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HDD
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HDDrive
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An IBM hard disk drive with the metal cover removed. The platters are highly reflective. Date Invented: September 13, 1956 Invented By: Reynold Johnson Connects to: Motherboard via one of PATA SATA SCSI External enclosure via one of USB 1.1/ 2 FireWire 400/ 800 eSATA Form Factor: Desktop Laptop The inside of a hard disk drive displaying the read/write head traveling over the top platterA hard disk drive (HDD, also commonly shortened to hard drive and formerly known as a fixed disk) is a digitally encoded non-volatile storage device which stores data on rapidly rotating platters with magnetic surfaces. Strictly speaking, "drive" refers to an entire unit containing multiple platters, a read/write head assembly, driver electronics, and motor while "hard disk" (sometimes "platter") refers to the storage medium itself.

Hard disks were originally developed for use with computers. In the 21st century, applications for hard disks have expanded beyond computers to include video recorders, audio players, digital organizers, and digital cameras. In 2005 the first cellular telephones to include hard disks were introduced by Samsung and Nokia. The need for large-scale, reliable storage, independent of a particular device, led to the introduction of configurations such as RAID, hardware such as network attached storage (NAS) devices, and systems such as storage area networks (SANs) for efficient access to large volumes of data.

Contents [hide] 1 Technology 2 Capacity 2.1 Capacity measurements 3 History 4 Hard disk characteristics 5 Integrity 5.1 Landing zones 6 Access and interfaces 6.1 Disk families used in personal computers 7 Manufacturers 8 See also 9 References 10 External links

[edit] Technology The inside of a hard disk drive with the disk itself removed. To the left is the read-write arm. In the middle the electromagnets of the platter's motor can be seen.Hard disks record data by magnetizing a magnetic material in a pattern that represents the data. They read the data back by detecting the magnetization of the material. A typical hard disk 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 glass or aluminum, 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.

The platters are spun at very high speeds. Information is written to a platter as it rotates past mechanisms called read-and-write heads that fly very close 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 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.

A cross section of the magnetic surface in action. In this case the binary data encoded using frequency modulation.The magnetic surface of each platter is 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 hard disks each of these magnetic regions is composed of a few hundred magnetic grains. Each magnetic region forms a magnetic dipole which generates a highly localised magnetic field nearby. The write head magnetizes a magnetic region by generating a strong local magnetic field nearby. Early hard disks used the same inductor that was used to read the data as an electromagnet to create this field. Later, metal in Gap (MIG) heads were used, and today thin film heads are common. With these later technologies, the read and write head are separate mechanisms, but are on the same actuator arm.

Hard disks have a mostly sealed enclosure that protects the disk internals from dust, condensation, and other sources of contamination. The hard disk's read-write heads fly on an air bearing which is a cushion of air only nanometers above the disk surface. The disk surface and the disk's internal environment must therefore be kept immaculate to prevent damage from fingerprints, hair, dust, smoke particles and such, given the sub-microscopic gap between the heads and disk.

Using rigid platters and sealing the unit allows much tighter tolerances than in a floppy disk. Consequently, hard disks can store much more data than floppy disk and access and transmit it faster. In 2006, a typical workstation hard disk might store between 80 GB and 250 GB of data, rotate at 7,200 to 10,800 revolutions per minute (RPM), and have a sequential media transfer rate of over 50 MB/s. The fastest workstation and server hard disks spin at 15,000 RPM, and can achieve sequential media transfer speeds up to and beyond 80 MB/s. Laptop hard disks, which are physically smaller than their desktop counterparts, tend to be slower and have less capacity. Most spin at only 4,200 RPM or 5,400 RPM, whereas the newest top models spin at 7,200 RPM.

[edit] Capacity The capacity of hard disks has grown dramatically over time. The first commercial disk, the IBM RAMAC introduced in 1956, stored 5 million characters (about 5 megabytes) on fifty 24-inch diameter disks. (See early IBM disk storage.) With early personal computers in the 1980s, a disk with a 20 megabyte capacity was considered large. In the latter half of the 1990s, hard disks with capacities of 1 gigabyte and greater became available. As of 2006, the "smallest" desktop hard disk still in production has a capacity of 20 gigabytes, while the largest-capacity internal disks are a 3/4 terabyte (768 gigabytes), with external disks at or exceeding one terabyte by using multiple internal disks. These new internal disks increased their storage capacities with perpendicular recording.

The exponential increases in disk space and data access times for hard disks has enabled the commercial viability of consumer products that require large storage capacities, such as the Apple iPod digital music player, the TiVo personal video recorder, and web-based email programs.[1] This is also gradually but significantly altering how programmers think; in many programming tasks there is a time-space tradeoff, so as space becomes cheaper and cheaper relative to CPU cycles the appropriate choice about time versus space changes. For instance in database work it is now common practice to store precomputed views, transitive closures, and the like on disk in order to speed up queries; 20 years ago such profligate use of disk space would have been impractical.

A vice president of Seagate projects a future growth in disk density of 40% per year.[1] Access times have not kept up with throughput increases, which themselves haven't kept up with growth in storage capacity. The main way to increase either is to increase the number of read-write heads in a hard disk. Since flying heads are the most expensive component of hard disks, increasing their number per hard disk wouldn't help the situation. Currently, the most promising way to reduce access times and increase throughput are to replace rotating disks with nonvolatile random access memory (NVRAM) or, possibly, holographic technology.

[edit] Capacity measurements Hard disk manufacturers typically specify disk capacity using the SI definition of the prefixes "mega" and "giga." This is largely for historical reasons. Disks with multi-million byte capacity have been used since 1956, long before there were standard binary prefixes. (The IEC only standardized binary prefixes in 1999.) Many practitioners early on in the computer and semiconductor industries used the prefix kilo to describe 210 (1024) bits, bytes or words because 1024 is "close enough" to 1000. Similar usage has been applied to the prefixes "mega," "giga," "tera," and even "peta." Often this non-SI conforming usage is noted by a qualifier such as "1 kB = 1,024 bytes" but the qualifier is sometimes omitted, particularly in marketing literature.

Operating systems, such as Microsoft Windows, frequently report capacity using the binary interpretation of the prefixes, which results in a discrepancy between the disk manufacturer's stated capacity and what the system reports. The difference becomes much more noticeable in the multi-gigabyte range. For example, Microsoft's Windows 2000 reports disk capacity both in decimal to 12 or more significant digits and with binary prefixes to 3 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. However utilities provided by Windows define a gigabyte as 230, or 1,073,741,824, bytes, so the reported capacity of the disk will be closer to 28.0 GB. For this reason, many utilities that report capacity have begun to use the aforementioned IEC standard binary prefixes (e.g. KiB, MiB, GiB) since their definitions are unambiguous.

Some people mistakenly attribute the discrepancy in reported and specified capacities to reserved space used for file system and partition accounting information. However, for large (several GiB) filesystems, this data rarely occupies more than a few MiB, and therefore cannot possibly account for the apparent "loss" of tens of GBs.

The capacity of a hard disk 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).

[edit] History Main article: history of hard disks IBM 62PC "Piccolo" HDD, circa 1979 - an early 8" diskFor many years, hard disks were large, cumbersome devices, more suited to use in the protected environment of a data center or large office than in a harsh industrial environment (due to their delicacy), or small office or home (due to their size and power consumption). Before the early 1980s, most hard disks had 8-inch (20 cm) or 14-inch (35 cm) platters, required an equipment rack or a large amount of floor space (especially the large removable-media disks, which were often referred to as "washing machines"), and in many cases needed high-current or even three-phase power hookups due to the large motors they used. Because of this, hard disks were not commonly used with microcomputers until after 1980, when Seagate Technology introduced the ST-506, the first 5.25-inch hard disk, with a capacity of 5 megabytes. In fact, in its factory configuration, the original IBM PC (IBM 5150) was not equipped with a hard disk.

Most microcomputer hard disks in the early 1980s were not sold under their manufacturer's names, but by OEMs as part of larger peripherals (such as the Corvus Disk System and the Apple ProFile). The IBM PC/XT had an internal hard disk, however, and this started a trend toward buying "bare" disks (often by mail order) and installing them directly into a system. Hard disk makers started marketing to end users as well as OEMs, and by the mid-1990s, hard disks had become available on retail store shelves.

While internal disks became the system of choice on PCs, external hard disks remained popular for much longer on the Apple Macintosh and other platforms. Every Mac made between 1986 and 1998 has a SCSI port on the back, making external expansion easy. External SCSI disks were also popular with older microcomputers such as the Apple II series, and were also used extensively in servers, a usage which is still popular today. The appearance in the late 1990s of high-speed external interfaces such as USB and FireWire has made external disk systems popular among PC users once again, especially for users who move large amounts of data between two or more locations, and most hard disk makers now make their disks available in external cases.

[edit] Hard disk characteristics 5.25" MFM 110 MB hard disk (2.5" IDE 6495 MB hard disk, US & UK pennies for comparison)Capacity, usually quoted in gigabytes. (older hard disks used to quote their smaller capacities in megabytes) Physical size, usually quoted in inches: Almost all hard disks today are of either the 3.5" or 2.5" varieties, used in desktops and laptops, respectively. 2.5" disks are usually slower and have less capacity but use less power and are more tolerant of movement. An increasingly common size is the 1.8" disks used in portable MP3 players and subnotebooks, which have very low power consumption and are highly shock-resistant. Additionally, there is the 1" form factor designed to fit the dimensions of CF Type II, which is also usually used as storage for portable devices including digital cameras. 1" was a de facto form factor led by IBM's Microdrive, but is now generically called 1" due to other manufacturers producing similar products. There is also a 0.85" form factor produced by Toshiba for use in mobile phones and similar applications. The size designations can be slightly confusing, for example a 3.5" disk has a case that is 4" wide. Furthermore, server-class hard disks also come in both 3.5" and 2.5" form factors. Reliability, usually given in terms of Mean Time Between Failures (MTBF): SATA 1.0 disks support speeds up to 10,000 rpm and MTBF levels up to 1 million hours under an eight-hour, low-duty cycle. Fibre Channel (FC) disks support up to 15,000 rpm and an MTBF of 1.4 million hours under a 24-hour duty cycle. Number of I/O operations per second: Modern disks can perform around 50 random access or 100 Sequential access operations per second. Power consumption (especially important in battery-powered laptops). audible noise in dBA (although many still report it in bels, not decibels). G-shock rating (surprisingly high in modern disks). Transfer Rate: Inner Zone: from 44.2 MB/s to 74.5 MB/s. Outer Zone: from 74.0 MB/s to 111.4 MB/s. Random access time: from 5 ms to 15 ms. —The preceding unsigned comment was added by Srinivasreddygujjula (talk • contribs) 04:26, 7 December 2006 (UTC).