Hard disk drive

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Hard Disk Drive

An IBM hard disk drive with the metal cover removed. The platters are highly reflective. A screwdriver bit is placed into one of six screws that clamp the stack of platters and spacers. In the center, below the screws and clamping plate, is the motor that spins the platters.
Date Invented: / September 13, 1956
Invented By: / An IBM team led by Rey Johnson
Connects to:
  • Host adapter (on PCs often integrated into motherboard) via one of
  • PATA (IDE) interface
  • SATA interface
  • SAS interface
  • SCSI interface (popular on servers)
  • FC interface (almost exclusively found on servers)

Market Segments:
  • Desktop
  • Mobile
  • Enterprise
  • Consumer
  • Other/Miscellaneous

A hard disk drive (HDD), commonly referred to as a hard drive, hard disk or fixed disk drive,[1] is a non-volatile storage device which stores digitally encoded data on rapidly rotating platters with magnetic surfaces. Strictly speaking, "drive" refers to a device distinct from its medium, such as a tape drive and its tape, or a floppy disk drive and its floppy disk. Early HDDs had removable media; however, an HDD today is typically a sealed unit (except for a filtered vent hole to equalize air pressure) with fixed media.[2]

A HDD is a rigid-disk drive, although it is probably never referred to as such. By way of comparison, a so-called "floppy" drive (more formally, a diskette drive) has a disc that is flexible. Originally, the term "hard" was temporary slang, substituting "hard" for "rigid", before these drives had an established and universally-agreed-upon name. Some time ago, IBM's internal company term for an HDD was "file".

HDDs (introduced in 1956 as data storage for an IBM accounting computer[3]) were originally developed for use with computers, see History of hard disk drives.

In the 21st century, applications for HDDs have expanded beyond computers to include digital video recorders, digital audio players, personal digital assistants, digital cameras and video game consoles. In 2005 the first mobile phones to include HDDs were introduced by Samsung and Nokia.[4] The need for large-scale, reliable storage, independent of a particular device, led to the introduction of configurations such as RAID arrays, network attached storage (NAS) systems and storage area network (SAN) systems that provide efficient and reliable access to large volumes of data.

Contents

[hide]
  • 1Technology
  • 1.1Architecture
  • 2Capacity and access speed
  • 2.1Capacity measurements
  • 3Form factors
  • 4Other characteristics
  • 5Access and interfaces
  • 5.1Disk interface families used in personal computers
  • 6Integrity
  • 6.1Landing zones and load/unload technology
  • 6.2Disk failures and their metrics
  • 7Manufacturers
  • 8See also
  • 9Notes and References
  • 10External links

[edit]Technology

HDDs record data by magnetizing ferromagnetic material directionally, to represent either a 0 or a 1 binary digit. They read the data back by detecting the magnetization of the material. A typical HDD design consists of a spindle which holds one or more flat circular disks called platters, onto which the data is recorded. The platters are made from a non-magnetic material, usually aluminum alloy or glass, and are coated with a thin layer of magnetic material. Older disks used iron(III) oxide as the magnetic material, but current disks use a cobalt-based alloy.

A cross section of the magnetic surface in action. In this case the binary data is encoded using frequency modulation:

The platters are spun at very high speeds (details follow). Information is written to 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. 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 older designs) a stepper motor. Stepper motors were outside the head-disk chamber, and preceded voice-coil drives. The latter, for a while, had a structure similar to that of a loudspeaker; the coil and heads moved in a straight line, along a radius of the platters. The present-day structure differs in several respects from that of the earlier voice-coil drives, but the same interaction between the coil and magnetic field still applies, and the term is still used.

Older drives read the data on the platter by sensing the rate of change of the magnetism in the head; these heads had small coils, and worked (in principle) much like magnetic-tape playback heads, although not in contact with the recording surface. 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 that in earlier types, and was dubbed "giant" magnetoresistance (GMR). This refers to the degree of effect, not the physical size, of the head — the heads themselves are extremely tiny, and are too small to be seen without a microscope. GMR read heads are now commonplace.[citation needed]

HD heads are kept from contacting the platter surface by the air that is extremely close to the platter; that air moves at, or close to, the platter speed.[citation needed] 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. It's a type of air bearing.

The magnetic surface of each platter is conceptually divided into many small sub-micrometre-sized magnetic regions, each of which is used to encode a single binary unit of information. In today's HDDs, each of these magnetic regions is composed of a few hundred magnetic grains. Each magnetic region forms a magnetic dipole which generates a highly localized magnetic field nearby. The write head magnetizes a region by generating a strong local magnetic field. Early HDDs used an electromagnet both to generate this field and to read the data by using electromagnetic induction. Later versions of inductive heads included metal in Gap (MIG) heads and thin film heads. 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.[5]

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-thick layer of the non-magnetic element ruthenium, and the two layers are magnetized in opposite orientation, thus reinforcing each other.[6] Another technology used to overcome thermal effects to allow greater recording densities is perpendicular recording, which has been used in many hard drives as of 2007[7][8][9].

See File System for how operating systems access data on HDDs and other storage devices.

[edit]Architecture

A hard disk drive with the platters and spindle motor hub removed showing the copper colored motor coils surrounding a bearing at the center of the spindle motor.

The motor has an external rotor; the stator windings are copper-colored. The spindle bearing is in the center. To the left of center is the actuator with a read-write head under the tip of its very end (near center); the orange stripe along the side of the arm, a thin printed-circuit cable, connects the read-write head to the hub of the actuator. The flexible, somewhat 'U'-shaped, ribbon cable barely visible below and to the left of the actuator arm is the flexible section, one end on the hub, that continues the connection from the head to the controller board on the opposite side.

The head support arm is very light, but also rigid; in modern drives, acceleration at the head reaches 250 g's.

The silver-colored structure at the upper left is the top plate of the permanent-magnet and moving coil "motor" that swings the heads to the desired position. Beneath this plate is the moving coil, attached to the actuator hub, and beneath that is a thin neodymium-iron-boron (NIB) high-flux magnet. That magnet is mounted on the bottom plate of the "motor".

The 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's wound on a form, making it self-supporting. Much of the coil, sides of the arrowhead, which points to the actuator bearing center, interacts with the magnetic field to develop a tangential force to rotate the actuator. Considering that current flows (at a given time) radially outward along one side of the arrowhead, and radially inward on the other, the surface of the magnet is half N pole, half S pole; the dividing line is midway, and radial.

[edit]Capacity and access speed

PC hard disk drive capacity (in GB). The vertical axis is logarithmic, so the fit line corresponds to exponential growth.

Using rigid disks and sealing the unit allows much tighter tolerances than in a floppy disk drive. Consequently, hard disk drives can store much more data than floppy disk drives and can access and transmit it faster. As of January 2008:

  • A typical desktop HDD, might store between 120 and 300 GB of data (based on US market data[10]), rotate at 7,200 revolutions per minute (RPM) and have a media transfer rate of 1 Gbit/s or higher. (1 GB = 109 B; 1 Gbit/s = 109 bit/s)
  • The highest capacity HDDs are 1 TB[11].
  • The fastest “enterprise” HDDs spin at 10,000 or 15,000 rpm, and can achieve sequential media transfer speeds above 1.6 Gbit/s.[12] Drives running at 10,000 or 15,000 rpm use smaller platters because of air drag and therefore generally have lower capacity than the highest capacity desktop drives.
  • Mobile, i.e., laptop HDDs, which are physically smaller than their desktop and enterprise counterparts, tend to be slower and have less capacity. A typical mobile HDD spins at 5,400 rpm, with 7,200 rpm models available for a slight price premium. Because of the smaller disks, mobile HDDs generally have lower capacity than the highest capacity desktop drives.

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.[13] 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, email, and streaming video (Google, Yahoo!, YouTube, etc.).

The main way to decrease access time is to increase rotational speed, while the main way to increase throughput and storage capacity is to increase areal density. A vice president of Seagate Technology projects a future growth in disk density of 40% per year.[14]Access times have not kept up with throughput increases, which themselves have not kept up with growth in storage capacity.

The first 3.5" HDD marketed as able to store 1 TB was the Hitachi Deskstar 7K1000. It contains five platters at approximately 200 GB each, providing 935.5 GiB of usable space.[15] Hitachi has since been joined by Samsung (Samsung SpinPoint F1, which has 3 × 334 GB platters), Seagate and Western Digital in the 1 TB drive market.[16][17]

Form factor / Width / Largest capacity / Platters (Max)
5.25" FH / 146 mm / 47 GB[18] (1998) / 14
5.25" HH / 146 mm / 19.3 GB[19] (1998) / 4[20]
3.5" / 102 mm / 1 TB[15] (2007) / 5
2.5" / 69.9 mm / 500 GB[21] (2008) / 3
1.8" (PCMCIA) / 54 mm / 160 GB[22] (2007)
1.8" (ATA-7 LIF) / 53.8 mm
1.3" / 36.4 mm / 40 GB[23] (2008) / 1

[edit]Capacity measurements

/ It has been suggested that Disk Overhead be merged into this article or section. (Discuss)

A disassembled and labeled 1997 hard drive.

The capacity of an HDD can be calculated by multiplying the number of cylinders by the number of heads by the number of sectors by the number of bytes/sector (most commonly 512). Drives with ATA interface bigger and more than eight gigabytes behave as if they were structured into 16383 cylinders, 16 heads, and 63 sectors, for compatibility with older operating systems. Unlike in the 1980s, the cylinder, head, sector (C/H/S) counts reported to the CPU by a modern ATA drive are no longer actual physical parameters since the reported numbers are constrained by historic operating-system interfaces and with zone bit recording the actual number of sectors varies by zone. Disks with SCSI interface address each sector with a unique integer number; the operating system remains ignorant of their head or cylinder count.

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 drive has anywhere near 32 platters.

Hard disk drive manufacturers specify disk capacity using the SI prefixesmega-, giga- and tera-, and their abbreviations M, G and T. Byte is typically abbreviated B.

Most operating-system tools report capacity using the same abbreviations but actually use binary prefixes. For instance, the prefix mega-, which normally means 106 (1,000,000), in the context of data storage can mean 220 (1,048,576), which is nearly 5% more. Similar usage has been applied to prefixes of greater magnitude. This results in a discrepancy between the disk manufacturer's stated capacity and the apparent capacity of the drive when examined through most operating-system tools. The difference becomes even more noticeable (7%) for a gigabyte. For example, Microsoft Windows reports disk capacity both in decimal-based units to 12 or more significant digits and with binary-based units to three significant digits. Thus a disk specified by a disk manufacturer as a 30 GB disk might have its capacity reported by Windows 2000 both as "30,065,098,568 bytes" and "28.0 GB". The disk manufacturer used the SI definition of "giga", 109 to arrive at 30 GB; however, because the utilities provided by Windows, Mac and some Linux distributions define a gigabyte as 1,073,741,824 bytes (230 bytes, often referred to as a gibibyte, or GiB), the operating system reports capacity of the disk drive as (only) 28.0 GB.