A hard disk drive (HDD; also hard drive, hard
disk, or disk drive)[2]
is a device for storing and retrieving digital information, primarily
computer data. It consists of one or more rigid (hence "hard") rapidly
rotating discs (often referred to as platters), coated with magnetic material and with magnetic heads arranged to write data to the
surfaces and read it from them.
Hard drives are classified as non-volatile, random
access, digital, magnetic, data storage devices. Introduced by IBM
in 1956, hard disk drives have decreased in cost and physical size over
the years while dramatically increasing in capacity and speed.
Hard
disk drives have been the dominant device for secondary storage of data in general purpose computers since the
early 1960s.[3]
They have maintained this position because advances in their recording
capacity, cost, reliability, and speed have kept pace with the
requirements for secondary storage.2.Technology :
A hard disk drive records data by magnetizing a thin film of ferromagnetic material on a disk. User data
is encoded into a run-length limitedcode[8]
and the encoded data written as a pattern of sequential magnetic
transitions on the disk. The data is represented by the time between
transitions. The self-clocking nature of the run-length limited codes
used enables the clocking of the data during reads. The data is read
from the disk by detecting the transitions and then decoding the written
run-length limited data back to the user data.
A typical HDD design consists of a spindle[9]
that holds flat circular disks, also called platters,
which hold the recorded data. The platters are made from a non-magnetic
material, usually aluminum alloy, glass, or ceramic, and are coated
with a shallow layer of magnetic material typically 10–20 nm in depth, with an outer layer of carbon for
protection.[10][11][12]
For reference, a standard piece of copy paper is 0.07–0.18 millimetre
(70,000–180,000 nm).[13]
The platters in contemporary HDDs are spun at speeds varying from
4,200 rpm in energy-efficient portable devices, to 15,000 rpm for high
performance servers.[15]
The first hard drives spun at 1,200 rpm[16]
and, for many years, 3,600 rpm was the norm.[17]
Information is written to and read from a platter as it rotates past
devices called read-and-write heads
that operate very close (tens of nanometers in new drives) over the
magnetic surface. The read-and-write head is used to detect and modify
the magnetization of the material immediately under it. In modern drives
there is one head for each magnetic platter surface on the spindle,
mounted on a common arm. An actuator arm (or access arm) moves the heads
on an arc (roughly radially) across the platters as they spin, allowing
each head to access almost the entire surface of the platter as it
spins. The arm is moved using a voice
coil actuator or in some older designs a stepper
motor.
The magnetic surface of each platter is conceptually divided into many
small sub-micrometer-sized magnetic regions referred to as magnetic domains.
In older disk designs the regions were oriented horizontally and
parallel to the disk surface, but beginning about 2005, the orientation
was changed to perpendicular to allow for closer
magnetic domain spacing. Due to the polycrystalline
nature of the magnetic material each of these magnetic regions is
composed of a few hundred magnetic grains.
Magnetic grains are typically 10 nm in size and each form a single magnetic domain. Each magnetic region in
total forms a magnetic dipole which generates a magnetic field.
For reliable storage of data, the recording material needs to resist
self-demagnetization, which occurs when the magnetic domains repel each
other. Magnetic domains written too densely together to a weakly
magnetizable material will degrade over time due to physical rotation of
one or more domains to cancel out these forces. The domains rotate
sideways to a halfway position that weakens the readability of the
domain and relieves the magnetic stresses. Older hard disks used iron(III) oxide as the magnetic material, but current disks
use a cobalt-based
alloy.[18]
A write head magnetizes a region by generating a strong local magnetic
field. Early HDDs used an electromagnet
both to magnetize the region and to then read its magnetic field by
using electromagnetic induction. Later
versions of inductive heads included metal in Gap (MIG) heads and thin
film heads. As data density increased, read heads using magnetoresistance
(MR) came into use; the electrical resistance of the head changed
according to the strength of the magnetism from the platter. Later
development made use of spintronics;
in these heads, the magnetoresistive effect was much greater than in
earlier types, and was dubbed "giant" magnetoresistance
(GMR). In today's heads, the read and write elements are separate, but
in close proximity, on the head portion of an actuator arm. The read
element is typically magneto-resistive[19] while the write element is
typically thin-film inductive.
The heads are kept from contacting the platter surface by the air
that is extremely close to the platter; that air moves at or near the
platter speed. The record and playback head are mounted on a block
called a slider, and the surface next to the platter is shaped to keep
it just barely out of contact. This forms a type of air bearing.
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