Disk storage

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Disk storage or disc storage is a general category of storage mechanisms, in which data are digitally recorded by various electronic, magnetic, optical, or mechanical methods on a surface layer deposited of one or more planar, round and rotating platters. A disk drive is a device implementing such a storage mechanism with fixed or removable media; with removable media the device is usually distinguished from the media as in compact disc drive and the compact disc. Notable types are the hard disk drive (which contain a non-removable disc), the floppy disk drive and its removable floppy disk, various optical disc drives and associated media.

Contents

Background

Musical and audio information was originally recorded by analog methods (see Sound recording and reproduction). Similarly the first video disc used analog recording. Analog recording has been mostly replaced by digital optical technology where the data is recorded in a digital format as optical information.

The first commercial disk storage device, that is the first commercial digital disk storage device, was the IBM RAMAC 350 shipped in 1956 as a part of the IBM 305 RAMAC computing system. Disk storage is now used in both computer storage and consumer electronic storage (e.g., audio CD and video DVD).

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.

Today disk storage devices typically have a single head that moves across a disk surface; earlier there were fixed head devices with multiple heads per surface but today they are no longer being manufactured. Movable head devices store more data per sensor and usually more per area of the medium. Fixed head devices avoid the seek time, while the head moves to the data.


Access methods

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

Originally the disk capacity was quite low and has been improved in one of several ways. Improvements in mechanical design and manufacture allowed smaller and more accurate heads, meaning that more tracks could be used on each of the platters. Advancements in data compression methods created more information in each of the individual sectors, and this was instrumental in allowing drives to store smaller units of data. sectors of data, the smallest unit stored, were reduced and so less wasted space was created.

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