Disk storage

Disk storage is a general category of a computer storage mechanisms, in which data is recorded on planar, round and rotating surfaces (disks, discs, or platters). A disk drive is a peripheral device used to collect information from. Main implementations are hard disks, floppy disks and optical discs. Nowadays the term disk storage almost exclusively refers to hard disk storage.


In the 1950s and the early 1960s single data bits were stored as magnetic charges in a magnetic core memory.

Then the scientists at IBM in San Jose, California created a rotating drum that was coated in a magnetically polarizable film that could be used to store data by changing and sensing magnetic polarization. The drum was later superseded by disks, because of their lower mass and inertia. Rey Johnson, an inventor who worked for IBM for many years, is said to be the "father" of the disk drive.

The random-access, low-density storage of disks was developed to complement the already used sequential-access high-density storage provided by magnetic tape. Vigorous innovation in disk storage technology, coupled with less vigorous innovation in tape storage, has reduced the density and cost per bit gap between disk and tape, reducing the importance of tape as a complement to disk.

In the beginning, there were movable head devices, usually disks, and fixed head devices, usually drums. Movable head devices store more data per magnetic sensor and usually more per area of the medium. Fixed head devices avoid the seek time, while the head moves to the data. Fixed head devices have not been common since integrated circuit random access memory was developed. So the usual storage devices of, for example, an IBM 360 were discrete transistor registers, magnetic core random access memory (ram), fixed head drums, movable head disk packs (several disks with the heads connected mechanically), and magnetic tape, in order of increasing time to access a random data element.

Audio recordings

In musical and audio data storage, the first devices were also drum shaped, called phonograph cylinders, which were popularized by Thomas Edison. In the 1910s these were replaced as the dominant medium of sound recording by analogue disc records, commonly called gramophone records (in British English) or phonograph records (in American English). From the 1950s through the 1980s, audio recordings were also done on magnetic tape media of several types, although the vinyl record remained the most popular medium for home use. These were mostly replaced by compact disc technology, where the data is recorded in a digital format as optical information. This compact disc technology has been widely accepted, and data storage, using writable compact disks or CD-R devices is very common.

Access methods

Disk drives are block storage devices. Disk is divided into logical blocks (collection of sectors). Blocks are addressed using their Logical Block Addresses (LBA). Read or writing from/to disk happens at the granularity of blocks.

Mechanically, there are usually two types of motion: the constant rate rotation, which passes the data of a track sequentially under a read head, and the radial head motion or seek, which selects the track. Rotation is faster than seek, so the logical blocks are related in simple ways to the physical tracks.


Disk drive interface is the mechanism/protocol of communication between the rest of the system and the disk drive itself. Different interface types include SCSI and SAS for enterprise disks, ATA, PATA, SATA for desktop disks.


Since a magnetic dipole field decreases rapidly with distance from the magnetic material, the space between the head and medium must be controlled with more precision than the thermal expansion of parts of the disk drive. The head, therefor "flies" or is hydrodynamically lubricated by air. That is, the air pulled along by the disk forces the head away from the disk surface. When the disk stops, the head must either "land" or be pulled away. Defects, wear or foreign objects such as dust, can distort the disk and head surfaces, so they make contact and damage each other further. When this process runs away, we say there is a "head crash". This results in lost data and an inoperable device. Head crashes are one of the reasons that important data must be backed up on some other device (often audio or video media). When removable hard disks were used, disc damage was contagious, since either the head or the disk could propagate the damage.

Basic terminology

  • Rotation - how the disks spin. Two techniques are common:
    • Constant angular velocity (CAV) keeps the disk spinning at a fixed rate, measured in revolutions per minute (RPM). This means the heads cover more distance per unit of time on the outer tracks than on the inner tracks. This method is typical with computer hard drives.
    • Constant linear velocity (CLV) keeps the distance covered by the heads per unit time fixed. Thus the disk has to slow down as the arm moves to the outer tracks. This method is typical for CD drives.
  • Sector - an area of disk enclosed within a given central angle (a pie piece)
  • Platter - an individual disk (since confusingly, what is now commonly called a single hard disk is in fact a set of disks)

  • Head - the device that reads and writes the information - magnetic or optical - on the disk surface.
  • Arm - the mechanical assembly that supports the head as it moves in and out.
  • Seek time - average time needed to move the head to a new position(specific track).
  • Rotational delay - average time, once the arm is on the right track, before a head is over a desired sector.
  • Interleave - the spacing between sectors. Since early hard disks had enough buffer space to read only one sector at a time, sequential sectors were physically spaced on the media to enable the next sector to be in the correct position under the head once the host was ready to read it. At an interleave factor of 3:1, three full rotations would be required to read an entire track. Almost all hard disks since Compaq and Western Digital defined the AT Attachment standard have however used an interleave factor of 1:1. Floppy disks are still usually interleaved.

See also

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