Hard disk drive
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 (
platters) coated with magnetic material, and with
magnetic heads arranged to write data to the surfaces and read it from them.
History
Hard disk drives were introduced in 1956 as data storage for an IBM real-time transaction processing computer
[4] and were developed for use with general purpose
mainframe and
mini computers. The first IBM drive, the
350 RAMAC, was approximately the size of two refrigerators and stored 5 million 6-bit characters (the equivalent of 3.75 million 8-bit bytes) on a stack of 50 discs.
In 1961 IBM introduced the model 1311 disk drive, which was about the size of a washing machine and stored two million characters on a removable disk "pack." Users could buy additional packs and interchange them as needed, much like reels of magnetic tape. Later models of removable pack drives, from IBM and others, became the norm in most computer installations and reached capacities of 300 megabytes by the early 1980s.
In 1973, IBM introduced a new type of hard drive codenamed "Winchester." Its primary distinguishing feature was that the disk heads were not withdrawn completely from the stack of disk platters when the drive was powered down. Instead, the heads were allowed to "land" on a special area of the disk surface upon spin-down, "taking off" again when the disk was later powered on. This greatly reduced the cost of the head actuator mechanism, but precluded removing just the disks from the drive as was done with the disk packs of the day. Instead, the first models of "Winchester technology" drives featured a removable disk module, which included both the disk pack and the head assembly, leaving the actuator motor in the drive upon removal. Later "Winchester" drives abandoned the removable media concept and returned to non-removable platters.
Like the first removable pack drive, the first "Winchester" drives used platters 14 inches in diameter. A few years later, designers were exploring the possibility that physically smaller platters might offer advantages. Drives with non-removable eight-inch platters appeared, and then drives that fit in a "five and a quarter inch" form factor (a mounting width equivalent to that used by a five and a quarter inch
floppy disk drive). The latter were primarily intended for the then-fledgling personal computer market.
As the 1980s began, hard disk drives were a rare and very expensive additional feature on personal computers (PCs); however by the late '80s, their cost had been reduced to the point where they were standard on all but the cheapest PC.
Most hard disk drives in the early 1980s were sold to PC end users as an add on subsystem, not under the drive manufacturer's name but by systems integrators such as the Corvus Disk System or the systems manufacturer such as the Apple ProFile. The IBM PC/XT in 1983 included an internal standard 10MB hard disk drive, and soon thereafter internal hard disk drives proliferated on personal computers.
External hard disk drives remained popular for much longer on the Apple Macintosh. Every Mac made between 1986 and 1998 has a SCSI port on the back, making external expansion easy; also, "toaster" Compact Macs did not have easily accessible hard drive bays (or, in the case of the Mac Plus, any hard drive bay at all), so on those models, external SCSI disks were the only reasonable option.
Driven by
areal density doubling every two to four years since their invention, hard disk drives have changed in many ways. A few highlights include:
- Capacity per HDD increasing from 3.75 megabytes[4] to 4 terabytes or more, more than 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]Technology
Diagram labeling the major components of a computer hard disk drive
Overview of how a hard disk drive functions.
[edit]Magnetic recording
A hard disk drive records data by magnetizing a thin film of
ferromagnetic material on 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
run-length limited encoding
[8], which determines how the data is represented by the magnetic transitions.
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 aluminium alloy, glass, or ceramic, and are coated with a shallow layer of magnetic material typically 10–20
nm in depth, with an outer layer of carbon for protection.
[10][11][12] For reference, a standard piece of copy paper is 0.07–0.18 millimetre (70,000–180,000 nm).
[13]
Recording of single magnetisations of bits on an hdd-platter (recording made visible using CMOS-MagView).
[14]
Longitudinal recording (standard) & perpendicular recording diagram
The platters in contemporary HDDs are spun at speeds varying from 4,200
rpm in energy-efficient portable devices, to 15,000 rpm for high performance servers.
[15] The first hard drives spun at 1,200 rpm
[16] and, for many years, 3,600 rpm was the norm.
[17]Today, most consumer hard drives operate at a speed of 7,200 rpm.
Information is written to and read from a platter as it rotates past devices called
read-and-write heads that operate very close (often tens of nanometers) 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 coilactuator 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, (although these are not
magnetic domains in a rigorous physical sense), each of which has a mostly uniform magnetization. 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 true
magnetic domain. Each magnetic region in total forms a
magnetic dipole which generates a
magnetic field. 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.
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 rotation of the
magnetic moment 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, and a read head detects the magnetization of the regions. 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 read 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]Capacity
The capacity of an HDD may appear to the end user to be a different amount than the amount stated by a drive or system manufacturer due to amongst other things, different units of measuring capacity, capacity consumed in formatting the drive for use by an operating system and/or redundancy.
[edit]Units of storage capacity
Advertised capacity
by manufacturer
(using decimal multiples) | Expected capacity
by consumers in class action
(using binary multiples) | Reported capacity |
|---|
Windows
(using binary
multiples) | Mac OS X 10.6+
(using decimal
multiples) |
|---|
| With prefix | Bytes | Bytes | Diff. |
|---|
| 100 MB | 100,000,000 | 104,857,600 | 4.86% | 95.4 MB | 100.0 MB |
| 100 GB | 100,000,000,000 | 107,374,182,400 | 7.37% | 93.1 GB, 95,367 MB | 100.00 GB |
| 1 TB | 1,000,000,000,000 | 1,099,511,627,776 | 9.95% | 931 GB, 953,674 MB | 1,000.00 GB, 1,000,000 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 1,000, 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 1,024.
When the
unit prefixes like
kilo- denote
powers of 1,024 in the measure of memory capacities, the 1,024
n progression (for
n = 1, 2, ...) is as follows:
[30]
- kilo = 210 = 1,0241 = 1,024,
- mega = 220 = 1,0242 = 1,048,576,
- giga = 230 = 1,0243 = 1,073,741,824,
The practice of using prefixes assigned to
powers of 1,000 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 1,000 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 1,024 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 1,024 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 1,024; 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 1,024 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 1,000 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 1,000 definition and the
powers of 1,024 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 1,024, such as “
mebibyte (MiB)”, which exclusively denotes 2
20 or 1,048,576 bytes.
[43] In the over‑13 years that have since elapsed, the proposal has seen little adoption by the computer industry and the conventionally prefixed forms of “byte” continue to denote slightly different values depending on context.
[44][45]
[edit]Form factors
5¼″ full height 110 MB HDD
2½″ (8.5 mm) 6,495 MB HDD
Six hard drives with 8″, 5.25″, 3.5″, 2.5″, 1.8″, and 1″ hard disks with a ruler to show the length of platters and read-write heads.
Mainframe and minicomputer hard disks were of widely varying dimensions, typically in free standing cabinets the size of washing machines or designed to fit a
19" rack. 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. Thus hard disk drive
Form factors, initially followed those of 8-inch, 5.25-inch, and 3.5-inch floppy disk drives. Because there were no smaller floppy disk drives, smaller hard disk drive form factors developed from product offerings or industry standards.
- 8 inch: 9.5 in × 4.624 in × 14.25 in (241.3 mm × 117.5 mm × 362 mm)
In 1979, Shugart Associates' SA1000 was the first form factor compatible HDD, having the same dimensions and a compatible interface to the 8″ FDD.
- 5.25 inch: 5.75 in × 3.25 in × 8 in (146.1 mm × 82.55 mm × 203 mm)
This smaller form factor, first used in an HDD by Seagate in 1980,[53] was the same size as full-height 51⁄4-inch-diameter (130 mm) FDD, 3.25-inches high. This is twice as high as "half height"; i.e., 1.63 in (41.4 mm). Most desktop models of drives for optical 120 mm disks (DVD, CD) use the half height 5¼″ dimension, but it fell out of fashion for HDDs. The Quantum Bigfoot HDD was the last to use it in the late 1990s, with "low-profile" (≈25 mm) and "ultra-low-profile" (≈20 mm) high versions.
- 3.5 inch: 4 in × 1 in × 5.75 in (101.6 mm × 25.4 mm × 146 mm) = 376.77344 cm³
This smaller form factor is similar to that used in an HDD by Rodime in 1983,[54] which was the same size as the "half height" 3½″ FDD, i.e., 1.63 inches high. Today, the 1-inch high ("slimline" or "low-profile") version of this form factor is the most popular form used in most desktops.
- 2.5 inch: 2.75 in × 0.275–0.59 in × 3.945 in (69.85 mm × 7–15 mm × 100 mm) = 48.895–104.775 cm3
This smaller form factor was introduced by PrairieTek in 1988;[55] there is no corresponding FDD. It came to be widely used for hard disk drives in mobile devices (laptops, music players, etc.) and for solid-state drives, by 2008 replacing some 3.5 inch enterprise-class drives.[56] It is also used in the Playstation 3[57] and Xbox 360[citation needed] video game consoles. Drives 9.5 mm high became an unofficial standard for all except the largest-capacity laptop drives (usually having two platters inside); 12.5 mm-high drives, typically with three platters, are used for maximum capacity, but will not fit most laptop computers. Enterprise-class drives can have a height up to 15 mm.[58] Seagate released a 7mm drive aimed at entry level laptops and high end netbooks in December 2009.[59]
- 1.8 inch: 54 mm × 8 mm × 71 mm = 30.672 cm³
This form factor, originally introduced by Integral Peripherals in 1993, has evolved into the ATA-7 LIF with dimensions as stated. For a time it was increasingly used in digital audio players and subnotebooks, but its popularity decreased. There is a variant for 2–5GB sized HDDs that fit directly into a PC card expansion slot. These became popular for use in iPods and other HDD based MP3 players.
- 1 inch: 42.8 mm × 5 mm × 36.4 mm
This form factor was introduced in 1999 as IBM's Microdrive to fit inside a CF Type II slot. Samsung calls the same form factor "1.3 inch" drive in its product literature.[60]
- 0.85 inch: 24 mm × 5 mm × 32 mm
Toshiba announced this form factor in January 2004[61] for use in mobile phones and similar applications, including SD/MMC slot compatible HDDs optimized for video storage on 4G handsets. Toshiba manufactured a 4 GB (MK4001MTD) and an 8 GB (MK8003MTD) version[62][dead link] and holds the Guinness World Record for the smallest hard disk drive.[63]
3.5-inch and 2.5-inch hard disks were the most popular sizes as of 2012.
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,
[64][65] which has no moving parts.
While these sizes are customarily described by an approximately correct figure in inches, actual sizes have long been specified in millimeters.
[edit]Current hard disk form factors
| Form factor | Width (mm) | Height (mm) | Largest capacity | Platters (max) | Per platter (GB) |
|---|
| 3.5″ | 102 | 19 or 25.4 | 4 TB[66][67][68][69] (2011) | 5 | 1000 GB |
| 2.5″ | 69.9 | 7,[70] 9.5,[71] 12.5,[72] or 15 | 2 TB[66][73][74] (2012) | 4 | 500 GB |
| 1.8″ | 54 | 5 or 8 | 320 GB[75] (2009) | 2 | 160 GB |
[edit]Obsolete hard disk form factors
| Form factor | Width (mm) | Largest capacity | Platters (max) | Per platter (GB) |
|---|
| 5.25″ FH | 146 | 47 GB[76] (1998) | 14 | 3.36 GB |
| 5.25″ HH | 146 | 19.3 GB[77] (1998) | 4[78] | 4.83 GB |
| 1.3″ | 43 | 40 GB[79] (2007) | 1 | 40 GB |
| 1″ (CFII/ZIF/IDE-Flex) | 42 | 20 GB (2006) | 1 | 20 GB |
| 0.85″ | 24 | 8 GB[80][81] (2004) | 1 | 8 GB |
[edit]Performance characteristics
[edit]Access time
The factors that limit the time to access the data on a hard disk drive (
Access time) are mostly related to the mechanical nature of the rotating disks and moving heads.
Seek time is a measure of how long it takes the head assembly to travel to the track of the disk that contains data.
Rotational latency is incurred because the desired disk sector may not be directly under the head when data transfer is requested. These two delays are on the order of milliseconds each. The
bit rate or data transfer rate (once the head is in the right position) creates delay which is a function of the number of blocks transferred; typically relatively small, but can be quite long with the transfer of large contiguous files. Delay may also occur if the drive disks are stopped to save energy, see
Power management.
An HDD's
Average Access Time is its average
Seek time which technically is the time to do all possible seeks divided by the number of all possible seeks, but in practice is determined by statistical methods or simply approximated as the time of a seek over one-third of the number of tracks
[82]
Defragmentation is a procedure used to minimize delay in retrieving data by moving related items to physically proximate areas on the disk.
[83] Some computer operating systems perform defragmentation automatically. Although automatic defragmentation is intended to reduce access delays, the procedure can slow response when performed while the computer is in use.
[84]
Access time can be improved by increasing rotational speed, thus reducing latency and/or by decreasing seek time. Increasing areal density increases
throughput by increasing data rate and by increasing the amount of data under a set of heads, thereby potentially reducing seek activity for a given amount of data. Based on historic trends, analysts predict a future growth in HDD areal density (and therefore capacity) of about 40% per year.
[85] Access times have not kept up with throughput increases, which themselves have not kept up with growth in storage capacity.
[edit]Seek time
Average
seek time ranges from 3
ms[86] for high-end server drives, to 15 ms for mobile drives, with the most common mobile drives at about 12
ms[87] and the most common desktop type typically being around 9 ms. The
first HDD had an average seek time of about 600 ms and by the middle 1970s HDDs were available with seek times of about
25 ms. Some early PC drives used a
stepper motor to move the heads, and as a result had seek times as slow as 80–120 ms, but this was quickly improved by
voice coil type actuation in the 1980s, reducing seek times to around 20 ms. Seek time has continued to improve slowly over time.
Some desktop and laptop computer systems allow the user to make a tradeoff between seek performance and drive noise. Faster seek rates typically require more energy usage to quickly move the heads across the platter, causing loud noises from the pivot bearing and greater device vibrations as the heads are rapidly accelerated during the start of the seek motion and decelerated at the end of the seek motion. Quiet operation reduces movement speed and acceleration rates, but at a cost of reduced seek performance.
[edit]Rotational latency
Rotational speed
[rpm] | Average
latency [ms] |
|---|
| 15,000 | 2 |
| 10,000 | 3 |
| 7,200 | 4.16 |
| 5,400 | 5.55 |
| 4,800 | 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.
[edit]Data transfer rate
As of 2010, a typical 7,200 rpm desktop hard drive has a sustained "disk-to-
buffer" data transfer rate up to 1,030
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.
[edit]Power consumption
Power consumption has become increasingly important, not only in mobile devices such as laptops but also in server and desktop markets. Increasing data center machine density has led to problems delivering sufficient power to devices (especially for spin up), and getting rid of the waste heat subsequently produced, as well as environmental and electrical cost concerns (see
green computing). Heat dissipation is tied directly to power consumption, and as drives age, disk
failure rates increase at higher drive temperatures.
[90] Similar issues exist for large companies with thousands of desktop PCs. Smaller form factor drives often use less power than larger drives. One interesting development in this area is actively controlling the seek speed so that the head arrives at its destination only just in time to read the sector, rather than arriving as quickly as possible and then having to wait for the sector to come around (i.e. the rotational latency).
[91] Many of the hard drive companies are now producing Green Drives that require much less power and cooling. Many of these Green Drives spin slower (<5,400 rpm compared to 7,200, 10,000 or 15,000 rpm) thereby generating less heat. Power consumption can also be reduced by parking the drive heads when the disk is not in use reducing friction, adjusting spin speeds,
[92] and disabling internal components when not in use.
[93]
Drives use more power, briefly, when starting up (spin-up). Although this has little direct effect on total energy consumption, the maximum power demanded from the power supply, and hence its required rating, can be reduced in systems with several drives by controlling when they spin up.
- On SCSI hard disk drives, the SCSI controller can directly control spin up and spin down of the drives.
- Some Parallel ATA (PATA) and Serial ATA (SATA) hard disk drives support power-up in standby or PUIS: each drive does not spin up until the controller or system BIOS issues a specific command to do so. This allows the system to be set up to stagger disk start-up and limit maximum power demand at switch-on.
- Some SATA II and later hard disk drives support staggered spin-up, allowing the computer to spin up the drives in sequence to reduce load on the power supply when booting.[94]
[edit]Power management
Most hard disk drives today support some form of power management which uses a number of specific power modes that save energy by reducing performance. When implemented an HDD will change between a full power mode to one or more power saving modes as a function of drive usage. Recovery from the deepest mode, typically called Sleep, may take as long as several seconds.
[95]
[edit]Audible noise
Measured in
dBA, audible noise is significant for certain applications, such as
DVRs, digital audio recording and
quiet computers. Low-noise disks typically use
fluid bearings, slower rotational speeds (usually 5,400 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.
[edit]Shock resistance
Shock resistance is especially important for mobile devices. Some laptops now include
active hard drive protection that parks the disk heads if the machine is dropped, hopefully before impact, to offer the greatest possible chance of survival in such an event. Maximum shock tolerance to date is 350
g for operating and 1,000 g for non-operating.
[96]
[edit]Disk interface families used in personal computers
Several Parallel ATA hard disk drives
Historical bit serial interfaces connect a hard disk drive (HDD) to a hard disk controller (HDC) with two cables, one for control and one for data. (Each drive also has an additional cable for power, usually connecting it directly to the power supply unit). The HDC provided significant functions such as serial/parallel conversion, data separation, and track formatting, and required matching to the drive (after formatting) in order to assure reliability. Each control cable could serve two or more drives, while a dedicated (and smaller) data cable served each drive.
- ST506 used MFM (Modified Frequency Modulation) for the data encoding method.
- ST412 was available in either MFM or RLL (Run Length Limited) encoding variants.
- Enhanced Small Disk Interface (ESDI) was an industry standard interface similar to ST412 supporting higher data rates between the processor and the disk drive.
Modern
bit serial interfaces connect a hard disk drive to a host bus interface adapter (today typically integrated into the "
south bridge") with one data/control cable. (As for historical
bit serial interfaces above, each drive also has an additional power cable, usually direct to the power supply unit.)
- Fibre Channel (FC) is a successor to parallel SCSI interface on enterprise market. It is a serial protocol. In disk drives usually the Fibre Channel Arbitrated Loop (FC-AL) connection topology is used. FC has much broader usage than mere disk interfaces, and it is the cornerstone of storage area networks (SANs). Recently other protocols for this field, like iSCSI and ATA over Ethernet have been developed as well. Confusingly, drives usually use coppertwisted-pair cables for Fibre Channel, not fibre optics. The latter are traditionally reserved for larger devices, such as servers or disk array controllers.
- Serial ATA (SATA). The SATA data cable has one data pair for differential transmission of data to the device, and one pair for differential receiving from the device, just like EIA-422. That requires that data be transmitted serially. A similar differential signaling system is used in RS485, LocalTalk, USB, Firewire, and differential SCSI.
- Serial Attached SCSI (SAS). The SAS is a new generation serial communication protocol for devices designed to allow for much higher speed data transfers and is compatible with SATA. SAS uses a mechanically identical data and power connector to standard 3.5-inch SATA1/SATA2 HDDs, and many server-oriented SAS RAID controllers are also capable of addressing SATA hard drives. SAS uses serial communication instead of the parallel method found in traditional SCSI devices but still uses SCSI commands.
Word serial interfaces connect a hard disk drive to a host bus adapter (today typically integrated into the "
south bridge") with one cable for combined data/control. (As for all
bit serial interfaces above, each drive also has an additional power cable, usually direct to the power supply unit.) The earliest versions of these interfaces typically had a 8 bit parallel data transfer to/from the drive, but 16-bit versions became much more common, and there are 32 bit versions. Modern variants have serial data transfer. The word nature of data transfer makes the design of a host bus adapter significantly simpler than that of the precursor HDD controller.
- Integrated Drive Electronics (IDE), later standardized under the name AT Attachment, with the alias P-ATA or PATA (Parallel ATA) retroactively added upon introduction of the new variant Serial ATA. The original name reflected the integration of the controller with the hard drive itself. (That integration was not new with IDE, having been done a few years earlier with SCSI drives.) Moving the HDD controller from the interface card to the disk drive helped to standardize the host/contoller interface, reduce the programming complexity in the host device driver, and reduced system cost and complexity. The 40-pin IDE/ATA connection transfers 16 bits of data at a time on the data cable. The data cable was originally 40-conductor, but later higher speed requirements for data transfer to and from the hard drive led to an "ultra DMA" mode, known as UDMA. Progressively swifter versions of this standard ultimately added the requirement for an 80-conductor variant of the same cable, where half of the conductors provides grounding necessary for enhanced high-speed signal quality by reducing cross talk. The interface for 80-conductor only has 39 pins, the missing pin acting as a key to prevent incorrect insertion of the connector to an incompatible socket, a common cause of disk and controller damage.
- EIDE was an unofficial update (by Western Digital) to the original IDE standard, with the key improvement being the use of direct memory access (DMA) to transfer data between the disk and the computer without the involvement of the CPU, an improvement later adopted by the official ATA standards. By directly transferring data between memory and disk, DMA eliminates the need for the CPU to copy byte per byte, therefore allowing it to process other tasks while the data transfer occurs.
- Small Computer System Interface (SCSI), originally named SASI for Shugart Associates System Interface, was an early competitor of ESDI. SCSI disks were standard on servers, workstations, Commodore Amiga, and Apple Macintosh computers through the mid-1990s, by which time most models had been transitioned to IDE (and later, SATA) family disks. Only in 2005 did the capacity of SCSI disks fall behind IDE disk technology, though the highest-performance disks are still available in SCSI, SAS and Fibre Channel only. The range limitations of the data cable allows for external SCSI devices. Originally SCSI data cables used single ended (common mode) data transmission, but server class SCSI could use differential transmission, eitherlow voltage differential (LVD) or high voltage differential (HVD). ("Low" and "High" voltages for differential SCSI are relative to SCSI standards and do not meet the meaning of low voltage and high voltage as used in general electrical engineering contexts, as apply e.g. to statutory electrical codes; both LVD and HVD use low voltage signals (3.3 V and 5 V respectively) in general terminology.)
| Acronym or abbreviation | Meaning | Description |
|---|
| SASI | Shugart Associates System Interface | Historical predecessor to SCSI. |
| SCSI | Small Computer System Interface | Bus oriented that handles concurrent operations. |
| SAS | Serial Attached SCSI | Improvement of SCSI, uses serial communication instead of parallel. |
| ST-506 | Seagate Technology | Historical Seagate interface. |
| ST-412 | Seagate Technology | Historical Seagate interface (minor improvement over ST-506). |
| ESDI | Enhanced Small Disk Interface | Historical; backwards compatible with ST-412/506, but faster and more integrated. |
| ATA [(PATA)Parallel Advanced Technology Attachment] | 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. |
[edit]Recovery of data from failed drive
Data from a failed drive can sometimes be partially or totally
recovered if the platters' magnetic coating is not totally destroyed. Specialised companies carry out data recovery, at significant cost, by opening the drives in a
clean room and using appropriate equipment to read data from the platters directly. If the electronics have failed, it is sometimes possible to replace the electronics board, though often drives of nominally exactly the same model manufactured at different times have different, incompatible, circuit boards.
Sometimes operation can be restored for long enough to recover data. Risky techniques are justifiable if the drive is otherwise dead. If a drive is started up once it may continue to run for a shorter or longer time but never start again, so as much data as possible is recovered as soon as the drive starts. A 1990s drive that does not start due to stiction can sometimes be started by tapping it or rotating the body of the drive rapidly by hand. Another technique which is sometimes known to work is to cool the drive, in a waterproof wrapping, in a domestic freezer. There is much useful information about this in blogs and forums,
[100] but professionals also resort to this method with some success.
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External removable drives
External removable hard disk drives
[114] typically connect via
USB.
Plug and play drive functionality offers system compatibility, and features large storage options and portable design. External hard disk drives are available in 2.5" and 3.5" sizes, and as of March 2012 their capacities generally range from 160GB to 2TB. Common sizes are 160GB, 250GB, 320GB, 500GB, 640GB, 1TB, and 2TB.
[115][116]
External hard disk drives are available as preassembled integrated products, or may be assembled by combining an external enclosure (with USB or other interface) with a separately-purchased drive.
Features such as biometric security or multiple interfaces are available at a higher cost.
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