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encyclopedia
A hard disk drive (HDD, also commonly
shortened to hard drive and formerly known as a fixed disk) is a
digitally encoded non-volatile storage device which stores data
on rapidly rotating platters with magnetic surfaces. Strictly
speaking, "drive" refers to an entire unit containing multiple
platters, a read/write head assembly, driver electronics, and
motor while "hard disk" (sometimes "platter") refers to the
storage medium itself.
Hard disks were originally developed for use with computers. In
the 21st century, applications for hard disks have expanded
beyond computers to include video recorders, audio players,
digital organizers, and digital cameras. In 2005 the first
cellular telephones to include hard disks were introduced by
Samsung and Nokia. The need for large-scale, reliable storage,
independent of a particular device, led to the introduction of
configurations such as RAID, hardware such as network attached
storage (NAS) devices, and systems such as storage area networks
(SANs) for efficient access to large volumes of data.
Technology
The inside of a hard disk drive
with the disk itself removed. To the left is the
read-write arm. In the middle the electromagnets of
the platter's motor can be seen.
Hard disks record data by magnetizing a
magnetic material in a pattern that represents the data. They
read the data back by detecting the magnetization of the
material. A typical hard disk design consists of a spindle which
holds one or more flat circular disks called platters, onto
which the data is recorded. The platters are made from a
non-magnetic material, usually glass or aluminum, and are coated
with a thin layer of magnetic material. Older disks used
iron(III) oxide as the magnetic material, but current disks use
a cobalt-based alloy.
The platters are spun at very high speeds.
Information is written to a platter as it rotates past
mechanisms called read-and-write heads that fly very close over
the magnetic surface. The read-and-write head is used to detect
and modify the magnetization of the material immediately under
it. There is one head for each magnetic platter surface on the
spindle, mounted on a common arm. An actuator 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.
A cross section of the magnetic
surface in action. In this case the binary data
encoded using frequency modulation.
The magnetic surface of each platter is
divided into many small sub-micrometre-sized magnetic regions,
each of which is used to encode a single binary unit of
information. In today's hard disks each of these magnetic
regions is composed of a few hundred magnetic grains. Each
magnetic region forms a magnetic dipole which generates a highly
localised magnetic field nearby. The write head magnetizes a
magnetic region by generating a strong local magnetic field
nearby. Early hard disks used the same inductor that was used to
read the data as an electromagnet to create this field. Later,
metal in Gap (MIG) heads were used, and today thin film heads
are common. With these later technologies, the read and write
head are separate mechanisms, but are on the same actuator arm.
Hard disks have a mostly sealed enclosure that protects the disk
internals from dust, condensation, and other sources of
contamination. The hard disk's read-write heads fly on an air
bearing which is a cushion of air only nanometers above the disk
surface. The disk surface and the disk's internal environment
must therefore be kept immaculate to prevent damage from
fingerprints, hair, dust, smoke particles and such, given the
sub-microscopic gap between the heads and disk.
Using rigid platters and sealing the unit allows much tighter
tolerances than in a floppy disk. Consequently, hard disks can
store much more data than floppy disk and access and transmit it
faster. In 2006, a typical workstation hard disk might store
between 80 GB and 250 GB of data, rotate at 7,200 to 10,800
revolutions per minute (RPM), and have a sequential media
transfer rate of over 50 MB/s. The fastest workstation and
server hard disks spin at 15,000 RPM, and can achieve sequential
media transfer speeds up to and beyond 80 MB/s. Laptop hard
disks, which are physically smaller than their desktop
counterparts, tend to be slower and have less capacity. Most
spin at only 4,200 RPM or 5,400 RPM, whereas the newest top
models spin at 7,200 RPM.
Capacity
PC hard disk capacity (in GB). The
plot is logarithmic, so the fit line corresponds to
exponential growth.
The capacity of hard disks has grown
dramatically over time. The first commercial disk, the IBM RAMAC
introduced in 1956, stored 5 million characters (about 5
megabytes) on fifty 24-inch diameter disks. (See early IBM disk
storage.) With early personal computers in the 1980s, a disk
with a 20 megabyte capacity was considered large. In the latter
half of the 1990s, hard disks with capacities of 1 gigabyte and
greater became available. As of 2006, the "smallest" desktop
hard disk still in production has a capacity of 20 gigabytes,
while the largest-capacity internal disks are a 3/4 terabyte
(768 gigabytes), with external disks at or exceeding one
terabyte by using multiple internal disks. These new internal
disks increased their storage capacities with perpendicular
recording.
The exponential increases in disk space and
data access times for hard disks has enabled the commercial
viability of consumer products that require large storage
capacities, such as the Apple iPod digital music player, the
TiVo personal video recorder, and web-based email programs.[1]
This is also gradually but significantly altering how
programmers think; in many programming tasks there is a
time-space tradeoff, so as space becomes cheaper and cheaper
relative to CPU cycles the appropriate choice about time versus
space changes. For instance in database work it is now common
practice to store precomputed views, transitive closures, and
the like on disk in order to speed up queries; 20 years ago such
profligate use of disk space would have been impractical.
A vice president of Seagate projects a future growth in disk
density of 40% per year.[1] Access times have not kept up with
throughput increases, which themselves haven't kept up with
growth in storage capacity. The main way to increase either is
to increase the number of read-write heads in a hard disk. Since
flying heads are the most expensive component of hard disks,
increasing their number per hard disk wouldn't help the
situation. Currently, the most promising way to reduce access
times and increase throughput are to replace rotating disks with
nonvolatile random access memory (NVRAM) or, possibly,
holographic technology.
Capacity measurements
Hard disk manufacturers typically specify disk capacity using
the SI definition of the prefixes "mega" and "giga." This is
largely for historical reasons. Disks with multi-million byte
capacity have been used since 1956, long before there were
standard binary prefixes. (The IEC only standardized binary
prefixes in 1999.) Many practitioners early on in the computer
and semiconductor industries used the prefix kilo to describe
210 (1024) bits, bytes or words because 1024 is "close enough"
to 1000. Similar usage has been applied to the prefixes "mega,"
"giga," "tera," and even "peta." Often this non-SI conforming
usage is noted by a qualifier such as "1 kB = 1,024 bytes" but
the qualifier is sometimes omitted, particularly in marketing
literature.
Operating systems, such as Microsoft Windows, frequently report
capacity using the binary interpretation of the prefixes, which
results in a discrepancy between the disk manufacturer's stated
capacity and what the system reports. The difference becomes
much more noticeable in the multi-gigabyte range. For example,
Microsoft's Windows 2000 reports disk capacity both in decimal
to 12 or more significant digits and with binary prefixes to 3
significant digits. Thus a disk specified by a disk manufacturer
as a 30 GB disk might have its capacity reported by Windows 2000
both as "30,065,098,568 bytes" and "28.0 GB." The disk
manufacturer used the SI definition of "giga," 109. However
utilities provided by Windows define a gigabyte as 230, or
1,073,741,824, bytes, so the reported capacity of the disk will
be closer to 28.0 GB. For this reason, many utilities that
report capacity have begun to use the aforementioned IEC
standard binary prefixes (e.g. KiB, MiB, GiB) since their
definitions are unambiguous.
Some people mistakenly attribute the discrepancy in reported and
specified capacities to reserved space used for file system and
partition accounting information. However, for large (several
GiB) filesystems, this data rarely occupies more than a few MiB,
and therefore cannot possibly account for the apparent "loss" of
tens of GBs.
The capacity of a hard disk can be calculated by multiplying the
number of cylinders by the number of heads by the number of
sectors by the number of bytes/sector (most commonly 512).
