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| History | |
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Main article: History of hard disk drives
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 Its primary distinguishing feature was that the disk heads were not withdrawn completely from the stack of disk platters when the drive was powered down.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 ($100/1 terabyte), a greater than 150-million-to-1 decrease.[7]
Average access time decreasing from over 100 milliseconds to a few milliseconds, a greater than 40-to-1 improvement.
Market application expanding from mainframe computers of the late 1950s to most mass storage applications including computers and consumer applications such as storage of entertainment content.
[edit] Tags:Ibm,Platters,Magnetic,Edit,History Of Hard Disk Drives,Mainframe,Floppy Disk,Us$,Mainframe Computers,Mass Storage,Nm,Magnet,Terabyte,Ram,Rom,Os, | |
| Magnetic recording | |
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See also: Magnetic storage
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 are encoded into a run-length limitedcode[8] and the encoded data written as a pattern of sequential magnetic transitions on the disk. The data are 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 are 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]
Magnetic cross section & frequency modulation encoded binary data
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]
[edit] Tags:Magnetic Storage,Ferromagnetic,Run-length Limitedcode,Frequency Modulation,Read-and-write Heads,Voice Coil,Stepper Motor,Micrometer,Magnetic Domains,Perpendicular,Polycrystalline,Magnetic Domain,Magnetic Dipole,Magnetic Field,Iron(iii) Oxide,Cobalt,Electromagnet,Electromagnetic Induction,Thin Film,Magnetoresistance,Spintronics,Magneto-resistive,Ruthenium,Perpendicular Recording, | |
| Components | |
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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.
[edit] Tags:Neodymium-iron-boron,Loudspeakers,Magnet Wire, | |
| Actuation of moving arm | |
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This section needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (November 2010)
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.
[edit] Tags:Verification,Reliable Sources,Challenged, | |
| Error handling | |
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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.
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| Future development | |
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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]
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| Capacity | |
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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.
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| Units of storage capacity | |
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See also: Binary prefix
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 1024n 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 220 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]
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| HDD formatting | |
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Main article: Disk 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.
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| Redundancy | |
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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 drives, data integrity and fault-tolerance requirements also reduce the realized capacity. For example, a RAID1 drive will be about half the total capacity as a result of data mirroring. For RAID5 drives with x drives you would lose 1/x of your space to parity. RAID drives are multiple drives that appear to be one drive to the user, but provides some 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]
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| HDD parameters to calculate capacity | |
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PC hard disk drive capacity (in GB) over time. The vertical axis is logarithmic, so the fit line corresponds to exponential growth.
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 constra Tags: | |
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