Editing Disk storage

{{More citations needed|date=March 2013}}
{{short description|General category of storage mechanisms}}

'''Disk storage''' (also sometimes called '''drive storage''') is a [[data storage]] mechanism based on a rotating disk. The recording employs various electronic, magnetic, optical, or mechanical changes to the disk's surface layer. A '''disk drive''' is a device implementing such a storage mechanism. Notable types are [[hard disk drive|hard disk drives]] (HDD), containing one or more non-removable rigid [[hard disk drive platter|platters]]; the [[floppy disk| floppy disk drive]] (FDD) and its removable [[floppy disk]]; and various [[optical disc drive]]s (ODD) and associated [[optical disc]] media.

(The [[Spelling of disc|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 magnetic disk drives#IBM 350|IBM 350 disk storage unit]]".)

[[File:SixHardDriveFormFactors.jpg|thumb|Six hard disk drives]]
[[File:Floppy Disk Drives 8 5 3.jpg|thumb|Three floppy disk drives]]
[[File:ASUS CD-ROM CD-S520-A4 20080821.jpg|thumb|A CD-ROM (optical) disc drive]]

== Background ==
[[Sound|Audio]] information was originally recorded by analog methods (see [[Sound recording and reproduction]]). Similarly the [[Laserdisc|first video disc]] used an analog recording method. In the music industry, analog recording has been mostly replaced by digital [[optical]] technology where the data is recorded in a digital format with optical information.

The first commercial digital disk storage device was the [[IBM 350]] which shipped in 1956 as a part of the [[IBM 305 RAMAC]] computing system. The [[random-access memory|random-access]], low-density storage of disks was developed to complement the already used [[sequential access|sequential-access]], high-density storage provided by [[tape drive]]s using [[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]] 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.<ref>{{cite web 
|url= http://users.sdsc.edu/~mcdonald/content/papers/dt_cost.pdf |archive-url=https://web.archive.org/web/20080725004034/http://users.sdsc.edu/~mcdonald/content/papers/dt_cost.pdf |archive-date=2008-07-25 |url-status=live
|title= Disk and Tape Storage Cost Models 
|last= Richard L. Moore
|date= May 3, 2007
|publisher= San Diego Supercomputer Center, UCSD
|access-date=20 February 2013|display-authors=etal}}</ref>

Disk storage is now used in both computer storage and consumer electronic storage, e.g., [[CD-ROM|audio CD]]s and video discs ([[Video CD|VCD]], [[DVD]] and [[Blu-ray]]).

Data on modern disks is stored in fixed length blocks, usually called [[Disk sector|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 [[Count key data#IBM.27s CKD DASD subsystems|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 ==
<!--
<gallery widths="180px" heights="135px" perrow="2" align="right">
Image:Samsung_HD753LJ_01-Top.jpg|{{center|A typical hard disk drive}}
Image:Samsung_HD753LJ_02-Bottom.jpg|{{center|Translation hardware on circuit board}}
Image:Samsung_HD753LJ_03-Opened.jpg|{{center|Top cover removed}}
Image:Hard_disk_platters_and_head.jpg|{{center|Disks (2) and actuator arms (4)}}
Image:Samsung_HD753LJ_05-Head.jpg|{{center|The head and disk}}
Image:Hard_disk_head.jpg|{{center|Head on the disk platter}}
</gallery>
Too HDD specific, putting better images in intro-->

Digital disk drives are [[Computer data storage|block storage devices]]. Each disk is divided into logical [[Block (data storage)|blocks]] (collection of sectors). Blocks are addressed using their [[Logical block addressing|logical block addresses]] (LBA). Read from or write 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 [[Cylinder-head-sector|cylinders, heads, and sectors]]. The sector unit is the smallest size of data to be stored in a hard disk drive, and each file will have many sector 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 [[Compact disc subcode|subcode]] 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.

{{anchor|CAV-CLV}}

==Rotation speed and track layout==
[[File:Comparison disk storage.svg|thumb|300px|Comparison of several forms of disk storage showing tracks (not-to-scale); green denotes start and red denotes end.<br /><nowiki>*</nowiki> Some CD-R(W) and DVD-R(W)/DVD+R(W) recorders operate in ZCLV, CAA or CAV modes.]]
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]] (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 [[Gramophone record|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, and Iomega [[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]] track that starts at the innermost 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 [[AT Attachment|ATA]] ([[Parallel Advanced Technology Attachment|PATA]]) and [[Serial ATA|SATA]] interfaces. Enterprise systems and high-end storage devices will typically use [[SCSI]], [[Serial Attached SCSI|SAS]], and [[Fibre Channel|FC]] interfaces in addition to some use of SATA.
<!-- Move to HDD article ? : ==Disk crashes==
Since a [[magnetic dipole]] field decreases rapidly with distance from the magnetic material, the space between the head and medium must be controlled with more precision than the thermal expansion of parts of the disk drive. The head therefore ''flies'' or glides on a layer of air by means of hydrodynamical [[lubrication]]. That is, the air pulled along by the disk forces the head away from the disk surface. When the disk stops, the head must either "land" or be pulled away.
Defects, wear or foreign objects such as dust, can distort the disk and head surfaces, so they make contact and damage each other further. When these elements build up over time, there is a chance that a "[[head crash]]" will occur. This results in lost data and an inoperable device.
Head crashes are one of the reasons that important data must be backed up on some other device (often audio or video media).
When removable hard disks were used, disc damage was contagious, since either the head or the disk could propagate the damage.
-->

== Basic terminology ==
<!-- This should be NOT specific to hard drives, there is a separate page for that! Please keep it general (hard disk, floppy, optical, ...). -->
;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 [[Comparison of high definition optical disc formats|DVD]]s.
; [[Spindle (hard disk drive)|Spindle]] : the spinning [[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]] (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 [[Compact disc|CD]] drives.
; Track : The circle of recorded data on a single recording surface of a platter.
<!--  too much HDD specific 
; Cylinder : In a multi-head drive, all the tracks under the heads for a given arm position can be read without seek delay. The operating system treats the contents of those tracks as a single cylinder. 
-->
; [[Disk sector|Sector]] : A segment of a track
; Low level [[disk format|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 time]] : Time needed to move the head to a new position (specific track).
; [[Rotational latency]] : Average time, once the arm is on the right track, before a head is over a desired sector.
; [[Bit rate|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 [[Bit rate#Gross bit rate|"gross" data transfer rate]]. 
<!--- Obsolete, even for floppies
; Interleave : The spacing between sectors. Sequential sectors were spaced on the media to enable the next sector to be in the correct position under the head once the host was ready to read it. At an interleave factor of 3:1, three full rotations would be required to read an entire track. Due to technological advances almost all hard disk drives since [[Compaq]] and [[Western Digital]] defined the [[AT Attachment]] standard have used an interleave factor of 1:1. Floppy disks are still usually interleaved.
-->
<!-- ==Standard Disk Sizes==
The IDEMA (International Disk Drive Equipment and Materials Association)<ref>
[http://www.idema.org/_smartsite/modules/local/data_file/show_file.php?cmd=download&data_file_id=1066 Document LBA01-02] LBA Count for IDE Hard Disk Drives</ref> defines the standard disk sizes for IDE drives as:
* Total size in bytes = LBA ([[Logical Block Addressing]]) count * 512 bytes per block
* LBA count = (97696368) + (1953504 * (Desired Capacity in Gbytes – 50.0))
{|align="center" class="wikitable"
|-bgcolor="#ccccff" -align="right"
| GB
|  LBA
|   bytes
|    kibibytes
|     mebibytes
|      gibibytes
|-align=right
| 50
|  97696368
|   50,020,540,416
|    48,848,184
|     47,703.3
|      46.585
|-align=right
| 60
|  117231408
|   60,022,480,896
|    58,615,704
|     57,241.9
|      55.900
|-align=right
| 80
|  156301488
|   80,026,361,856
|    78,150,744
|     76,319.0
|      74.530
|-align=right
| 100
|  195371568
|   100,030,242,816
|    97,685,784
|     95,396.2
|      93.160
|-align=right
| 120
|  234441648
|   120,034,123,776
|    117,220,824
|     114,473.4
|      111.790
|-align=right
| 160
|  312581808
|   160,041,885,696
|    156,290,904
|     152,627.8
|      149.051
|-align=right
| 180
|  351651888
|   180,045,766,656
|    175,825,944
|     171,705.0
|      167.681
|-align=right
| 200
|  390721968
|   200,049,647,616
|    195,360,984
|     190,782.2
|      186.311
|-align=right
| 250
|  488397168
|   250,059,350,016
|    244,198,584
|     238,475.1
|      232.886
|-align=right
| 300
|  586072368
|   300,069,052,416
|    293,036,184
|     286,168.1
|      279.461
|-align=right
| 320
|  625142448
|   320,072,933,376
|    312,571,224
|     305,245.4
|      298.091
|-align=right
| 400
|  781422768
|   400,088,457,216
|    390,711,384
|     381,554.0
|      372.611
|-align=right
| 500
|  976773168
|   500,107,862,016
|    488,386,584
|     476,940.0
|      465.762
|} -->

== See also ==
* [[Disk array]]
* [[Disk drive performance characteristics]]
* [[Disk read-and-write head]]
* [[Magnetic storage]]
* [[RAID]]
* [[USB flash drive]]

== References ==
{{Reflist|30em}}

{{Authority control}}


[[Category:Computer storage devices]]
[[Category:Rotating disc computer storage media]]