that are covered with a hard metal alloy instead of an iron-oxide compound. Plated disks can store more data than their oxide-coated counter-parts.

Platter A disk made of metal (or other rigid material) that is mounted inside a fixed disk drive. Most drives use more than one platter mounted on a single spindle (shaft) to provide more data storage surfaces in a smaller area.

POH Acronym for power on hours. The unit of measurement for mean time between failure (MTBF), expressed as the number of hours that the drive is powered on. See MTBF.

RAM Acronym for random access memory. An integrated circuit memory chip that allows information to be stored and retrieved by a microprocessor or controller. The information can be stored or accessed in any order, and all storage locations are equally accessible.

RAM disk A "phantom" drive created by setting aside a section of RAM as if it were a group of regular sectors. Access to a RAM disk is very fast but data is lost when the system is reset or turned off.

Read after write A mode of operation that has the computer read back each sector immediately after it is written on the disk, checking that the data read back is the same as recorded. This slows disk operations, but raises reliability.

Read verify A disk mode where the disk reads in data to the con-troller, but the controller only checks for errors and does not pass the data on to the system.

Removable disk Generally said of disk drives where the disk itself is meant to be removed, and in particular of hard disks using disks mounted in cartridges. Their ad-vantage is that multiple disks can be used to increase the amount of stored material, and that once removed, the disk can be stored away to prevent unauthorized use.

RLL Acronym for run length limited. A method used on some hard disks to encode data into magnetic pulses. RLL requires more processing, but stores almost 50 percent more data per disk than the older MFM (modified frequency modulation) method.

ROM Acronym for read only memory. Integrated circuit chip containing programs and data that can be accessed and read but cannot be modified.

Rotational latency The delay between when the controller starts looking for a specific block of data on a track and when that block rotates around to where it can be read by the read/write head. On average, it is half of the time needed for a full rotation (which depends on the rotational speed, or rpm, of the disk).

SCSI Acronym for small computer system interface. An American National Standards Institute (ANSI) interface between the computer and peripheral controllers. Apple Macintosh systems and many UNIX operating system workstations use the SCSI interface.

Sector On a PC hard drive, the minimum segment of track length that can be assigned to store information. On Macintosh and UNIX drives, sectors usually are grouped into blocks or logical blocks that function as the smallest data unit permitted. Since these blocks often are defined as a single sector, the terms block and sector are sometimes used interchangeably in this context. (Note: The usage of the term block in connection with the physical configuration of the disk is different from its meaning at the system level.)

Seek The movement of a read/ write head to a specific data track.

Servo data Magnetic markings written on the media that guide the read/write heads to the proper position.

Servo surface A separate surface containing only positioning and disk timing information but no data. Used only in a dedicated servo system.

Settle time The interval between the arrival of the read/write head at a specific track, and the lessening of the residual movement to a level sufficient for reliable reading or writing.

Shock rating A rating (expressed in Gs) of how much shock a disk drive can sustain without damage. Operating and non-operating shock levels are usually specified separately.

Soft error A faulty data reading that does not recur if the same data is reread from the disk or corrected by ECC. Usually caused by power fluctuations or noise spikes.

Soft-sectored Disks that mark the beginning of each sector of data within a track by a magnetic pattern.

Spindle The drive's center shaft, on which the hard disk platters are mounted.

Stepper A type of motor that moves in discrete amounts with each electrical pulse. Steppers were originally the most common type of actuator engine, since they can be geared to advance a read/ write head one track per step. However, they are not as fast, reliable, or durable as the voice coil actuators found in Quantum disk drives.

Substrate The material underneath the magnetic coating of a disk. Common substrates include aluminum for hard disks, glass for optical disks, and mylar for floppy disks.

Surface The top or bottom side of the platter that is coated with the magnetic material for recording data. On some drives one surface may be reserved for positioning information.

Thin film A type of coating allowing very thin layers of magnetic material used on hard disks and read/write heads. Hard disks with thin film surfaces can store greater amounts of data.

TPI Acronym for tracks per inch. The number of tracks written within each inch of the disk's surface, used as a measure of how closely the tracks are packed on a disk surface. Also known as track density.

Track One of the many concentric magnetic circular patterns written on a disk surface as a guide for storing and reading data.

Track-to-track seek time The time required for the read/write heads to move to an adjacent track. Transfer rate The rate at which the disk drive sends and receives data from the controller. The sustained transfer rate includes the time required for system processing, head switches, and seeks, and accurately reflects the drive's true performance. The burst mode transfer rate is a much higher figure that refers only to the movement of data directly into RAM.

Unformatted capacity The total number of usable bytes on a disk, including the space that will be required later to record location, boundary definitions, and timing information. See also formatted capacity.

Voice coil A fast and reliable actuator motor that works like a loudspeaker, with the force of a magnetic coil causing a proportionate movement of the head. Voice coil actuators are more durable than their stepper counterparts, since fewer parts are subject to daily stress and wear and also provide higher performance. Voice coil technology is used in all Quantum drives.

Winchester disks Former code name for an early IBM hard disk model. Sometimes still used to refer to hard drives in general. Write once An optical disk technology that allows the drive to store and read back data but prevents the drive from erasing information once it has been written.

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