Disk storage
Disk storage is a category of
data storage mechanisms for
computers; data are recorded on planar surfaces or 'disks' for temporary or permanent storage. A
disk drive is a
peripheral device used to read from and write to a disk.
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 The drum was superseded by disks, as the lower mass and inertia allowed smaller and lighter devices.
Reynold Johnson an inventor who worked for IBM for many years is said to be the "father" of the disk drive.
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 innovationin 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 oftape as a complement to disk.
In disk storage, there are the two primary access methods.
Block storage means that the disk is divided into normally equal-sized blocks which are accessed at random by the operating system.
File storage contains an
abstraction of files and directories which can be used to refer to storage content. Another access method,
content-addressable storage (CAS) uses a hashing algorithm to refer to pieces of data.
* 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.
* 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.
*
More Bits in Pits - article by JR Minkel
*
floppy disk*
hard disk*
compact disc*
Early IBM disk storage*
Kryder's law*
RAID*
Solid state disk*
Fragmentation_%28computer%29*
ATA*
SCSI*
Serial ATA*
AoE
