Hard disk
Strictly speaking, "drive" refers to an entire unit containing hard
disk, 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.
Hard disks record information 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 1Tb
of data, rotate at 7,200 to 10,000 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
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
(750 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.
This 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 1073741824, 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).
History
IBM 62PC "Piccolo" HDD, circa 1979 -
an early 8" diskFor many years, hard disks were large, cumbersome
devices, more suited to use in the protected environment of a data
center or large office than in a harsh industrial environment (due to
their delicacy), or small office or home (due to their size and power
consumption). Before the early 1980s, most hard disks had 8-inch (20 cm)
or 14-inch (35 cm) platters, required an equipment rack or a large
amount of floor space (especially the large removable-media disks, which
were often referred to as "washing machines"), and in many cases needed
high-current or even three-phase power hookups due to the large motors
they used. Because of this, hard disks were not commonly used with
microcomputers until after 1980, when Seagate Technology introduced the
ST-506, the first 5.25-inch hard disk, with a capacity of 5 megabytes.
In fact, in its factory configuration, the original IBM PC (IBM 5150)
was not equipped with a hard disk.
Most microcomputer hard disks in the early 1980s were not sold under
their manufacturer's names, but by OEMs as part of larger peripherals
(such as the Corvus Disk System and the Apple ProFile). The IBM PC/XT
had an internal hard disk, however, and this started a trend toward
buying "bare" disks (often by mail order) and installing them directly
into a system. Hard disk makers started marketing to end users as well
as OEMs, and by the mid-1990s, hard disks had become available on retail
store shelves.
While internal disks became the system of choice on PCs, external
hard disks remained popular for much longer on the Apple Macintosh and
other platforms. Every Mac made between 1986 and 1998 has a SCSI port on
the back, making external expansion easy. External SCSI disks were also
popular with older microcomputers such as the Apple II series, and were
also used extensively in servers, a usage which is still popular today.
The appearance in the late 1990s of high-speed external interfaces such
as USB and FireWire has made external disk systems popular among PC
users once again, especially for users who move large amounts of data
between two or more locations, and most hard disk makers now make their
disks available in external cases.
Hard disk characteristics
5.25" MFM 110 MB
hard disk (2.5" IDE 6495 MB hard disk, US & UK pennies for
comparison)Capacity, usually quoted in gigabytes. (older hard disks used
to quote their smaller capacities in megabytes)
Physical size, usually quoted in inches:
Almost
all hard disks today are of either the 3.5" or 2.5" varieties, used in
desktops and laptops, respectively. 2.5" disks are usually slower and
have less capacity but use less power and are more tolerant of movement.
An increasingly common size is the 1.8" disks used in portable MP3
players and subnotebooks, which have very low power consumption and are
highly shock-resistant. Additionally, there is the 1" form factor
designed to fit the dimensions of CF Type II, which is also usually used
as storage for portable devices including digital cameras. 1" was a de
facto form factor led by IBM's Microdrive, but is now generically called
1" due to other manufacturers producing similar products. There is also
a 0.85" form factor produced by Toshiba for use in mobile phones and
similar applications. The size designations can be slightly confusing,
for example a 3.5" disk has a case that is 4" wide. Furthermore,
server-class hard disks also come in both 3.5" and 2.5" form factors.
Reliability, usually given in terms of Mean Time Between Failures (MTBF):
SATA
1.0 disks support speeds up to 10,000 rpm and MTBF levels up to 1
million hours under an eight-hour, low-duty cycle. Fibre Channel (FC)
disks support up to 15,000 rpm and an MTBF of 1.4 million hours under a
24-hour duty cycle.
Number of I/O operations per second:
Modern disks can perform around 50 random access or 100 Sequential access operations per second.
Power consumption (especially important in battery-powered laptops).
audible noise in dBA (although many still report it in bels, not decibels).
G-shock rating (surprisingly high in modern disks).
Transfer Rate:
Inner Zone: from 44.2 MB/s to 74.5 MB/s.
Outer Zone: from 74.0 MB/s to 111.4 MB/s.
Random access time: from 5 ms to 15 ms.
Integrity
Close-up of a hard disk head
suspended above the disk platter together with its mirror image in the
smooth surface of the magnetic platter.The hard disk's spindle system
relies on air pressure inside the enclosure to support the heads at
their proper flying height while the disk is in motion. A hard disk
requires a certain range of air pressures in order to operate properly.
The connection to the external environment and pressure occurs through a
small hole in the enclosure (about 1/2 mm in diameter), usually with a
carbon filter on the inside (the breather filter, see below). If the air
pressure is too low, there will not be enough lift for the flying head,
the head will not be at the proper height, and there is a risk of head
crashes and data loss. Specially manufactured sealed and pressurized
disks are needed for reliable high-altitude operation, above about
10,000 feet (3,000 m). This does not apply to pressurized enclosures,
like an airplane pressurized cabin. Modern disks include temperature
sensors and adjust their operation to the operating environment.
Very high humidity for extended periods can cause accelerated wear of
the heads and platters by corrosion. If the disk uses "Contact
Start/Stop" (CSS) technology to park its heads on the platters when not
operating, increased humidity can also lead to increased stiction (the
tendency for the heads to stick to the platter surface). This can cause
physical damage to the platter and spindle motor and can also lead to
head crash. Breather holes can be seen on all disks — they usually have a
warning sticker next to them, informing the user not to cover the
holes. The air inside the operating disk is constantly moving too, being
swept in motion by friction with the spinning platters. This air passes
through an internal recirculation (or "recirc") filter to remove any
leftover contaminants from manufacture, any particles or chemicals that
may have somehow entered the enclosure, and any particles or outgassing
generated internally in normal operation.
Due to the extremely close spacing between the heads and the disk
surface, any contamination of the read-write heads or platters can lead
to a head crash — a failure of the disk in which the head scrapes across
the platter surface, often grinding away the thin magnetic film. For
giant magnetoresistive (GMR) heads in particular, a minor head crash
from contamination (that does not remove the magnetic surface of the
disk) will still result in the head temporarily overheating, due to
friction with the disk surface, and can render the data unreadable for a
short period until the head temperature stabilizes (so called "thermal
asperity," a problem which can partially be dealt with by proper
electronic filtering of the read signal). Head crashes can be caused by
electronic failure, a sudden power failure, physical shock, wear and
tear, corrosion, or poorly manufactured platters and heads. In most
desktop and server disks, when powering down, the heads are moved to a
landing zone, an area of the platter usually near its inner diameter
(ID), where no data is stored. This area is called the CSS (Contact
Start/Stop) zone. However, especially in old models, sudden power
interruptions or a power supply failure can sometimes result in the
device shutting down with the heads in the data zone, which increases
the risk of data loss. In fact, it used to be procedure to "park" the
hard disk before shutting down your computer. Newer disks are designed
such that either a spring (at first) or (more recently) rotational
inertia in the platters is used to safely park the heads in the case of
unexpected power loss.
The hard disk's electronics control the movement of the actuator and
the rotation of the disk, and perform reads and writes on demand from
the disk controller. Modern disk firmware is capable of scheduling reads
and writes efficiently on the platter surfaces and remapping sectors of
the media which have failed. Also, most major hard disk and motherboard
vendors now support self-monitoring, analysis, and reporting technology
(S.M.A.R.T.), by which impending failures can be predicted, allowing
the user to be alerted to prevent data loss.
Landing zones
Microphotograph of a hard
disk head. The size of the front face (which is the "trailing face" of
the slider) is about 0.3 mm × 1.0 mm. The (not visible) bottom face of
the slider is about 1.0 mm × 1.25 mm (so called "nano" size) and faces
the platter. One functional part of the head is the round, orange
structure in the middle - the lithographically defined copper coil of
the write transducer. Also note the electric connections by wires bonded
to gold-plated pads.Around 1995 IBM pioneered a technology where the
landing zone is made by a precision laser process (Laser Zone Texture =
LZT) producing an array of smooth nanometer-scale "bumps" in the ID
landing zone, thus vastly improving stiction and wear performance. This
technology is still widely in use today (2006). A few years after LZT,
initially for mobile applications (i.e. laptop etc.), and later also for
the other HDD types, IBM introduced "head unloading" technology, where
the heads are lifted off the platters onto plastic "ramps" near the
outer disk edge, thus eliminating the risk of stiction altogether and
greatly improving non-operating shock performance. All HDD manufacturers
use these two technologies to this day. Both have a list of advantages
and drawbacks in terms of loss of storage space, relative difficulty of
mechanical tolerance control, cost of implementation, etc.
IBM created a technology for their Thinkpad line of laptop computers
called the Active Protection System. When a sudden, sharp movement is
detected by the built-in motion sensor in the Thinkpad, internal hard
disk heads automatically unload themselves into the parking zone to
reduce the risk of any potential data loss or scratches made. Apple
later also utilized this technology in their Powerbook, iBook, MacBook
Pro, and MacBook line, known as the Sudden Motion Sensor.
Spring tension from the head mounting constantly pushes the heads
towards the platter. While the disk is spinning, the heads are supported
by an air bearing and experience no physical contact or wear. In CSS
drives the sliders carrying the head sensors (often also just called
heads) are designed to reliably survive a number of landings and
takeoffs from the media surface, though wear and tear on these
microscopic components eventually takes its toll. Most manufacturers
design the sliders to survive 50,000 contact cycles before the chance of
damage on startup rises above 50%. However, the decay rate is not
linear—when a disk is younger and has fewer start-stop cycles, it has a
better chance of surviving the next startup than an older,
higher-mileage disk (as the head literally drags along the disk's
surface until the air bearing is established). For example, the Maxtor
DiamondMax series of desktop hard disks are rated to 50,000 start-stop
cycles. This means that no failures attributed to the head-platter
interface were seen before at least 50,000 start-stop cycles during
testing.
Access and interfaces
Hard disks are generally
accessed over one of a number of bus types, including ATA (IDE, EIDE),
Serial ATA (SATA), SCSI, SAS, IEEE 1394, USB, and Fibre Channel.
Back in the days of the ST-506 interface, the data encoding scheme
was also important. The first ST-506 disks used Modified Frequency
Modulation (MFM) encoding (which is still used on the common "1.44 MB"
(1440 KiB) 3.5-inch floppy), and transferred data at a rate of 5
megabits per second. Later on, controllers using 2,7 RLL (or just "RLL")
encoding increased the transfer rate by half, to 7.5 megabits per
second; it also increased disk capacity by half.
Many ST-506 interface disks were only certified by the manufacturer
to run at the lower MFM data rate, while other models (usually more
expensive versions of the same basic disk) were certified to run at the
higher RLL data rate. In some cases, the disk was overengineered just
enough to allow the MFM-certified model to run at the faster data rate;
however, this was often unreliable and was not recommended. (An
RLL-certified disk could run on a MFM controller, but with 1/3 less data
capacity and speed.)
Enhanced Small Disk Interface (ESDI) also supported multiple data
rates (ESDI disks always used 2,7 RLL, but at 10, 15 or 20 megabits per
second), but this was usually negotiated automatically by the disk and
controller; most of the time, however, 15 or 20 megabit ESDI disks
weren't downward compatible (i.e. a 15 or 20 megabit disk wouldn't run
on a 10 megabit controller). ESDI disks typically also had jumpers to
set the number of sectors per track and (in some cases) sector size.
SCSI originally had just one speed, 5 MHz (for a maximum data rate of
5 megabytes per second), but later this was increased dramatically. The
SCSI bus speed had no bearing on the disk's internal speed because of
buffering between the SCSI bus and the disk's internal data bus;
however, many early disks had very small buffers, and thus had to be
reformatted to a different interleave (just like ST-506 disks) when used
on slow computers, such as early IBM PC compatibles and Apple
Macintoshes.
ATA disks have typically had no problems with interleave or data
rate, due to their controller design, but many early models were
incompatible with each other and couldn't run in a master/slave setup
(two disks on the same cable). This was mostly remedied by the
mid-1990s, when ATA's specification was standardised and the details
began to be cleaned up, but still causes problems occasionally
(especially with CD-ROM and DVD-ROM disks, and when mixing Ultra DMA and
non-UDMA devices).
Serial ATA does away with master/slave setups entirely, placing each
disk on its own channel (with its own set of I/O ports) instead.
FireWire/IEEE 1394 and USB(1.0/2.0) hard disks are external units
containing generally ATA or SCSI disks with ports on the back allowing
very simple and effective expansion and mobility. Most FireWire/IEEE
1394 models are able to daisy-chain in order to continue adding
peripherals without requiring additional ports on the computer itself.
Disk families used in personal computers
Notable disk families include:
MFM (Modified Frequency Modulation) disks required that the controller electronics be compatible with the disk electronics.
RLL
(Run Length Limited) disks were named after the modulation technique
that made them an improvement on MFM. They required large cables between
the controller in the PC and the hard disk, the disk did not have a
controller, only a modulator/demodulator.
ESDI (Enhanced Small Disk
Interface) was an interface developed by Maxtor to allow faster
communication between the PC and the disk than MFM or RLL.
Integrated Drive Electronics (IDE) was later renamed to ATA, and then PATA.
The
name comes from the way early families had the hard disk controller
external to the disk. Moving the hard disk controller from the interface
card to the disk helped to standardize interfaces, reducing cost and
complexity.
The data cable was originally 40 conductor, but UDMA modes from the
later disks requires using an 80 conductor cable (note that the 80
conductor cable still uses a 40 position connector.)
The interface changed from 40 pins to 39 pin. The missing pin acts as
a key to prevent incorrect insertion of the connector, a common cause
of disk and controller damage.
SCSI (Small Computer System Interface) was an early competitor with
ESDI, originally named SASI for Shugart Associates. SCSI disks were
standard on servers, workstations, and Apple Macintosh computers through
the mid-90s, by which time most models had been transitioned to IDE
(and later, SATA) family disks. Only in 2005 did the capacity of SCSI
disks fall behind IDE disk technology, though the highest-performance
disks are still available in SCSI and Fibre Channel only. The length
limitations of the data cable allows for external SCSI devices.
Originally SCSI data cables used single ended data transmission, but
server class SCSI could use differential transmission, and then Fibre
Channel (FC) interface, and then more specifically the Fibre Channel
Arbitrated Loop (FC-AL), connected SCSI hard disks using fibre optics.
FC-AL is the cornerstone of storage area networks, although other
protocols like iSCSI and ATA over Ethernet have been developed as well.
SATA
(Serial ATA). The SATA data cable has only one data pair for the
differential transmission of data to the device, and one pair for
receiving from the device. That requires that data be transmitted
serially. The same differential transmission system is used in RS485,
LocalTalk, USB, Firewire,and differential SCSI. In 2005/2006 parlance,
the 40 pin IDE/ATA is called "PATA" or parallel ATA, which means that
there are 16 bits of data transferred in parallel at a time on the data
cable.
SAS (Serial Attached SCSI). The SAS is a new generation
serial communication protocol for devices designed to allow for much
higher speed data transfers and is compatible with SATA. SAS uses serial
communication instead of the parallel method found in traditional SCSI
devices but still uses SCSI commands for interacting with SAS
EIDE
was an unofficial update (by Western Digital) to the original IDE
standard, with the key improvement being the use of DMA to transfer data
between the disk and the computer, an improvement later adopted by the
official ATA standards. DMA is used to transfer data without the CPU or
program being responsible to transfer every word. That leaves the
CPU/program/operating system to do other tasks while the data transfer
occurs.
Acronym Meaning Description
SASI Shugart Associates System Interface Predecessor to SCSI
SCSI Small Computer System Interface Bus oriented that handles concurrent operations.
ST-412 Seagate interface
ST-506 Seagate interface (improvement over ST-412)
ESDI Enhanced Small Disk Interface Faster and more integrated than ST-412/506, but still backwards compatible
ATA
Advanced Technology Attachment Successor to ST-412/506/ESDI by
integrating the disk controller completely onto the device. Incapable of
concurrent operations.
As of 2005, over 98% of the world's hard disks are manufactured by
just a handful of large firms: Seagate, Maxtor (acquired by Seagate in
May 2006), Western Digital, Samsung, and Hitachi which owns the former
disk manufacturing division of IBM. Fujitsu continues to make mobile-
and server-class disks but exited the desktop-class market in 2001.
Toshiba is a major manufacturer of 2.5-inch and 1.8-inch notebook disks.
Dozens of former hard disk manufacturers have gone out of business,
merged, or closed their hard disk divisions; as capacities and demand
for products increased, profits became hard to find, and there were
shakeouts in the late 1980s and late 1990s. The first notable casualty
of the business in the PC era was Computer Memories Inc. or CMI; after
an incident with faulty 20 MB AT disks in 1985.[2] CMI's reputation
never recovered, and they exited the hard disk business in 1987. Another
notable failure was MiniScribe, who went bankrupt in 1990 after it was
found that they had "cooked the books" and inflated sales numbers for
several years. Many other smaller companies (like Kalok, Microscience,
LaPine, Areal, Priam and PrairieTek) also did not survive the shakeout,
and had disappeared by 1993; Micropolis was able to hold on until 1997,
and JTS, a relative latecomer to the scene, lasted only a few years and
was gone by 1999, after attempting to manufacture hard disks in India
using a second hand factory.[citation needed] Rodime was also an
important manufacturer during the 1980s, but stopped making disks in the
early 1990s amid the shakeout and now concentrates on technology
licensing; they hold a number of patents related to 3.5-inch form factor
hard disks.
