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Hard disk

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A hard disk drive is a type of storage device made up of hard disk platters, a spindle, read and write heads, read and write arms, electrical motors, and integrated electronics contained inside an airtight enclosure.

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Hard vs. floppy

Using rigid platters and sealing the unit allows much tighter tolerances that in a floppy disk. Consequently, hard disks can store much more data than floppy disk, and access and transmit it faster. In 2003, a typical workstation hard disk might store between 60 GB and 120 GB of data, rotate at 5400 to 10,000 rpm (revolutions per minute), and have an average transfer rate of about 30 MByte/sec.

Hard disk hardware

Typical hard drives of the mid-1990s.
(show full size)

The disk drive is a type of disk storage that stores and retrieves digital data from a planar magnetic surface. Information is written to the disk by transmitting an electromagnetic flux through an antenna or write head that is very close to a magnetically polarizable material that changes its polarization due to the flux. The information can be read back in a reverse manner, as the magnetic fields cause electrical change in the coil or read head that passes over it.

A typical hard disk drive design consists of a central axis or spindle upon which the platters spin at a constant speed. Moving along and between the platters on a common armature are the read-write heads, with one head for each platter face. The armature moves the heads radially across the platters as they spin, allowing each head access to the entirety of its platter.

The integrated electronics[?] control the movement of the read-write armature and the rotation of the disk, and perform reads and writes upon demand from the disk controller. Some modern drive electronics are capable of scheduling reads and writes efficiently across the disk, and of remapping sectors of the disk which have failed.

The sealed enclosure protects the drive internals from dust, condensation, and other sources of contamination. Any contamination of the read-write heads or disk platters can lead to a head crash—a failure of the disk in which the head scrapes across the platter surface, grinding away the thin magnetic film. Head crashes can also be caused by electronic failure, wear and tear, or poorly manufactured disks.

Access and interfaces

A hard disk is generally accessed over one of a number of bus types, including ATA (IDE, EIDE), SCSI, FireWire/IEEE 1394, and Fibre Channel. From late 2002 Serial ATA was introduced to improve upon the performance of ATA.


There are three primary factors that determine hard drive performance: seek time, latency and data transfer rate, plus several subsiduary factors:

  • Seek time is a measure of the speed with which the drive can position its read/write heads over any particular data track. Because neither the starting position of the head nor the distance from there to the desired track is fixed, seek time varies greatly, and it is almost always measured as an average seek time, though full-track (the longest possible) and track-to-track (the shortest possible) seeks are also quoted sometimes. The standard way to measure seek time is to time a large number of disk accesses to random locations, subtract the latency (see below) and take the mean. Note, however, that two different drives with identical average seek times can display quite different performance characteristics. Seek time is always measured in milliseconds (ms), and often regarded as the single most important determinant of drive performance, though this claim is hotly debated. (More on seek time.)

  • All drives have rotational latency: the time that elapses between the moment when the read/write head settles over the desired data track and the moment when the first byte of the required data appeares under the head. For any individual read or write operation, latency is random between zero (if the first data sector happpens to be directly under the head at the exact moment that the head is ready to begin reading or writing) or anything up to the full rotational period of the drive (for a typical 7200 RPM drive, just under 8.4ms). However, on average, latency is always equal to one half of the rotational period. Thus, all 5400 RPM drives of any make or model have 5.56ms latency; all 7200 RPM drives, 4.17ms; and all 15,000 RPM drives have 2.0ms latency. Like seek time, latency is considered a critical performance factor and is always measured in milliseconds. (More on latency.)

  • Access time is simply the sum of the seek time and the latency. It is important not to mistake seek time figures for access time figures!

  • The internal data rate is the speed with which the drive's internal read channel can transfer data from the magnetic media. (Or, less commonly, in the reverse direction.) Previously a very important factor in drive performance, it remains significant but less so than in prior years, as all modern drives have very high internal data rates. Internal data rates are normally measured in Megabits per second (Mb/s—note the lower-case "b".)

  • The external data rate is the speed with which the drive can transfer data from its buffer to the host computer system. Although in theory this is vital, in practice it is usually a non-issue. It is a relatively trivial matter to design an electronic interface capable of outpacing any possible mechanical read/write mechanism, and it is routine for computer makers to include a hard drive controller interface that is significantly faster than the drive it will be attached to. As a general rule, modern ATA and SCSI interfaces are capable of dealing with at least twice as much data as any single drive can deliver; they are, after all, designed to handle two or more drives per bus even though a consumer desktop computer almost always mounts only one. For a single-drive computer, the difference between ATA-100 and ATA-133, for example, is primarily one of marketing rather than performance. No drive yet manufactured can utilise the full bandwidth of an ATA-100 interface, and few are able to send more data than an ATA-66 interface can accept. The external data rate is usually measured in Megabytes per second. (MB/s—note the upper-case "B".)

  • Command overhead is the time it takes the drive electronics to interpret instructions from the host computer and issue commands to the read/write mechanism. In modern drives it is negligible.

Operating system use of hard disks hard disk drive partitioning, master boot record, drive letter assignment

addressing modes

There are two modes of addressing the data blocks on the hard disk. The older one is the CHS addressing (Cylinder-Head-Sector) and the more recent one the LBA (Logical Block Addressing). In the LBA-mode the hard disk is like an array of blocks for the programmer of an operating system. The calculation which cylinder and head to use and which sector to read is done by the firmware of the hard disk controller.

Manufacturers Most of the world's hard disks are manufactured by a handful of firms: Seagate, Maxtor, Western Digital, Samsung[?], and the former drive manufacturing division of IBM, now sold to Hitachi. Fujitsu continue to make specialist SCSI drives but exited the mass market in 2001. Toshiba are a major maker of 2.5 inch notebook drives.
Dozens of former hard drive manufacturers have gone out of business, merged, or closed their hard drive divisions, notably Conner (now part of Seagate), Quantum (now a tape drive specialist with the hard drive division sold to Maxtor), Micropolis and JTS (both closed down), and Miniscribe (part of Maxtor).

Further reading

All Wikipedia text is available under the terms of the GNU Free Documentation License

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