Disk storage
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Disk storage is a group of data storage mechanisms for computers; data are transferred to planar surfaces or disks for temporary or permanent storage.
In the early 1960s single data bits were stored as magnetic charges in magnetic core memory. The scientists at IBM in San Jose, California successfully 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 The drum was superseded by disks, as the lower mass and inertia allowed smaller and lighter devices.
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.
The random-access, low-density storage of disks has historically been complemented by the 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.
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 rotations 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.
- Low level formatting - establishing the tracks and sectors.
- Platter - individual disk. A disk drive may have several platters.
- Track - the circle of recorded data on a single recording surface for a single arm position.
- Sector -- tracks are further divided into sectors. A single read or write operation covers an entire sector.
- Cylinder -In a multi-head drive, all the tracks under the heads for a given arm position can be read without seek delay. The operating system treats the contents of those tracks as a single cylinder.
- Head - the device that reads and writes the information - magnetic or optical - on the disk surface.
- Arm - the mechanical assembly that supports the heads as they move in and out.
- Seek time - average time needed to move the heads to a new track.
- rotational delay -average time, once the arm is on the right track before a head is over a desired sector.