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Hard disk

A hard disk (commonly known as a HDD (hard disk drive) or 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 digital video recorders, digital audio players, personal digital assistants, and digital cameras. In 2005 the first mobile phones to include hard disks were introduced by Samsung Group 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

Technology

A cross section of the magnetic surface in action. In this case the binary data encoded using frequency modulation.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 (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 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.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, m e t al 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 750 GB of data (as of local US market by December 2006), rotate at 7,200 to 10,800 revolutions per minute (RPM), and have a sequential media transfer rate of over 100 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 300 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.

Capacity

PC hard disk capacity (in GB). The plot is logarithmic, so the fit line corresponds to exponential growth.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 platters. (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 lowest-capacity desktop computer hard disk still in production has a capacity of 20 gigabytes, while the largest-capacity internal disks are a 3/4 terabyte (768 gigabytes) on 4 platters. On January 5, 2007, Hitachi Global Storage Technologies announced they would ship a five platter, 1 terabytes (1000 gigabytes) hard disk drive in the 1st quarter of 2007.

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

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. Access times have not kept up with throughput increases, which themselves have not 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 memory technology.