Disk Storage

Disk Storage Category

Quick reference for disk terminology

Winchester Disk

  • The term “Winchester” comes from an early type of disk drive developed by IBM that stored 30MB and had a 30-millisecond access time; so its inventors called it a Winchester in honour of the .30-calibre rifle of the same name. Although modern disk drives are faster and hold more data, the basic technology is the same, so “Winchester” has become synonymous with “hard”.

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.

VFAT

  • Virtual File Allocation Table: the 32-bit file system that Windows 95 uses to manage information stored on disks. An extension of the FAT file system, VFAT supports long filenames and 32-bit Protected Mode access while retaining compatibility with FAT volumes.

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.

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.

Track

  • Sub-division of the recording area of storage media, such as magnetic disks, optical discs and magnetic tape.

TPI

  • Tracks Per Inch: the number of tracks written within each inch of a storage medium’s recording surface. In the context of hard disk drives, EIDE drives generally have a higher TPI than SCSI drives. Also referred to as Track Density.

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.

Thermal Recalibration

  • The periodic sensing of the temperature in hard disk drives so as to make minor adjustments to the alignment servo and data platters. In an AV drive, this process is performed only in idle periods so that there is no interruption in reading and writing long streams of digital video data.

TFI

  • Thin film inductive heads use a minute coil deposited onto a thin film using the photo-etching techniques employed to create integrated circuits; as the magnetic flux of the platter cuts the coil it induces a detectable current.

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.

Sustained Transfer Rate

  • The amount of data a drive can continuously read or write per second.

ST506

  • Introduced in 1979, Seagate’s ST506 was the first hard disk drive for personal computers. Supporting 5.25in full-height drives with a capacity of between 5MB and 40MB, the ST506 interface became an industry standard for the IBM PC and its successors, eventually being superseded by the IDE interface.

SSA

  • Serial Storage Architecture: a peripheral interface from IBM whose ring configuration allows remaining devices to function if one fails. SCSI software can be mapped over SSA allowing existing SCSI devices to be used.

Spindle

  • The drive’s centre shaft, on which the hard disk platters are mounted.

Spindle Speed

  • Velocity at which the disk media spins within a hard disk, measured in rpm (revolutions per minute). By the late 1990s EIDE hard disks generally features a 5,400rpm or 7,200 mechanism, while SCSI drives were usually either 7,200rpm or 10,000rpm.

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.

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.

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.

Servo Data

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

Servo Motor

  • A closed-loop control system used to adjust head position and/or tape speed.

Servo Platter

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

Seek Time

  • The time taken for the actuator to move the heads to the correct cylinder in order to access data.

Sector

  • Describes the minimum segment of track length that can be assigned to store data. Magnetic disks are typically divided into tracks, each which contains a number of sectors. A sector contains a predetermined amount of data, such as 512 bytes. CDs can contain [(75 sectors per second) x (60 seconds per minute) x (number of minutes on disc)] sectors, the capacity of a sector depending on what physical format and mode is used for recording.

SCA

  • Single Connector Attachment: Same speed SCSI interface as LVD, but integrates power and I/O information into a single 80-pin connector. Used in high-end servers to allow hard disks to be hot-swapped in a RAID array.

RLL

  • 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. The “run length” is the maximum number of consecutive 0s before a 1 bit is recorded.

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 advantage 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 unauthorised use.

Read/Write Head

  • A device which uses induction to “write” a data pattern onto magnetic media; and which uses either inductance or magnetoresistance to “read” the data back. Heads come in many different shapes and forms, and are used for both contact and non-contact type recording.

Read Verify

  • A disk mode where the disk reads in data to the controller, but the controller only checks for errors and does not pass the data on to the system.

Read Channel

  • A drive’s read channel performs the vital job of converting the head’s analogue signal into accurate digital data.

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.

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.

RAID

  • Redundant Array of Inexpensive Disks: when configured for performance a RAID writes and reads data in parallel from multiple drive simultaneously. In theory data can be moved at the speed of one drive multiplied by the number of drives working in parallel, although in practice management overheads reduce this significantly.

PRML

  • Partial Response Maximum Likelihood: a technique used to differentiate a valid signal from noise which achieves improved accuracy by looking at entire waveforms rather than just peaks in isolation, using digital signal processing (DSP) to reconstruct recorded data. On magnetic disks PRML uses RLL encoding to provide a ratio of user data to recorded data of 8:9.

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.

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.

Physical Format

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

Performance

  • A measure of the speed of the drive during normal operation. Factors affecting performance are seek times, transfer rate, and command overhead.
  • Partition
  • A portion of a hard disk accessible as a single logical volume, perhaps dedicated to a particular operating system or application.

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.

MR

  • Magneto-resistive heads detect the magnetic flux of a platter by using a sliver of a special material whose resistance changes according to the strength of an applied magnetic field; MR heads are more sensitive than TFI heads, allowing higher areal densities.

Microdrive

  • An ultra-miniature hard disk technology from IBM that uses a single one-inch diameter platter to provide either 170MB or 340MB storage capacity and either one or two GMR heads, the Microdrive is built into a Type II CompactFlash form factor.

MFM

  • Modified Frequency Modulation: the data storage system used by floppy disk drives and older early hard disk drives. Had twice the capacity of the earlier FM method but was slower than the competing RLL scheme.

LVD

  • Low Voltage Differential: the lastest type of SCSI used by hard disk drives in entry-level servers and workstations. Connects via a coloured ribbon cable and a 68-pin socket. Also known as Ultra Wide 2.

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. Also known as initialisation.

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.

LBA

  • Logical Block Addressing: the scheme by which the BIOS passes an operating system request for a given sector to a modern hard drive.

Landing Zone

  • The non-data area set-aside on a hard drive platter for the heads to rest when the system powers down.

Kerr Effect

  • A change in rotation of light reflected off a magnetic field. The polarity of a magneto-optic bit causes the laser to shift one degree clockwise or counterclockwise.

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).

Interleave Factor

  • Refers to a technique used by older hard disk drives to arrange sectors in a non-contiguous way so as to reduce rotational latency and thereby increase read/write performance. The interleave factor specifies the physical spacing between consecutive logical sectors.

HVD

  • High Voltage Differential: the logic signalling system originally defined in the SCSI-2 standard. HVD has a maximum logic voltage of 5V and uses a paired plus and minus signal level to reduce the effects of noise on the SCSI bus. It was functionally replaced by LVD (Low Voltage Differential) in the SCSI-3 variant of the standard. HVD and LVD SCSI are not directly compatible but can be interconnected by the use of a special adapter.

Home

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

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.

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.

HDA

  • Head Disk Assembly: The mechanical components of a disk drive (minus the electronics), which includes the actuators, access arms, read/write heads and platters. Typically housed in a sealed unit.

Hard Error

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

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 aluminium or glass platter. Magnetic read/write heads are mounted on an actuator that resembles a record needle pickup arm.

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.

GMR

  • Giant Magnetoresistive technology uses various thin film layers to produce a greater change in resistance and is even more sensistive than standard magnetoresistive technology. See also Magnetoresistive.

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.

Fly 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.

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.

Floppy Drive

  • Practically all PCs come with a floppy disk drive. 3.5in high density 1.44MB floppy disks are now the standard. They come in hard plastic cases and have replaced the older, literally floppy, 5.25in disks.

Ferrite

  • A ferromagnetic compound of ferric oxide used in the construction of magnetic recording heads and media.

FCI

  • Flux Changes per Inch. See also BPI.

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.

Fast Multiword DMA

  • An alternative protocol to PIO modes for a controller to send and receive data to and from a drive.

FAT

  • File Allocation Table: the file system used by DOS and Windows to manage files stored on hard disks, floppy disks, and other disk media. The file system takes its name from an on-disk data structure known as the file allocation table, which records where individual portions of each file are located on the disk. Earlier versions of Windows used the 16-bit version known as FAT16. Windows 98 has the option of using FAT32, which supports larger partition sizes and smaller cluster sizes, thereby improving disk performance and increasing available disk space. See also VFAT.

Embedded Servo

  • The method most disks use to help the head locate tracks accurately; servo fields are interspersed with the real data, acting like runway lights for the head to line up on.

EDAP

  • Extended Data Availability and Protection: Created by the RAID Advisory Board in 1997, EDAP introduces a classification system for the resilience of the entire storage system and that is not confined to disk-based storage alone. Availability of an EDAP-certified system is sustainable even in the event of failure, the degree of resiliency provided being reflected in the level of EDAP capability attributed to the system.

Drive Geometry

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

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.

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.

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.

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.

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 expansion card that interprets the commands between the processor and the disk drive.

Cluster

  • A group of sectors on a hard disk drive that is addressed as one logical unit by the operating system.

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.

Cache Buffer

  • An intermediate storage capacity between the processor and the disk drive used to store data likely to be requested next. Also known as Data Buffer. See also Look Ahead.

Burst Transfer Rate

  • The maximum amount of data per second a drive can supply intermittently; this is limited by the disk interface and is typically 16.6 MBps (using PIO Mode 4).

Boot Sector

  • Reserved sectors on disk that are used to load the operating system. On start-up, the computer looks for the master book record (MBR), which is typically the first sector in the first partition of the disk. The MBR contains pointers to the first sector of the partition that contains the operating system, and that sector contains the instructions that cause the computer to “boot” the operating system (from the phrase “pulling yourself up from your bootstraps”).

Bernoulli Drive

  • Named after a Swiss scientist who discovered the principle of aerodynamic lift, principal characteristic of a Bernoulli drive is that the flexible disk floats between the read/write heads, so there is no actual contact between the disk and the heads and, being flexible, it is less susceptible than a hard disk to head crashes.

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.

Bad Block

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

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.

AV Drive

  • Audio Video drive: a hard disk drive that is optimised for audio and video applications. Transferring analogue high-fidelity audio and video signals onto a digital disk and playing them back at high performance levels requires a drive that can sustain continuous reads and writes without interruption. AV drives are designed to avoid thermal recalibration during reading and writing so that lengthy transfers digital video data will not be interrupted, and frames will not be lost.

Areal Density

  • The amount of data that’s stored on a hard disk per square inch, and equal to the tracks per inch multiplied by the bits per inch along each track. In the context of tape storage, the number of flux transitions per square unit of recordable area, or bits per square inch.

Actuator

  • The internal mechanism that moves the read/write head to the proper track. Typically 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.

Access Time

  • Time interval between the instant that a piece of information is requested from a memory or peripheral device and the instant the information is supplied by the device. Access time includes the actual seek time, rotational latency, and command processing overhead time.

Zones

  • Because outer tracks are longer than inner tracks they can store more data; consequently disks are divided into zones, each zone having a certain number of sectors per track.

SATA

  • Serial Advanced Technology Attachment: a new standard for connecting hard drives into computer systems. An evolution of the Parallel ATA physical storage interface, SATA is based on serial signalling technology, a single cable with a minimum of four wires creating a point-to-point connection between devices. The first implementation of SATA supported a transfer rate of 150 MBps.

T&L

  • Transform and Lighting: two separate engines on the GPU that provide for a powerful, balanced PC platform and enable extremely high polygon count scenes. Transform performance determines how complex objects can be and how many can appear in a scene without sacrificing frame rate. Lighting techniques add to a scene’s realism by changing the appearance of objects based on light sources.

AHCI

  • Advanced Host Controller Interface: developed by an industry group chaired by Intel, AHCI provides a standard interface to system driver/OS software for discovering and implementing such advanced SATA features as native command queuing (NCQ) and hot plug and power management.

NCQ

  • Native command queuing: a technology designed to increase performance of SATA hard disks by allowing the disk firmware to internally optimise the order in which read and write commands are executed. For NCQ to be enabled, it must be supported and turned on in the SATA controller driver and in the hard drive itself.

Shader

  • A shader is an algorithm which mathematically describes how an individual material is rendered to an object and how light interacts with its overall appearance.

GPU

  • Graphics Processing Unit: a single-chip processor that creates lighting effects and transforms objects every time a 3D scene is
  • redrawn. Off-loading these mathematically-intensive tasks from the main processor greatly increases overall system performance.

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