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Disk storage (also sometimes called drive storage) is a general category of storage mechanisms where data is recorded by various electronic, magnetic, optical, or mechanical changes to a surface layer of one or more rotating disks. A disk drive is a device implementing such a storage mechanism. Notable types are the
hard disk drive A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device File:Reel-to-reel recorder tc-630.jpg, On a reel-to-reel tape recorder (Sony TC-630), the recorder is data storage equipment an ...

hard disk drive
(HDD) containing a non-removable disk, the
floppy disk drive
 floppy disk drive
(FDD) and its removable
floppy disk A floppy disk or floppy diskette (sometimes casually referred to as a floppy or diskette) is a type of disk storage Disk storage (also sometimes called drive storage) is a general category of storage mechanisms where data is recorded by various ...

floppy disk
, and various
optical disc drive In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithm of an algorithm (Euclid's algorithm) for calculating the greatest ...
s (ODD) and associated
optical disc In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithm of an algorithm (Euclid's algorithm) for calculating the greatest ...
media. (The spelling disk and disc are used interchangeably except where trademarks preclude one usage, e.g. the Compact Disc logo. The choice of a particular form is frequently historical, as in IBM's usage of the ''disk'' form beginning in 1956 with the " IBM 350 disk storage unit").


Background

Audio Audio most commonly refers to sound In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, its Motion ( ...

Audio
information was originally recorded by analog methods (see
Sound recording and reproduction Sound recording and reproduction is an electrical Electricity is the set of physical phenomena associated with the presence and motion Image:Leaving Yongsan Station.jpg, 300px, Motion involves a change in position In physics, motion ...
). Similarly the
first video disc
first video disc
used an analog recording method. In the music industry, analog recording has been mostly replaced by digital
optical Optics is the branch of physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities ...
technology where the data is recorded in a digital format with optical information. The first commercial digital disk storage device was the
IBM 350 IBM International Business Machines Corporation (IBM) is an American multinational technology company headquartered in Armonk, New York, with operations in over 170 countries. The company began in 1911, founded in Endicott, New York, as th ...
which shipped in 1956 as a part of the
IBM 305 RAMAC The IBM 305 RAMAC was the first commercial computer that used a moving-head hard disk drive A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device File:Reel-to-reel recorder tc-630. ...
computing system. The
random-access Random access (more precisely and more generally called direct access) is the ability to access an arbitrary element of a sequence in equal time or any datum from a population of addressable elements roughly as easily and efficiently as any other ...
, low-density storage of disks was developed to complement the already used sequential-access, high-density storage provided by
tape drive A tape drive is a that on a . is typically used for offline, archival data storage. Tape media generally has a favorable unit cost and a long archival stability. A tape drive provides storage, unlike a , which provides storage. A disk drive ...
s using
magnetic tape Magnetic tape is a medium for , made of a thin, magnetizable coating on a long, narrow strip of . It was developed in Germany in 1928, based on . Devices that record and playback audio and video using magnetic tape are s and s respectively. A ...

magnetic tape
. Vigorous innovation in disk storage technology, coupled with less vigorous innovation in tape storage, has reduced the difference in acquisition cost per
terabyte The byte is a unit of digital information that most commonly consists of eight bit The bit is a basic unit of information in computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It ...
between disk storage and tape storage; however, the total cost of ownership of data on disk including power and management remains larger than that of tape. Disk storage is now used in both computer storage and consumer electronic storage, e.g.,
audio CD Compact Disc Digital Audio (CDDA or CD-DA), also known as Digital Audio Compact Disc or simply as Audio CD, is the standardization, standard format for audio compact discs. The standard is defined in the ''Red Book'', one of a series of Rainbow ...

audio CD
s and video discs (
VCD Video CD (abbreviated as VCD, and also known as Compact Disc Digital Video) is a home video Home video is prerecorded media sold or rented for home viewing. The term originates from the VHS VHS (Video Home System) is a standard Sta ...

VCD
,
standard DVD
standard DVD
and
Blu-ray The Blu-ray Disc (BD), often known simply as Blu-ray, is a digital Digital usually refers to something using digits, particularly binary digits. Technology and computing Hardware *Digital electronics Digital electronics is a field of elect ...

Blu-ray
). Data on modern disks is stored in fixed length blocks, usually called sectors and varying in length from a few hundred to many thousands of bytes. Gross disk drive capacity is simply the number of disk surfaces times the number of blocks/surface times the number of bytes/block. In certain legacy IBM CKD drives the data was stored on magnetic disks with variable length blocks, called records; record length could vary on and between disks. Capacity decreased as record length decreased due to the necessary gaps between blocks.


Access methods

Digital disk drives are block storage devices. Each disk is divided into logical blocks (collection of sectors). Blocks are addressed using their logical block addresses (LBA). Read from or writing to disk happens at the granularity of blocks. Originally the disk capacity was quite low and has been improved in one of several ways. Improvements in mechanical design and manufacture allowed smaller and more precise heads, meaning that more tracks could be stored on each of the disks. Advancements in data compression methods permitted more information to be stored in each of the individual sectors. The drive stores data onto cylinders, heads, and sectors. The sectors unit is the smallest size of data to be stored in a hard disk drive and each file will have many sectors units assigned to it. The smallest entity in a CD is called a frame, which consists of 33 bytes and contains six complete 16-bit stereo samples (two bytes × two channels × six samples = 24 bytes). The other nine bytes consist of eight CIRC error-correction bytes and one
subcode Subcode or subchannel data (called "control bytes" in the CD-ROM A CD-ROM (, compact disc read-only memory) is a pre-pressed optical compact disc that contains data. Computers can read—but not write to or erase—CD-ROMs, i.e. it is a type of ...
byte used for control and display. The information is sent from the computer processor to the BIOS into a chip controlling the data transfer. This is then sent out to the hard drive via a multi-wire connector. Once the data is received onto the circuit board of the drive, they are translated and compressed into a format that the individual drive can use to store onto the disk itself. The data is then passed to a chip on the circuit board that controls the access to the drive. The drive is divided into sectors of data stored onto one of the sides of one of the internal disks. An HDD with two disks internally will typically store data on all four surfaces. The hardware on the drive tells the actuator arm where it is to go for the relevant track and the compressed information is then sent down to the head which changes the physical properties, optically or magnetically for example, of each byte on the drive, thus storing the information. A file is not stored in a linear manner, rather, it is held in the best way for quickest retrieval.


Rotation speed and track layout

Mechanically there are two different motions occurring inside the drive. One is the rotation of the disks inside the device. The other is the side-to-side motion of the head across the disk as it moves between tracks. There are two types of disk rotation methods: *
constant linear velocity In optical storage, constant linear velocity (CLV) is a qualifier for the rated speed of an optical disc drive In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes ...
(used mainly in optical storage) varies the rotational speed of the optical disc depending upon the position of the head, and * constant angular velocity (used in HDDs, standard FDDs, a few optical disc systems, and vinyl audio records) spins the media at one constant speed regardless of where the head is positioned. Track positioning also follows two different methods across disk storage devices. Storage devices focused on holding computer data, e.g., HDDs, FDDs, Iomega
zip drive The Zip drive is a removable floppy disk storage system that was introduced by Iomega in late 1994. Considered medium-to-high-capacity at the time of its release, Zip disks were originally launched with capacities of 100  MB, then 250& ...

zip drive
s, use concentric tracks to store data. During a sequential read or write operation, after the drive accesses all the sectors in a track it repositions the head(s) to the next track. This will cause a momentary delay in the flow of data between the device and the computer. In contrast, optical audio and video discs use a single
spiral In , a spiral is a which emanates from a point, moving farther away as it revolves around the point. Helices Two major definitions of "spiral" in the are:
spiral
track that starts at the inner most point on the disc and flows continuously to the outer edge. When reading or writing data there is no need to stop the flow of data to switch tracks. This is similar to vinyl records except vinyl records started at the outer edge and spiraled in toward the center.


Interfaces

The disk drive interface is the mechanism/protocol of communication between the rest of the system and the disk drive itself. Storage devices intended for desktop and mobile computers typically use
ATA ATA or Ata may refer to: Computing * AT Attachment (ATA/ATAPI), the old name of Parallel ATA Parallel ATA (PATA), originally , also known as ATA or Integrated Drive Electronics, IDE is standardization, standard Electrical connector, interface ...
( PATA) and
SATA Serial ATA (SATA, abbreviated from Serial AT Attachment) is a interface that connects to s such as s, s, and s. Serial ATA succeeded the earlier (PATA) standard to become the predominant interface for storage devices. Serial ATA industry co ...

SATA
interfaces. Enterprise systems and high-end storage devices will typically use
SCSI Small Computer System Interface (SCSI, ) is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI standards define commands, protocols, electrical, optical and logical interface ...
, SAS, and
FC
FC
interfaces in addition to some use of SATA.


Basic terminology

* Disk - Generally refers to magnetic media and devices. * Disc - Required by trademarks for certain optical media and devices. * Platter – An individual recording disk. A hard disk drive contains a set of platters. Developments in optical technology have led to multiple recording layers on
DVD The DVD (common abbreviation for Digital Video Disc or Digital Versatile Disc) is a digital Digital usually refers to something using digits, particularly binary digits. Technology and computing Hardware *Digital electronics Digital electr ...
s. * Spindle – the spinning
axle An axle or axletree is a central shaft for a rotating A rotation is a circular movement of an object around a center (or point) of rotation. The plane (geometry), geometric plane along which the rotation occurs is called the ''rotation p ...

axle
on which the platters are mounted. * Rotation – Platters rotate; two techniques are common: ** Constant angular velocity (CAV) keeps the disk spinning at a fixed rate, measured in revolutions per minute (RPM). This means the heads cover more distance per unit of time on the outer tracks than on the inner tracks. This method is typical with computer hard drives. **
Constant linear velocity In optical storage, constant linear velocity (CLV) is a qualifier for the rated speed of an optical disc drive In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes ...
(CLV) keeps the distance covered by the heads per unit time fixed. Thus the disk has to slow down as the arm moves to the outer tracks. This method is typical for
CD
CD
drives. * Track – The circle of recorded data on a single recording surface of a platter. * Sector – A segment of a track * Low level formatting – Establishing the tracks and sectors. * Head – The device that reads and writes the information—magnetic or optical—on the disk surface. * Arm – The mechanical assembly that supports the head as it moves in and out. *
Seek timeHigher performance in hard disk drive A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device File:Reel-to-reel recorder tc-630.jpg, On a reel-to-reel tape recorder (Sony TC-630), the ...
– Time needed to move the head to a new position (specific track). *
Rotational latencyHigher performance in hard disk drive A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device File:Reel-to-reel recorder tc-630.jpg, On a reel-to-reel tape recorder (Sony TC-630), the ...
– Average time, once the arm is on the right track, before a head is over a desired sector. * Data transfer rate - The rate at which user data bits are transferred from or to the medium. Technically, this would more accurately be entitled the "gross" data transfer rate.


See also

*
Disk array A disk array is a disk storage Disk storage (also sometimes called drive storage) is a general category of storage mechanisms where data is recorded by various electronic, magnetic, optical, or mechanical changes to a surface layer of one or more r ...
* Disk drive performance characteristics *
Disk read-and-write head Disk read/write heads are the small parts of a disk drive Disk storage (also sometimes called drive storage) is a general category of storage mechanisms where data is recorded by various electronic, magnetic, optical, or mechanical changes to ...
*
Magnetic storage Magnetic storage or magnetic recording is the storage of data Data (; ) are individual facts A fact is something that is truth, true. The usual test for a statement of fact is verifiability—that is whether it can be demonstrated t ...
*
RAID Raid or RAID may refer to: Attack * Raid (military), a sudden attack behind the enemy's lines without the intention of holding ground * Corporate raid, a type of hostile takeover in business * Panty raid, a prankish raid by male college students ...

RAID
*
USB flash drive A USB flash drive (i.e. thumb drive) is a that includes with an integrated interface. It is typically removable, rewritable and much smaller than an . Most weigh less than . Since first appearing on the market in late 2000, as with virtuall ...

USB flash drive


References

{{Authority control Computer storage devices Rotating disc computer storage media