Video of modern hard disk drive operation (cover removed) |
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| Date invented | 24 December 1954[1] |
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| 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]
Contents |
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 Its primary distinguishing feature was that the disk heads were not withdrawn completely from the stack of disk platters when the drive was powered down.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:
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]
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]
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).
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.
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.
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.
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 are 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]
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.
| 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]
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]
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.
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]
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.
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.
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.
| Form factor | Width (mm) | Height (mm) | Largest capacity | Platters (Max) |
|---|---|---|---|---|
| 3.5″ | 102 | 19 or 25.4 | 4 TB[65][66][67][68] (2011) | 5 |
| 2.5″ | 69.9 | 7,[69] 9.5,[70] 11.5,[71] or 15 | 1.5 TB[65][72][73][74] (2010) | 4 |
| 1.8″ | 54 | 5 or 8 | 320 GB[75] (2009) | 3 |
| 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 |
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.
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.
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 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.
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 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.
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]
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 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]
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.
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.
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.)
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.
| 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. |
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).
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.
When data of which there is no copy (via backup, RAID, etc.) is on a failed drive, it can sometimes be recovered partially or totally 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.
It is sometimes possible for a user to restore operation 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]
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.
A landing zone is an area of the platter usually near its inner diameter (ID), where no data are 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.
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.
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. 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). 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. 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.
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:
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]
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.).
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]
| Manufacturer \ Market segment | Mobile | Desktop | Enterprise | Consumer electronics | Total market share in units shipped |
|---|---|---|---|---|---|
| Western Digital | 30% | 31.2% | |||
| Seagate | 20% | 29.2% | |||
| Hitachi GST[note 1] | 19% | 18.1% | |||
| Toshiba | 17% | 10.8% | |||
| Samsung[note 2] | 14% | 10.7% | |||
| Units shipped 4Q10 (millions) | 72.70 | 64.06 | 8.16 | 22.39 | 167.31 |
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.
The LBAs on a logical unit shall begin with zero and shall be contiguous up to the last logical block on the logical unit.
—Information technology – Serial Attached SCSI – 2 (SAS-2), INCITS 457 Draft 2, 8 May 2009, chapter 4.1 Direct-access block device type model overview
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