Hard disk drive
Hard disk drive
Video of modern hard disk drive operation (cover removed) |
| Date invented |
24 December 1954[1] |
| Invented by |
IBM team led by Rey Johnson |
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.
[3]
History
Hard disk drives were introduced in 1956 as data storage for an IBM real time transaction processing computer
[4] and were developed for use with general purpose
mainframe and
mini
computers. The first IBM drive, the 350 RAMAC, was approximately the
size of two refrigerators and stored 5 million 6-bit characters (the
equivalent of 3.75 million 8-bit bytes) on a stack of 50 discs.
In 1961 IBM introduced the model 1311 disk drive, which was about the
size of a washing machine and stored two million characters on a
removable disk "pack." Users could buy additional packs and interchange
them as needed, much like reels of magnetic tape. Later models of
removable pack drives, from IBM and others, became the norm in most
computer installations and reached capacities of 300 megabytes by the
early 1980s.
In 1973, IBM introduced a new type of hard drive codenamed
"Winchester." Its primary distinguishing feature was that the disk heads
were not withdrawn completely from the stack of disk platters when the
drive was powered down. Instead, the heads were allowed to "land" on a
special area of the disk surface upon spin-down, "taking off" again when
the disk was later powered on. This greatly reduced the cost of the
head actuator mechanism, but precluded removing just the disks from the
drive as was done with the disk packs of the day. Instead, the first
models of "Winchester technology" drives featured a removable disk
module, which included both the disk pack and the head assembly, leaving
the actuator motor in the drive upon removal. Later "Winchester" drives
abandoned the removable media concept and returned to non-removable
platters.
Like the first removable pack drive, the first "Winchester" drives
used platters 14 inches in diameter. A few years later, designers were
exploring the possibility that physically smaller platters might offer
advantages. Drives with non-removable eight-inch platters appeared, and
then drives that fit in a "five and a quarter inch" form factor (a
mounting width equivalent to that used by a five and a quarter inch
floppy disk drive). The latter were primarily intended for the then-fledgling personal computer market.
As the 1980s began, hard disk drives were a rare and very expensive
additional feature on personal computers (PCs); however by the late
'80s, their cost had been reduced to the point where they were standard
on all but the cheapest PC.
Most hard disk drives in the early 1980s were sold to PC end users as
an add on subsystem, not under the drive manufacturer's name but by
systems integrators such as the Corvus Disk System or the systems
manufacturer such as the Apple ProFile. The IBM PC/XT in 1983 included
an internal standard 10MB hard disk drive, and soon thereafter internal
hard disk drives proliferated on personal computers.
External hard disk drives remained popular for much longer on the
Apple Macintosh. Every Mac made between 1986 and 1998 has a SCSI port on
the back, making external expansion easy; also, "toaster" Compact Macs
did not have easily accessible hard drive bays (or, in the case of the
Mac Plus, any hard drive bay at all), so on those models, external SCSI
disks were the only reasonable option.
Driven by areal density doubling every two to four years since their
invention, hard disk drives have changed in many ways. A few highlights
include:
- Capacity per HDD increasing from 3.75 megabytes[4] to 3 terabytes or more, about a million times larger.
- Physical volume of HDD decreasing from 68 ft3[4] or about 2,000 litre (comparable to a large side-by-side refrigerator), to less than 20 ml[5] (1.2 in3), a 100,000-to-1 decrease.
- Weight decreasing from 2,000 lbs[4] (~900 kg) to 48 grams[5] (~0.1 lb), a 20,000-to-1 decrease.
- Price decreasing from about US$15,000 per megabyte[6] to less than $0.0001 per megabyte ($100/1 terabyte), a greater than 150-million-to-1 decrease.[7]
- Average access time decreasing from over 100 milliseconds to a few milliseconds, a greater than 40-to-1 improvement.
- Market application expanding from mainframe computers of the late 1950s to most mass storage applications including computers and consumer applications such as storage of entertainment content.
Technology
Magnetic recording
Diagram labeling the major components of a computer hard disk drive
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]
Recording of single magnetisations of bits on an hdd-platter (recording made visible using CMOS-MagView).
[14]
Longitudinal recording (standard) & perpendicular recording diagram
The platters in contemporary HDDs are spun at speeds varying from
4200 rpm in energy-efficient portable devices, to 15,000 rpm for high
performance servers.
[15] The first hard drives spun at 1200 rpm
[16] and, for many years, 3600 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 while the write element is typically thin-film inductive.
[19]
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.
In modern drives, the small size of the magnetic regions creates the
danger that their magnetic state might be lost because of thermal
effects. To counter this, the platters are coated with two parallel
magnetic layers, separated by a 3-atom layer of the non-magnetic element
ruthenium, and the two layers are magnetized in opposite orientation, thus reinforcing each other.
[20] Another technology used to overcome thermal effects to allow greater recording densities is
perpendicular recording, first shipped in 2005,
[21] and as of 2007 the technology was used in many HDDs.
[22][23][24]
Components
HDD with disks and motor hub removed exposing copper colored stator
coils surrounding a bearing in the center of the spindle motor. Orange
stripe along the side of the arm is thin printed-circuit cable, spindle
bearing is in the center and the actuator is in the lower left.
A typical hard disk drive has two electric motors; a disk motor that
spins the disks and an actuator (motor) that positions the read/write
head assembly across the spinning disks.
The disk motor has an external rotor attached to the disks; the stator windings are fixed in place.
Opposite the actuator at the end of the head support arm is the
read-write head (near center in photo); thin printed-circuit cables
connect the read-write heads to amplifier electronics mounted at the
pivot of the actuator. A flexible, somewhat U-shaped, ribbon cable, seen
edge-on below and to the left of the actuator arm continues the
connection to the controller board on the opposite side.
The head support arm is very light, but also stiff; in modern drives, acceleration at the head reaches 550
g.
The silver-colored structure at the lower left of the first image is
the top plate of the actuator, a permanent-magnet and moving coil motor
that swings the heads to the desired position (it is shown removed in
the second image). The plate supports a squat
neodymium-iron-boron (NIB) high-flux
magnet. Beneath this plate is the moving coil, often referred to as the
voice coil by analogy to the coil in
loudspeakers,
which is attached to the actuator hub, and beneath that is a second NIB
magnet, mounted on the bottom plate of the motor (some drives only have
one magnet).
A disassembled and labeled 1997 hard drive. All major components were
placed on a mirror, which created the symmetrical reflections.
The voice coil itself is shaped rather like an arrowhead, and made of doubly coated copper
magnet wire.
The inner layer is insulation, and the outer is thermoplastic, which
bonds the coil together after it is wound on a form, making it
self-supporting. The portions of the coil along the two sides of the
arrowhead (which point to the actuator bearing center) interact with the
magnetic field,
developing a tangential force that rotates the actuator. Current
flowing radially outward along one side of the arrowhead and radially
inward on the other produces the
tangential force.
If the magnetic field were uniform, each side would generate opposing
forces that would cancel each other out. Therefore the surface of the
magnet is half N pole, half S pole, with the radial dividing line in the
middle, causing the two sides of the coil to see opposite magnetic
fields and produce forces that add instead of canceling. Currents along
the top and bottom of the coil produce radial forces that do not rotate
the head.
Actuation of moving arm
Head stack with an actuator coil on the left and read/write heads on the right
The hard drive'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.
Feedback of the drive electronics is accomplished by means of special
segments of the disk dedicated to servo feedback. These are either
complete concentric circles (in the case of dedicated servo technology),
or segments interspersed with real data (in the case of embedded servo
technology). The servo feedback optimizes the signal to noise ratio of
the GMR sensors by adjusting the voice-coil of the actuated arm. The
spinning of the disk also uses a servo motor. Modern disk firmware is
capable of scheduling reads and writes efficiently on the platter
surfaces and remapping sectors of the media which have failed.
Error handling
Modern drives make extensive use of
Error Correcting Codes (ECCs), particularly
Reed–Solomon error correction.
These techniques store extra bits, determined by mathematical formulas,
for each block of data; the extra bits allow many errors to be
corrected invisibly. The extra bits themselves take up space on the hard
drive, but allow higher recording densities to be employed without
causing uncorrectable errors, resulting in much larger storage capacity.
[25] In the newest drives of 2009,
low-density parity-check codes (LDPC) were supplanting Reed-Solomon; LDPC codes enable performance close to the
Shannon Limit and thus provide the highest storage density available.
[26]
Typical hard drives attempt to "remap" the data in a physical sector
that is failing to a spare physical sector—hopefully while the errors in
the bad sector are still few enough that the ECC can recover the data
without loss. The
S.M.A.R.T.
system counts the total number of errors in the entire hard drive fixed
by ECC and the total number of remappings, as the occurrence of many
such errors may predict hard drive failure.
Future development
Due to bit-flipping errors and other issues, perpendicular recording
densities may be supplanted by other magnetic recording technologies.
Toshiba is promoting
bit-patterned recording (BPR),
[27] while Xyratex is developing
heat-assisted magnetic recording (HAMR).
[28]
October 2011:
TDK
has developed a special laser that heats up a hard's disk's surface
with a precision of a few dozen nanometers. TDK also used the new
material in the magnetic head and redesigned its structure to expand the
recording density. This new technology apparently makes it possible to
store one
terabyte on one platter and for the initial hard drive TDK plans to include two platters.
[29]
Capacity
The capacity of an HDD may appear to the end user to be a different
amount than the amount stated by a drive or system manufacturer due to
amongst other things, different units of measuring capacity, capacity
consumed in formatting the drive for use by an operating system and/or
redundancy.
Units of storage capacity
Advertised capacity
by manufacturer
(using decimal multiples) |
Expected capacity
by consumers in class action
(using binary multiples) |
Reported capacity |
Windows
(using binary
multiples) |
Mac OS X 10.6+
(using decimal
multiples) |
| With prefix |
Bytes |
Bytes |
Diff. |
| 100 MB |
100,000,000 |
104,857,600 |
4.86% |
95.4 MB |
100.0 MB |
| 100 GB |
100,000,000,000 |
107,374,182,400 |
7.37% |
93.1 GB, 95,367 MB |
100.00 GB |
| 1 TB |
1,000,000,000,000 |
1,099,511,627,776 |
9.95% |
931 GB, 953,674 MB |
1000.00 GB, 1000,000 MB |
The capacity of hard disk drives is given by manufacturers in
megabytes (1 MB = 1,000,000 bytes),
gigabytes (1 GB = 1,000,000,000 bytes) or
terabytes (1 TB = 1,000,000,000,000 bytes).
[30][31] This numbering convention, where prefixes like
mega- and
giga- denote
powers of 1000,
is also used for data transmission rates and DVD capacities. However,
the convention is different from that used by manufacturers of
memory (
RAM,
ROM) and CDs, where prefixes like
kilo- and
mega- mean
powers of 1024.
When the
unit prefixes like
kilo- denote
powers of 1024 in the measure of memory capacities, the 1024
n progression (for
n = 1, 2, ...) is as follows:
[30]
- kilo = 210 = 10241 = 1024,
- mega = 220 = 10242 = 1,048,576,
- giga = 230 = 10243 = 1,073,741,824,
- tera = 240 = 10244 = 1,099,511,627,776,
and so forth.
The practice of using prefixes assigned to
powers of 1000 within the hard drive and computer industries dates back to the early days of computing.
[32] By the 1970s million, mega and M were consistently being used in the
powers of 1000 sense to describe HDD capacity.
[33][34][35]
As HDD sizes grew the industry adopted the prefixes “G” for giga and
“T” for tera denoting 1,000,000,000 and 1,000,000,000,000 bytes of HDD
capacity respectively.
Likewise, the practice of using prefixes assigned to
powers of 1024 within the computer industry also traces its roots to the early days of computing
[36] By the early 1970s using the prefix “K” in a
powers of 1024 sense to describe memory was common within the industry.
[37][38]
As memory sizes grew the industry adopted the prefixes “M” for mega and
“G” for giga denoting 1,048,576 and 1,073,741,824 bytes of memory
respectively.
Computers do not internally represent HDD or memory capacity in
powers of 1024; reporting it in this manner is just a convention.
[39] Creating confusion, operating systems report HDD capacity in different ways. Most operating systems, including the
Microsoft Windows operating systems use the
powers of 1024
convention when reporting HDD capacity, thus an HDD offered by its
manufacturer as a 1 TB drive is reported by these OSes as a 931 GB HDD.
Apple's current OSes, beginning with
Mac OS X 10.6 (“
Snow Leopard”), use
powers of 1000 when reporting HDD capacity, thereby avoiding any discrepancy between what it reports and what the manufacturer advertises.
In the case of “mega-,” there is a nearly 5% difference between the
powers of 1000 definition and the
powers of 1024
definition. Furthermore, the difference is compounded by 2.4% with each
incrementally larger prefix (gigabyte, terabyte, etc.) The discrepancy
between the two conventions for measuring capacity was the subject of
several
class action suits against HDD manufacturers. The plaintiffs argued that the use of decimal measurements effectively misled consumers
[40][41]
while the defendants denied any wrongdoing or liability, asserting that
their marketing and advertising complied in all respects with the law
and that no Class Member sustained any damages or injuries.
[42]
In December 1998, an international
standards organization attempted to address these dual definitions of the conventional prefixes by proposing unique
binary prefixes and prefix symbols to denote multiples of 1024, such as “
mebibyte (MiB)”, which exclusively denotes 2
20 or 1,048,576 bytes.
[43]
In the over‑13 years that have since elapsed, the proposal has seen
little adoption by the computer industry and the conventionally prefixed
forms of “byte” continue to denote slightly different values depending
on context.
[44][45]
HDD formatting
The presentation of an HDD to its host is determined by its
controller. This may differ substantially from the drive's native
interface particularly in
mainframes or
servers.
Modern HDDs, such as SAS
[46] and SATA
[47]
drives, appear at their interfaces as a contiguous set of logical
blocks; typically 512 bytes long but the industry is in the process of
changing to 4,096 byte logical blocks; see
Advanced Format.
[48]
The process of initializing these logical blocks on the physical disk platters is called
low level formatting which is usually performed at the factory and is not normally changed in the field.
[49]
High level formatting then writes the
file system structures into selected logical blocks to make the remaining logical blocks available to the host
OS and its applications.
[50] The operating system
file system
uses some of the disk space to organize files on the disk, recording
their file names and the sequence of disk areas that represent the file.
Examples of data structures stored on disk to retrieve files include
the
MS DOS file allocation table (FAT) and
UNIX inodes,
as well as other operating system data structures. As a consequence not
all the space on a hard drive is available for user files. This file
system overhead is usually less than 1% on drives larger than 100 MB.
Redundancy
In modern HDDs spare capacity for defect management is not included
in the published capacity; however in many early HDDs a certain number
of sectors were reserved for spares, thereby reducing capacity available
to end users.
In some systems, there may be hidden
partitions used for system recovery that reduce the capacity available to the end user.
For
RAID
subsystems, data integrity and fault-tolerance requirements also reduce
the realized capacity. For example, a RAID1 subsystem will be about
half the total capacity as a result of data mirroring. RAID5 subsystems
with x drives, would lose 1/x of capacity to parity. RAID subsystems are
multiple drives that appear to be one drive or more drives to the user,
but provides a great deal of fault-tolerance. Most RAID vendors use
some form of
checksums
to improve data integrity at the block level. For many vendors, this
involves using HDDs with sectors of 520 bytes per sector to contain 512
bytes of user data and 8 checksum bytes or using separate 512 byte
sectors for the checksum data.
[51]
HDD parameters to calculate capacity
Because modern disk drives appear to their interface as a contiguous
set of logical blocks their gross capacity can be calculated by
multiplying the number of blocks by the size of the block. This
information is available from the manufacturers specification and from
the drive itself through use of special utilities invoking low level
commands
[46][47]
The gross capacity of older HDDs can be calculated by multiplying for each
zone of the drive the number of
cylinders by the number of heads by the number of
sectors/zone by the number of bytes/sector (most commonly 512) and then summing the totals for all zones. Some modern
ATA drives will also report
cylinder, head, sector
(C/H/S) values to the CPU but they are no longer actual physical
parameters since the reported numbers are constrained by historic
operating-system interfaces.
The old C/H/S scheme has been replaced by
logical block addressing.
In some cases, to try to "force-fit" the C/H/S scheme to large-capacity
drives, the number of heads was given as 64, although no modern drive
has anywhere near 32 platters.
Form factors
5¼″ full height 110 MB HDD
2½″ (8.5 mm) 6495 MB HDD
Six hard drives with 8″, 5.25″, 3.5″, 2.5″, 1.8″, and 1″ hard disks with
a ruler to show the length of platters and read-write heads.
Mainframe and minicomputer hard disks were of widely varying
dimensions, typically in free standing cabinets the size of washing
machines (e.g.
HP 7935 and
DEC RP06 Disk Drives) or designed so that dimensions enabled placement in a
19" rack (e.g.
Diablo Model 31). In 1962,
IBM introduced its
model 1311
disk, which used 14 inch (nominal size) platters. This became a
standard size for mainframe and minicomputer drives for many years,
[52] but such large platters were never used with microprocessor-based systems.
With increasing sales of microcomputers having built in
floppy-disk drives (FDDs),
HDDs that would fit to the FDD mountings became desirable, and this led
to the evolution of the market towards drives with certain
Form factors,
initially derived from the sizes of 8-inch, 5.25-inch, and 3.5-inch
floppy disk drives. Smaller sizes than 3.5 inches have emerged as
popular in the marketplace and/or been decided by various industry
groups.
- 8 inch: 9.5 in × 4.624 in × 14.25 in (241.3 mm × 117.5 mm × 362 mm)
In 1979, Shugart Associates' SA1000 was the first form factor compatible HDD, having the same dimensions and a compatible interface to the 8″ FDD.
- 5.25 inch: 5.75 in × 3.25 in × 8 in (146.1 mm × 82.55 mm × 203 mm)
This smaller form factor, first used in an HDD by Seagate in 1980,[53] was the same size as full-height 5+1⁄4-inch-diameter
(130 mm) FDD, 3.25-inches high. This is twice as high as "half height";
i.e., 1.63 in (41.4 mm). Most desktop models of drives for optical
120 mm disks (DVD, CD) use the half height 5¼″ dimension, but it fell
out of fashion for HDDs. The Quantum Bigfoot HDD was the last to use it in the late 1990s, with "low-profile" (≈25 mm) and "ultra-low-profile" (≈20 mm) high versions.
- 3.5 inch: 4 in × 1 in × 5.75 in (101.6 mm × 25.4 mm × 146 mm) = 376.77344 cm³
This smaller form factor is similar to that used in an HDD by Rodime in 1983,[54]
which was the same size as the "half height" 3½″ FDD, i.e., 1.63 inches
high. Today, the 1-inch high ("slimline" or "low-profile") version of
this form factor is the most popular form used in most desktops.
- 2.5 inch: 2.75 in × 0.275–0.59 in × 3.945 in (69.85 mm × 7–15 mm × 100 mm) = 48.895–104.775 cm3
This smaller form factor was introduced by PrairieTek in 1988;[55] there is no corresponding FDD. It is widely used today for solid-state drives
and for hard disk drives in mobile devices (laptops, music players,
etc.) and as of 2008 replacing 3.5 inch enterprise-class drives.[56] It is also used in the Playstation 3[57] and Xbox 360[citation needed]
video game consoles. Today, the dominant height of this form factor is
9.5 mm for laptop drives (usually having two platters inside), but
higher capacity drives have a height of 12.5 mm (usually having three
platters). Enterprise-class drives can have a height up to 15 mm.[58] Seagate released a 7mm drive aimed at entry level laptops and high end netbooks in December 2009.[59]
- 1.8 inch: 54 mm × 8 mm × 71 mm = 30.672 cm³
This form factor, originally introduced by Integral Peripherals in 1993,
has evolved into the ATA-7 LIF with dimensions as stated. It was
increasingly used in digital audio players and subnotebooks, but is rarely used today. An original variant exists for 2–5GB sized HDDs that fit directly into a PC card expansion slot. These became popular for their use in iPods and other HDD based MP3 players.
- 1 inch: 42.8 mm × 5 mm × 36.4 mm
This form factor was introduced in 1999 as IBM's Microdrive to fit inside a CF Type II slot. Samsung calls the same form factor "1.3 inch" drive in its product literature.[60]
- 0.85 inch: 24 mm × 5 mm × 32 mm
Toshiba announced this form factor in January 2004[61] for use in mobile phones and similar applications, including SD/MMC slot compatible HDDs optimized for video storage on 4G handsets. Toshiba currently sells a 4 GB (MK4001MTD) and 8 GB (MK8003MTD) version [1][dead link] and holds the Guinness World Record for the smallest hard disk drive.[62]
3.5-inch and 2.5-inch hard disks currently dominate the market.
By 2009 all manufacturers had discontinued the development of new
products for the 1.3-inch, 1-inch and 0.85-inch form factors due to
falling prices of
flash memory,
[63][64] which is slightly more stable and resistant to damage from impact and/or dropping.
The inch-based nickname of all these form factors usually do not
indicate any actual product dimension (which are specified in
millimeters for more recent form factors), but just roughly indicate a
size relative to disk diameters, in the interest of historic continuity.
Current hard disk form factors
Obsolete hard disk form factors
| Form factor |
Width (mm) |
Largest capacity |
Platters (Max) |
| 5.25″ FH |
146 |
47 GB[76] (1998) |
14 |
| 5.25″ HH |
146 |
19.3 GB[77] (1998) |
4[78] |
| 1.3″ |
43 |
40 GB[79] (2007) |
1 |
| 1″ (CFII/ZIF/IDE-Flex) |
42 |
20 GB (2006) |
1 |
| 0.85″ |
24 |
8 GB[80][81] (2004) |
1 |
Performance characteristics
Access time
The factors that limit the time to access the data on a hard disk drive (
Access time) are mostly related to the mechanical nature of the rotating disks and moving heads.
Seek time is a measure of how long it takes the head assembly to travel to the track of the disk that contains data.
Rotational latency
is incurred because the desired disk sector may not be directly under
the head when data transfer is requested. These two delays are on the
order of milliseconds each. The
bit rate
or data transfer rate (once the head is in the right position) creates
delay which is a function of the number of blocks transferred; typically
relatively small, but can be quite long with the transfer of large
contiguous files. Delay may also occur if the drive disks are stopped to
save energy, see
Power management.
An HDD's
Average Access Time is its average
Seek time
which technically is the time to do all possible seeks divided by the
number of all possible seeks, but in practice is determined by
statistical methods or simply approximated as the time of a seek over
one-third of the number of tracks
[82]
Defragmentation is a procedure used to minimize delay in retrieving data by moving related items to physically proximate areas on the disk.
[83]
Some computer operating systems perform defragmentation automatically.
Although automatic defragmentation is intended to reduce access delays,
the procedure can slow response when performed while the computer is in
use.
[84]
Access time
can be improved by increasing rotational speed, thus reducing latency
and/or by decreasing seek time. Increasing areal density increases
throughput
by increasing data rate and by increasing the amount of data under a
set of heads, thereby potentially reducing seek activity for a given
amount of data. Based on historic trends, analysts predict a future
growth in HDD areal density (and therefore capacity) of about 40% per
year.
[85] Access times have not kept up with throughput increases, which themselves have not kept up with growth in storage capacity.
Interleave
Low-level formatting software to find highest performance interleave choice for 10
MB IBM PC XT hard disk drive.
Sector interleave is a mostly obsolete device characteristic related
to access time, dating back to when computers were too slow to be able
to read large continuous streams of data. Interleaving introduced gaps
between data sectors to allow time for slow equipment to get ready to
read the next block of data. Without interleaving, the next logical
sector would arrive at the read/write head before the equipment was
ready, requiring the system to wait for another complete disk revolution
before reading could be performed.
However, because interleaving introduces intentional physical delays
into the drive mechanism, setting the interleave to a ratio higher than
required causes unnecessary delays for equipment that has the
performance needed to read sectors more quickly. The interleaving ratio
was therefore usually chosen by the end-user to suit their particular
computer system's performance capabilities when the drive was first
installed in their system.
Modern technology is capable of reading data as fast as it can be
obtained from the spinning platters, so hard drives usually have a fixed
sector interleave ratio of 1:1, which is effectively no interleaving
being used.
Seek time
Average
seek time ranges from 3
ms[86] for high-end server drives, to 15 ms for mobile drives, with the most common mobile drives at about 12
ms[87] and the most common desktop type typically being around 9 ms. The
first HDD had an average seek time of about 600 ms and by the middle 1970s HDDs were available with seek times of about
25 ms. Some early PC drives used a
stepper motor to move the heads, and as a result had seek times as slow as 80–120 ms, but this was quickly improved by
voice coil type actuation in the 1980s, reducing seek times to around 20 ms. Seek time has continued to improve slowly over time.
Some desktop and laptop computer systems allow the user to make a
tradeoff between seek performance and drive noise. Faster seek rates
typically require more energy usage to quickly move the heads across the
platter, causing loud noises from the pivot bearing and greater device
vibrations as the heads are rapidly accelerated during the start of the
seek motion and decelerated at the end of the seek motion. Quiet
operation reduces movement speed and acceleration rates, but at a cost
of reduced seek performance.
Rotational latency
Rotational speed
[rpm] |
Average
latency [ms] |
| 15000 |
2 |
| 10000 |
3 |
| 7200 |
4.16 |
| 5400 |
5.55 |
| 4800 |
6.25 |
Latency is the delay for the rotation of the disk to bring the required
disk sector under the read-write mechanism. It depends on rotational speed of a disk, measured in
revolutions per minute
(rpm). Average rotational latency is shown in the table below, based on
the statistical relation that the average latency in milliseconds for
such a drive is one-half the rotational period.
Data transfer rate
As of 2010, a typical 7200 rpm desktop hard drive has a sustained "disk-to-
buffer" data transfer rate up to 1030 Mbits/sec.
[88]
This rate depends on the track location, so it will be higher for data
on the outer tracks (where there are more data sectors) and lower toward
the inner tracks (where there are fewer data sectors); and is generally
somewhat higher for 10,000 rpm drives. A current widely used standard
for the "buffer-to-computer" interface is 3.0 Gbit/s SATA, which can
send about 300 megabyte/s (10-bit encoding) from the buffer to the
computer, and thus is still comfortably ahead of today's disk-to-buffer
transfer rates. Data transfer rate (read/write) can be measured by
writing a large file to disk using special file generator tools, then
reading back the file. Transfer rate can be influenced by
file system fragmentation and the layout of the files.
[83]
HDD data transfer rate depends upon the rotational speed of the
platters and the data recording density. Because heat and vibration
limit rotational speed, advancing density becomes the main method to
improve sequential transfer rates.
[89]
While areal density advances by increasing both the number of tracks
across the disk and the number of sectors per track, only the latter
will increase the data transfer rate for a given rpm. Since data
transfer rate performance only tracks one of the two components of areal
density, its performance improves at a lower rate.
Power consumption
Power consumption
has become increasingly important, not only in mobile devices such as
laptops but also in server and desktop markets. Increasing data center
machine density has led to problems delivering sufficient power to
devices (especially for spin up), and getting rid of the waste heat
subsequently produced, as well as environmental and electrical cost
concerns (see
green computing). Heat dissipation is tied directly to power consumption, and as drives age, disk
failure rates increase at higher drive temperatures.
[90]
Similar issues exist for large companies with thousands of desktop PCs.
Smaller form factor drives often use less power than larger drives. One
interesting development in this area is actively controlling the seek
speed so that the head arrives at its destination only just in time to
read the sector, rather than arriving as quickly as possible and then
having to wait for the sector to come around (i.e. the rotational
latency).
[91]
Many of the hard drive companies are now producing Green Drives that
require much less power and cooling. Many of these Green Drives spin
slower (<5400 rpm compared to 7200, 10,000 or 15,000 rpm) thereby
generating less heat. Power consumption can also be reduced by parking
the drive heads when the disk is not in use reducing friction, adjusting
spin speeds,
[92] and disabling internal components when not in use.
[93]
Drives use more power, briefly, when starting up (spin-up). Although
this has little direct effect on total energy consumption, the maximum
power demanded from the power supply, and hence its required rating, can
be reduced in systems with several drives by controlling when they spin
up.
- On SCSI hard disk drives, the SCSI controller can directly control spin up and spin down of the drives.
- Some Parallel ATA (PATA) and Serial ATA (SATA) hard disk drives support power-up in standby
or PUIS: each drive does not spin up until the controller or system
BIOS issues a specific command to do so. This allows the system to be
set up to stagger disk start-up and limit maximum power demand at
switch-on.
- Some SATA II and later hard disk drives support staggered spin-up, allowing the computer to spin up the drives in sequence to reduce load on the power supply when booting.[94]
Power management
Most hard disk drives today support some form of power management
which uses a number of specific power modes that save energy by reducing
performance. When implemented an HDD will change between a full power
mode to one or more power saving modes as a function of drive usage.
Recovery from the deepest mode, typically called Sleep, may take as long
as several seconds.
[95]
Audible noise
Measured in
dBA, audible noise is significant for certain applications, such as
DVRs, digital audio recording and
quiet computers. Low noise disks typically use
fluid bearings, slower rotational speeds (usually 5400 rpm) and reduce the seek speed under load (
AAM)
to reduce audible clicks and crunching sounds. Drives in smaller form
factors (e.g. 2.5 inch) are often quieter than larger drives.
Shock resistance
Shock resistance is especially important for mobile devices. Some laptops now include
active hard drive protection
that parks the disk heads if the machine is dropped, hopefully before
impact, to offer the greatest possible chance of survival in such an
event. Maximum shock tolerance to date is 350
g for operating and 1000 g for non-operating.
[96]
Access and interfaces
Hard disk drives are accessed over one of a number of bus types, including as of 2011
[update] parallel
ATA (PATA, also called IDE or
EIDE; described before the introduction of SATA as ATA),
Serial ATA (SATA),
SCSI,
Serial Attached SCSI (SAS), and
Fibre Channel. Bridge circuitry is sometimes used to connect hard disk drives to buses with which they cannot communicate natively, such as
IEEE 1394,
USB and
SCSI.
For the now obsolete
ST-506 interface, the data
encoding scheme as written to the disk surface was also important. The first ST-506 disks used
Modified Frequency Modulation (MFM) encoding, and transferred data at a rate of 5
megabits per second. Later controllers using 2,7
RLL
(or just "RLL") encoding caused 50% more data to appear under the heads
compared to one rotation of an MFM drive, increasing data storage and
data transfer rate by 50%, to 7.5 megabits per second.
Many ST-506 interface disk drives were only specified by the
manufacturer to run at the 1/3 lower MFM data transfer rate compared to
RLL, while other drive models (usually more expensive versions of the
same drive) were specified to run at the higher RLL data transfer rate.
In some cases, a drive in practice had sufficient margin to allow the
MFM specified model to run at the faster RLL data transfer rate,
although not officially supporting this mode. Also, any RLL-certified
drive could run on any MFM controller, but with 1/3 less data capacity
and as much as 1/3 less data transfer rate compared to its RLL
specifications.
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 drive and controller; most of the
time, however, 15 or 20 megabit ESDI disk drives were not downward
compatible (i.e. a 15 or 20 megabit disk drive would not run on a
10 megabit controller). ESDI disk drives typically also had jumpers to
set the number of sectors per track and (in some cases) sector size.
Modern hard drives present a consistent interface to the rest of the
computer, no matter what data encoding scheme is used internally.
Typically a
DSP in the electronics inside the hard drive takes the raw analog voltages from the read head and uses
PRML and
Reed–Solomon error correction[97]
to decode the sector boundaries and sector data, then sends that data
out the standard interface. That DSP also watches the error rate
detected by
error detection and correction, and performs
bad sector remapping, data collection for
Self-Monitoring, Analysis, and Reporting Technology, and other internal tasks.
SCSI originally had just one signaling frequency of 5
MHz for a maximum data rate of 5
megabytes/second
over 8 parallel conductors, 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 drive's internal data
bus; however, many early disk drives 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
Commodore Amiga,
IBM PC compatibles and
Apple Macintoshes.
(Parallel) ATA interfaces were designed to support two drives on each
channel, connected as master and slave on a single cable. Disks
typically had no problems with interleave or data rate, due to their
controller design, but many early models were incompatible with each
other and could not run with two devices on the same physical cable.
This was mostly remedied by the mid-1990s, when ATA's specification was
standardized 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 supports one drive per channel and per cable, with its own set of I/O ports, avoiding master/slave problems.
FireWire/IEEE 1394 and USB(1.0/2.0/3.0) hard drives consist of
enclosures containing generally ATA or Serial ATA disks with built-in
adapters to these external buses.
Disk interface families used in personal computers
Several Parallel ATA hard disk drives
Historical
bit serial interfaces connect a hard disk drive
(HDD) to a hard disk controller (HDC) with two cables, one for control
and one for data. (Each drive also has an additional cable for power,
usually connecting it directly to the power supply unit). The HDC
provided significant functions such as serial/parallel conversion, data
separation, and track formatting, and required matching to the drive
(after formatting) in order to assure reliability. Each control cable
could serve two or more drives, while a dedicated (and smaller) data
cable served each drive.
- ST506 used MFM (Modified Frequency Modulation) for the data encoding method.
- ST412 was available in either MFM or RLL (Run Length Limited) encoding variants.
- Enhanced Small Disk Interface
(ESDI) was an industry standard interface similar to ST412 supporting
higher data rates between the processor and the disk drive.
Modern
bit serial interfaces connect a hard disk drive to a host bus interface adapter (today typically integrated into the "
south bridge") with one data/control cable. (As for historical
bit serial interfaces above, each drive also has an additional power cable, usually direct to the power supply unit.)
- Fibre Channel (FC) is a successor to parallel SCSI interface on enterprise market. It is a serial protocol. In disk drives usually the Fibre Channel Arbitrated Loop (FC-AL) connection topology is used. FC has much broader usage than mere disk interfaces, and it is the cornerstone of storage area networks (SANs). Recently other protocols for this field, like iSCSI and ATA over Ethernet have been developed as well. Confusingly, drives usually use copper
twisted-pair cables for Fibre Channel, not fibre optics. The latter are
traditionally reserved for larger devices, such as servers or disk array controllers.
- Serial ATA
(SATA). The SATA data cable has one data pair for differential
transmission of data to the device, and one pair for differential
receiving from the device, just like EIA-422. That requires that data be transmitted serially. A similar differential signaling system is used in RS485, LocalTalk, USB, Firewire, and differential SCSI.
- Serial Attached SCSI
(SAS). 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 a mechanically identical data and power
connector to standard 3.5-inch SATA1/SATA2 HDDs, and many
server-oriented SAS RAID controllers are also capable of addressing SATA
hard drives. SAS uses serial communication instead of the parallel
method found in traditional SCSI devices but still uses SCSI commands.
Word serial interfaces connect a hard disk drive to a host bus adapter (today typically integrated into the "
south bridge") with one cable for combined data/control. (As for all
bit serial interfaces
above, each drive also has an additional power cable, usually direct to
the power supply unit.) The earliest versions of these interfaces
typically had a 8 bit parallel data transfer to/from the drive, but
16-bit versions became much more common, and there are 32 bit versions.
Modern variants have serial data transfer. The word nature of data
transfer makes the design of a host bus adapter significantly simpler
than that of the precursor HDD controller.
- Integrated Drive Electronics
(IDE), later renamed to ATA, with the alias P-ATA or PATA ("parallel
ATA") retroactively added upon introduction of the new variant Serial ATA.
The original name reflected the innovative integration of HDD
controller with HDD itself, which was not found in earlier disks. Moving
the HDD controller from the interface card to the disk drive helped to
standardize interfaces, and to reduce the cost and complexity. The
40-pin IDE/ATA connection transfers 16 bits of data at a time on the
data cable. The data cable was originally 40-conductor, but later higher
speed requirements for data transfer to and from the hard drive led to
an "ultra DMA" mode, known as UDMA.
Progressively swifter versions of this standard ultimately added the
requirement for a 80-conductor variant of the same cable, where half of
the conductors provides grounding necessary for enhanced high-speed signal quality by reducing cross talk.
The interface for 80-conductor only has 39 pins, the missing pin acting
as a key to prevent incorrect insertion of the connector to an
incompatible socket, a common cause of disk and controller damage.
- EIDE was an unofficial update (by Western Digital) to the original IDE standard, with the key improvement being the use of direct memory access (DMA) to transfer data between the disk and the computer without the involvement of the CPU,
an improvement later adopted by the official ATA standards. By directly
transferring data between memory and disk, DMA eliminates the need for
the CPU to copy byte per byte, therefore allowing it to process other
tasks while the data transfer occurs.
- Small Computer System Interface
(SCSI), originally named SASI for Shugart Associates System Interface,
was an early competitor of ESDI. SCSI disks were standard on servers,
workstations, Commodore Amiga, and Apple Macintosh
computers through the mid-1990s, 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, SAS and Fibre
Channel only. The range limitations of the data cable allows for
external SCSI devices. Originally SCSI data cables used single ended
(common mode) data transmission, but server class SCSI could use
differential transmission, either low voltage differential (LVD) or high voltage differential
(HVD). ("Low" and "High" voltages for differential SCSI are relative to
SCSI standards and do not meet the meaning of low voltage and high
voltage as used in general electrical engineering contexts, as apply
e.g. to statutory electrical codes; both LVD and HVD use low voltage
signals (3.3 V and 5 V respectively) in general terminology.)
| Acronym or abbreviation |
Meaning |
Description |
| SASI |
Shugart Associates System Interface |
Historical predecessor to SCSI. |
| SCSI |
Small Computer System Interface |
Bus oriented that handles concurrent operations. |
| SAS |
Serial Attached SCSI |
Improvement of SCSI, uses serial communication instead of parallel. |
| ST-506 |
Seagate Technology |
Historical Seagate interface. |
| ST-412 |
Seagate Technology |
Historical Seagate interface (minor improvement over ST-506). |
| ESDI |
Enhanced Small Disk Interface |
Historical; backwards compatible with ST-412/506, but faster and more integrated. |
| ATA (PATA) |
Advanced Technology Attachment |
Successor to ST-412/506/ESDI by integrating the disk controller completely onto the device. Incapable of concurrent operations. |
| SATA |
Serial ATA |
Modification of ATA, uses serial communication instead of parallel. |
Integrity
Close-up HDD head resting on disk platter
Due to the extremely close spacing between the heads and the disk
surface, hard disk drives are vulnerable to being damaged by a
head crash—a
failure
of the disk in which the head scrapes across the platter surface, often
grinding away the thin magnetic film and causing data loss. Head
crashes can be caused by electronic failure, a sudden power failure,
physical shock, contamination of the drive's internal enclosure, wear
and tear,
corrosion, or poorly manufactured platters and heads.
The HDD's spindle system relies on air pressure inside the
disk enclosure to support the heads at their proper
flying height
while the disk rotates. Hard disk drives require 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 0.5 mm in breadth), usually with a filter on the inside (the
breather filter).
[98]
If the air pressure is too low, then there is not enough lift for the
flying head, so the head gets too close to the disk, 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 3,000 m (9,800 ft).
[99]
Modern disks include temperature sensors and adjust their operation to
the operating environment. Breather holes can be seen on all disk
drives—they usually have a sticker next to them, warning the user not to
cover the holes. The air inside the operating drive 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. Very high humidity for extended periods can corrode the heads
and platters.
For
giant magnetoresistive
(GMR) heads in particular, a minor head crash from contamination (that
does not remove the magnetic surface of the disk) still results 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).
Modes of failure
Hard drives may fail in a number of ways. Failure may be immediate
and total, progressive, or limited. Data may be totally destroyed, or
partially or totally recoverable.
Earlier drives tended to develop
bad sectors
with use and wear, which could be "mapped out" so that they did not
affect operation; this was considered normal unless many bad sectors
developed in a short period. Later drives map out bad sectors
automatically and invisibly to the user; S.M.A.R.T. information logs
these problems. A drive with bad sectors may usually continue to be
used.
Other failures which may be either progressive or limited are usually
considered to be a reason to replace a drive; the value of data
potentially at risk usually far outweighs the cost saved by continuing
to use a drive which may be failing. Repeated but recoverable read or
write errors, unusual noises, excessive and unusual heating, and other
abnormalities, are warning signs.
- Head crash: a head may contact the rotating platter due to
mechanical shock or other reason. At best this will cause irreversible
damage and data loss where contact was made. In the worst case the
debris scraped off the damaged area may contaminate all heads and
platters, and destroy all data on all platters. If damage is initially
only partial, continued rotation of the drive may extend the damage
until it is total.[10]
- Bad sectors: some magnetic sectors may become faulty without
rendering the whole drive unusable. This may be a limited occurrence or a
sign of imminent failure.
- Stiction: after a time the head may not "take off" when started up as it tends to stick to the platter, a phenomenon known as stiction.
This is usually due to unsuitable lubrication properties of the platter
surface, a design or manufacturing defect rather than wear. This
occasionally happened with some designs until the early 1990s.
- Circuit failure: components of the electronic circuitry may fail making the drive inoperable.
- Bearing and motor failure: electric motors may fail or burn out, and bearings may wear enough to prevent proper operation.
- Miscellaneous mechanical failures: parts, particularly moving parts,
of any mechanism can break or fail, preventing normal operation, with
possible further damage caused by fragments.
Recovery of data from failed drive
Data from a failed drive can sometimes be partially or totally
recovered
if the platters' magnetic coating is not totally destroyed. Specialised
companies carry out data recovery, at significant cost, by opening the
drives in a
clean room
and using appropriate equipment to read data from the platters
directly. If the electronics have failed, it is sometimes possible to
replace the electronics board, though often drives of nominally exactly
the same model manufactured at different times have different,
incompatible, circuit boards.
Sometimes operation can be restored for long enough to recover data.
Risky techniques are justifiable if the drive is otherwise dead. If a
drive is started up once it may continue to run for a shorter or longer
time but never start again, so as much data as possible is recovered as
soon as the drive starts. A 1990s drive that does not start due to
stiction can sometimes be started by tapping it or rotating the body of
the drive rapidly by hand. Another technique which is sometimes known to
work is to cool the drive, in a waterproof wrapping, in a domestic
freezer. There is much useful information about this in blogs and
forums,
[100] but professionals also resort to this method with some success.
[101]
Landing zones and load/unload technology
Read/write head from circa-1998
Fujitsu 3.5" hard disk (approx. 2.0 mm x 3.0 mm)
Microphotograph of an older generation hard disk drive head and slider (1990s)
During normal operation heads in HDDs fly above the data recorded on
the disks. Modern HDDs prevent power interruptions or other malfunctions
from landing its heads in the data zone by either physically moving (
parking) the heads to a special
landing zone on the platters that is not used for data storage, or by physically locking the heads in a suspended (
unloaded)
position raised off the platters. Some early PC HDDs did not park the
heads automatically when power was prematurely disconnected and the
heads would land on data. In some other early units the user manually
parked the heads by running a program to park the HDD's heads.
Landing zones
A
landing zone is an area of the platter usually near its
inner diameter (ID), where no data is stored. This area is called the
Contact Start/Stop (CSS) zone. Disks are designed such that either a
spring or, more recently, rotational
inertia in the platters is used to park the heads in the case of unexpected power loss. In this case, the
spindle motor temporarily acts as a
generator, providing power to the actuator.
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 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 had 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 Seagate Barracuda 7200.10 series of
desktop hard disks are rated to 50,000 start-stop cycles, in other words
no failures attributed to the head-platter interface were seen before
at least 50,000 start-stop cycles during testing.
[102]
Around 1995 IBM pioneered a technology where a landing zone on the disk is made by a precision laser process (
Laser Zone Texture = LZT) producing an array of smooth nanometer-scale "bumps" in a landing zone,
[103] thus vastly improving
stiction
and wear performance. This technology is still largely in use today
(2011), predominantly in desktop and enterprise (3.5 inch) drives. In
general, CSS technology can be prone to increased stiction (the tendency
for the heads to stick to the platter surface), e.g. as a consequence
of increased humidity. Excessive stiction can cause physical damage to
the platter and slider or spindle motor.
Unloading
Load/Unload technology relies on the heads being lifted off
the platters into a safe location, thus eliminating the risks of wear
and stiction altogether. The first HDD
RAMAC
and most early disk drives used complex mechanisms to load and unload
the heads. Modern HDDs use ramp loading, first introduced by
Memorex in 1967,
[104] to load/unload onto plastic "ramps" near the outer disk edge.
All HDDs today still use one of these two technologies listed above.
Each has a list of advantages and drawbacks in terms of loss of storage
area on the disk, relative difficulty of mechanical tolerance control,
non-operating shock robustness, cost of implementation, etc.
Addressing shock robustness,
IBM also 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
accelerometer
in the Thinkpad, internal hard disk heads automatically unload
themselves to reduce the risk of any potential data loss or scratch
defects.
Apple later also utilized this technology in their
PowerBook,
iBook,
MacBook Pro, and
MacBook line, known as the
Sudden Motion Sensor.
Sony,
[105] HP with their HP 3D DriveGuard
[106] and
Toshiba[107] have released similar technology in their notebook computers.
Metrics of failures
Most major hard disk and motherboard vendors now support
S.M.A.R.T. (Self-Monitoring, Analysis, and Reporting Technology), which measures drive characteristics such as
operating temperature,
spin-up time, data error rates, etc. Certain trends and sudden changes
in these parameters are thought to be associated with increased
likelihood of drive failure and data loss.
However, S.M.A.R.T. parameters alone may not be useful for predicting individual drive failures.
[108]
Unpredictable breakdown of the inherently fragile device—and all
mechanisms must eventually fail—may occur at any time in normal use,
with potential loss of all data, which is reliably recoverable only if
another copy is stored by using a
RAID array or making
backup
copies (the purpose of RAID and backup is different in the context of a
data storage system, but the distinction is not relevant to this
article). Recovery of some or even all data from a damaged drive is
sometimes, but not always possible, and is normally costly.
A 2007 study published by
Google
suggested very little correlation between failure rates and either high
temperature or activity level; however, the correlation between
manufacturer/model and failure rate was relatively strong. Statistics in
this matter are kept highly secret by most entities. Google did not
relate manufacturers' names with failure rates,
[108] though they have since revealed that they use Hitachi Deskstar drives in some of their servers.
[109]
While several S.M.A.R.T. parameters have an impact on failure
probability, a large fraction of failed drives do not produce predictive
S.M.A.R.T. parameters.
[108]
A common misconception is that, all else being equal, a colder hard
drive will last longer than a hotter one. The Google study seems to
imply the reverse—"lower temperatures are associated with higher failure
rates". Hard drives with S.M.A.R.T.-reported average temperatures below
27 °C (81 °F) had higher failure rates than hard drives with the highest reported average temperature of
50 °C (122 °F), failure rates at least twice as high as the optimum S.M.A.R.T.-reported temperature range of
36 °C (97 °F) to
47 °C (117 °F).
[108]
SCSI,
SAS, and
FC drives are more expensive than consumer-grade PATA and SATA drives, and usually used in
servers and
disk arrays, whereas inexpensive PATA and SATA drives were sold to the
home computer and desktop market and were perceived to be less reliable. This distinction is now becoming blurred.
The
mean time between failures (MTBF) of SATA drives is usually about 600,000 hours (some drives such as
Western Digital Raptor have rated 1.4 million hours MTBF),
[110] while SCSI drives are rated for upwards of 1.5 million hours.
[citation needed] However, independent research indicates that MTBF is not a reliable estimate of a drive's longevity.
[111]
MTBF is conducted in laboratory environments in test chambers and is an
important metric to determine the quality of a disk drive before it
enters high volume production. Once the drive product is in production,
the more valid metric is
annualized failure rate (AFR).
[citation needed]
AFR is the percentage of real-world drive failures after shipping.
Differences in reliability between drives with different interfaces are
due to marketing and issues in the drive itself; the interface in itself
is not a significant factor, but expensive server-grade drives where
reliability (determined by construction) and performance (determined by
interface) are more important than purchase price are designed both for
higher reliability and faster interface. Consequently SCSI and SAS
drives are designed for higher MTBF and reliability than consumer PATA
and SATA drives.
[citation needed]
However, there are SATA drives designed and produced for enterprise
markets, designed for reliability comparable to other enterprise-class
drives.
[112][113]
Typically as of 2007
[update]
enterprise drives (all enterprise drives, including SCSI, SAS,
enterprise SATA, and FC) experienced between 0.70%–0.78% annual failure
rates from the total installed drives.
[citation needed]
External removable drives
External removable hard disk drives offer independence from
system integration, establishing communication via connectivity options, such as
USB.
Plug and play
drive functionality offers system compatibility, and features large
volume data storage options, but maintains a portable design.
These drives with an ability to function and be removed
simplistically, have had further applications due their flexibility.
These include:
- Backup of files and information
External hard disk drives are available in two main sizes (physical size), 2.5" and 3.5".
[114]
Features such as biometric security or multiple interfaces are available at a higher cost.
[115]
Market segments
- Desktop HDDs typically store between 60 GB and 4 TB and rotate at 5400 to 10,000 rpm, and have a media transfer rate of 0.5 Gbit/s or higher (1 GB = 109 bytes; 1 Gbit/s = 109 bit/s). As of September 2011[update], the highest capacity consumer HDDs is 4 TB.[66]
- Mobile HDDs or laptop HDDs, smaller than their desktop and
enterprise counterparts, tend to be slower and have lower capacity.
Mobile HDDs spin at 4200 rpm, 5200 rpm, 5400 rpm, or 7200 rpm, with 5400
rpm being typical. 7200 rpm drives tend to be more expensive and have
smaller capacities, while 4200 rpm models usually have very high storage
capacities. Because of smaller platter(s), mobile HDDs generally have
lower capacity than their greater desktop counterparts.
- Enterprise HDDs are typically used with multiple-user computers running enterprise software.
Examples are: transaction processing databases, internet infrastructure
(email, webserver, e-commerce), scientific computing software, and
nearline storage management software. Enterprise drives commonly operate
continuously ("24/7") in demanding environments while delivering the
highest possible performance without sacrificing reliability. Maximum
capacity is not the primary goal, and as a result the drives are often
offered in capacities that are relatively low in relation to their cost.[116]
- The fastest enterprise HDDs spin at 10,000 or 15,000 rpm, and can achieve sequential media transfer speeds above 1.6 Gbit/s.[117] and a sustained transfer rate up to 1 Gbit/s.[117] Drives running at 10,000 or 15,000 rpm use smaller platters to mitigate increased power requirements (as they have less air drag) and therefore generally have lower capacity than the highest capacity desktop drives.
- Enterprise HDDs are commonly connected through Serial Attached SCSI (SAS) or Fibre Channel (FC). Some support multiple ports, so they can be connected to a redundant host bus adapter.
They can be reformatted with sector sizes larger than 512 bytes (often
520, 524, 528 or 536 bytes). The additional storage is can be used by
hardware RAID cards or to store a Data Integrity Field.
- Consumer electronics HDDs include drives embedded into digital video recorders and automotive vehicles.
The former are configured to provide a guaranteed streaming capacity,
even in the face of read and write errors, while the latter build to
resist larger amounts of shock.
The
exponential
increases in disk space and data access speeds of HDDs have enabled the
commercial viability of consumer products that require large storage
capacities, such as
digital video recorders and digital audio players.
[118]
In addition, the availability of vast amounts of cheap storage has made
viable a variety of web-based services with extraordinary capacity
requirements, such as free-of-charge web search,
web archiving, and video sharing (
Google,
Internet Archive, YouTube, etc.).
Manufacturers and sales
More than 200 companies have manufactured hard disk drives over time.
Worldwide revenue from shipments of HDDs is expected to reach $27.7 billion in 2010, up 18.4% from $23.4 billion in 2009
[119] corresponding to a 2010 unit shipment forecast of 674.6 million compared to 549.5 million units in 2009.
[120] As of 2011
[update], most hard drives are made by:
[121]
- ^ In the process of being acquired by Western Digital.
- ^ Acquired by Seagate in Dec 2011.
Icons
HDDs are traditionally symbolized as a stylized stack of platters or
as a cylinder and are found in diagrams or on lights to indicate hard
drive access.
In most modern
operating systems, hard drives are represented by an illustration or photograph of the drive enclosure, as shown in the examples below.
-
HDDs are commonly symbolized with a drive icon
-
RAID diagram icon symbolizing the
array of disks
-
1970s vintage disk pack with the cover removed
an
bermacam – macam software hacking wifi… tinggal kita mau belajar dan
menyimaknya saja. Plus ketelatenan. Coba saja ketik hacking wifi di
youtube, maka akan hadir berjibun caranya. Software yang mau saya bagi
adalah airowizard. Program ini merupakan bentuk yang lebih user
friendly dari aircrack. Berguna untuk membaca lalu lintas data packet
wifi baik unprotect maupun protect. Lalu menganalisanya dan menghacknya
untuk mencari passwordnya. Apabila secara manual penyadapan lalu
lintas data ini memerlukan waktu berjam – jam maupun berhari – hari
untuk membongkar password WEP/WPA2, maka airowizard hanya butuh 5 menit
saja. Sesuai kata – kata pembuatnya sih.Di dunia maya sangat sulit
mencari software ini karena banyak pihak yang sirik karena software ini
bisa membongkar password wifi secara cepat.
Sebenarnya
kalau anda suka cara – cara manual maka airowizard lebih nyaman dan
sakti bila digunakan lewat OS linux yaitu Backtrack 3. Backtrack memang
didesain untuk security. Asyiknya lagi backtrack juga tersedia dalam
versi USB sehingga bisa simpan pada USB kapasitas 1 GB.
![[icon]](http://en.wikipedia.org/wiki/File:Wiki_letter_w_cropped.svg){kind=small}
