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r ibm disk drives used only two read write heads per arm the 1301 used an array of 48 m heads comb each array moving horizontally as a single unit one head per surface used cylinder mode read write operations were supported and the heads flew about 250 micro inches about 6 μm above the platter surface motion of the head array depended upon a binary adder system of hydraulic actuators which assured repeatable positioning the 1301 cabinet was about the size of three large refrigerators placed side by side storing the equivalent of about 21 million eight bit bytes per module access time was about a quarter of a second also in 1962 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 non removable hdds were called fixed disk drives in 1963 ibm introduced the 1302 34 with twice the track capacity and twice as many tracks per cylinder as the 1301 the 1302 had one for model 1 or two for model 2 modules each containing a separate comb for the first 250 tracks and the last 250 tracks some high performance hdds were manufactured with one head per track e g the burroughs b 475 in 1964 and the ibm 2305 in 1970 so that no time was lost physically moving the heads to a track and the only latency was the time for the desired block of data to rotate into position under the head 35 known as fixed head or head per track disk drives they were very expensive and are no longer in production 36 1970s edit in 1973 ibm introduced a new type of hdd code named winchester its primary distinguishing feature was that the disk heads were not withdrawn completely from the stack of disk platters when the drive was powered down instead the heads were allowed to land on a special area of the disk surface upon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 in 1974 ibm introduced the swinging arm actuator made feasible because the winchester recording heads function well when skewed to the recorded tracks the simple design of the ibm gv gulliver drive 37 invented at ibm s uk hursley labs became ibm s most licensed electro mechanical invention 38 of all time the actuator and filtration system being adopted in the 1980s eventually for all hdds and still universal over 50 years and 10 billion arms later like the first removable pack drive the first winchester drives used platters 14 inches 360 mm in diameter in 1978 ibm introduced a swing arm drive the ibm 0680 piccolo with eight inch platters exploring the possibility that smaller platters might offer advantages other eight inch drives followed then 5 1 4 in 130 mm drives sized to replace the contemporary floppy disk drives the latter were primarily intended for the then fledgling personal computer pc market 1980s 1990s edit over time as recording densities were greatly increased further reductions in disk diameter to 3 5 and 2 5 were found to be optimum powerful rare earth magnet materials became affordable during this period and were complementary to the swing arm actuator design to make possible the compact form factors of modern hdds by 1986 10 mb was the most commonly used hard disk size and most new corporate computers had hard drives most in the industry expected that 20 mb would soon become the new standard size 39 by the late 1980s their cost had been reduced to the point where they were standard on all but the cheapest computers most hdds in the early 1980s were sold to pc end users as an external add on subsystem the subsystem was not sold under the drive manufacturer s name but under the subsystem manufacturer s name such as corvus systems and tallgrass technologies or under the pc system manufacturer s name such as the apple profile the ibm pc xt in 1983 included an internal 10 mb hdd by 1985 internal hdds outsold external drives for personal computers when ibm began selling the xt without a hard disk hard disk manufacturers that had only sold to oem customers began selling to resellers which bundled xts with the non ibm hard disks hardcards became popular upgrades for existing computers 39 external hdds remained popular for much longer on the apple macintosh many macintosh computers made between 1986 and 1998 featured a scsi port on the back making external expansion simple older compact macintosh computers did not have user accessible hard drive bays indeed the macintosh 128k macintosh 512k and macintosh plus did not feature a hard drive bay at all so on those models external scsi disks were the only reasonable option for expanding upon any internal storage 21st century edit hdd improvements have been driven by increasing areal density listed in the table above applications expanded through the 2000s from the mainframe computers of the late 1950s to most mass storage applications including computers and consumer applications such as storage of entertainment content in the 2000s and 2010s nand began supplanting hdds in applications requiring portability or high performance in 2018 the largest hard drive had a capacity of 15 tb while the largest capacity ssd had a capacity of 100 tb 40 in 2018 hdds were forecast to reach 100 tb capacities around 2025 41 but as of 2019 update the expected pace of improvement was pared back to 50 tb by 2026 42 smaller form factors 1 8 inches and below were discontinued around 2010 the cost of solid state storage nand represented by moore s law is improving faster than hdds nand has a higher price elasticity of demand than hdds and this drives market growth 43 during the late 2000s and 2010s the product life cycle of hdds entered a mature phase and slowing sales may indicate the onset of the declining phase 44 during the 2020s nand adoption rate has significantly accelerated in no small part due to the rising use of the technology in consumer electronics while its performance has been improving faster than hdd performance the 2011 thailand floods damaged the manufacturing plants and impacted hard disk drive cost adversely between 2011 and 2013 45 in 2019 western digital closed its last malaysian hdd factory due to decreasing demand to focus on ssd production 46 all three remaining hdd manufacturers have had decreasing demand for their hdds since 2014 47 technology edit magnetic recording edit video overview of how hdds work a 3 5 and a 2 5 hdd with front covers removed to show their internals see also magnetic storage a modern hdd records data by magnetizing a thin film of ferromagnetic material n on both sides of a disk sequential changes in the direction of magnetization represent binary data bits the data is read from the disk by detecting the transitions in magnetization user data is encoded using an encoding scheme such as run length limited encoding o which determines how the data is represented by the magnetic transitions a typical hdd design consists of a spindle that holds flat circular disks called platters which hold the recorded data the platters are made from a non magnetic material usually aluminum alloy glass or ceramic they are coated with a shallow layer of magnetic material typically 10 20 nm in depth with an outer layer of carbon for protection 49 50 51 for reference a standard piece of copy paper is 0 07 0 18 mm 70 000 180 000 nm 52 thick 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 53 the first hdds spun at 1 200 rpm 5 and for many years 3 600 rpm was the norm 54 as of november 2019 update the platters in most consumer grade hdds spin at 5 400 or 7 200 rpm information is written to and read from a platter as it rotates past devices called read and write heads that are positioned to operate very close to the magnetic surface with their flying height often in the range of tens of nanometers the read and write head is used to detect and modify the magnetization of the material passing 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 early hard disk drives wrote data at some constant bits per second resulting in all tracks having the same amount of data per track but modern drives since the 1990s use zone bit recording increasing the write speed from inner to outer zone and thereby storing more data per track in the outer zones in modern drives the small size of the magnetic regions creates the danger that their magnetic state might be lost because of thermal effects thermally induced magnetic instability which is commonly known as the superparamagnetic limit to counter this the platters are coated with two parallel magnetic layers separated by a three atom layer of the non magnetic element ruthenium and the two layers are magnetized in opposite orientation thus reinforcing each other 55 another technology used to overcome thermal effects to allow greater recording densities is perpendicular recording pmr first shipped in 2005 56 and as of 2007 update used in certain hdds 57 58 59 perpendicular recording may be accompanied by changes in the manufacturing of the read write heads to increase the strength of the magnetic field created by the heads 60 in 2004 a higher density recording media was introduced consisting of coupled soft and hard magnetic layers so called exchange spring media magnetic storage technology also known as exchange coupled composite media allows good writability due to the write assist nature of the soft layer however the thermal stability is determined only by the hardest layer and not influenced by the soft layer 61 62 flux control mamr fc mamr allows a hard drive to have increased recording capacity without the need for new hard disk drive platter materials mamr hard drives have a microwave generating spin torque generator sto on the read write heads which allows physically smaller bits to be recorded to the platters increasing areal density normally hard drive recording heads have a pole called a main pole that is used for writing to the platters and adjacent to this pole is an air gap and a shield the write coil of the head surrounds the pole the sto device is placed in the air gap between the pole and the shield to increase the strength of the magnetic field created by the pole fc mamr technically doesn t use microwaves but uses technology employed in mamr the sto has a field generation layer fgl and a spin injection layer sil and the fgl produces a magnetic field using spin polarised electrons originating in the sil which is a form of spin torque energy 63 magnetic cross section frequency modulation encoded binary data hdd with front cover removed to show its operation recording of single magnetisations of bits on a 200 mb hdd platter recording made visible using cmos magview 64 longitudinal recording standard perpendicular recording diagram components edit the actuator is a permanent magnet and moving coil motor that swings the heads to the desired position the head support arm is very light but also stiff in modern drives acceleration at the head reaches 550 g a metal 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 have only one magnet the voice coil itself is shaped rather like an arrowhead and is 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 center of the actuator bearing then interact with the magnetic field of the fixed magnet 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 north pole and half south 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 the hdd s electronics controls the movement of the actuator and the rotation of the disk and transfers data to or from a 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 otherwise known as sector servo technology the servo feedback optimizes the signal to noise ratio of the gmr sensors by adjusting the voice coil motor to rotate the arm a more modern servo system also employs milli or micro actuators to more accurately position the read write heads 65 the spinning of the disks uses fluid bearing spindle motors modern disk firmware is capable of scheduling reads and writes efficiently on the platter surfaces and remapping sectors of the media that have failed diagram labeling the major components of a computer hdd an hdd with disks and motor hub removed exposing copper colored stator coils surrounding a bearing in the center of the spindle motor the orange stripe along the side of the arm is a thin printed circuit cable the spindle bearing is in the center and the actuator is in the upper left printed circuit board of a 2 5 inch samsung hard disk mp0402h head stack with an actuator coil on the left and read write heads on the right close up of a single read write head showing the side facing the platter the circuit board of a western digital hard drive attached to its chassis the read write arm error rates and handling edit modern drives make extensive ...
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