Data Storage

Computer storage, or '''computer memory''', refers to the Computer components, devices and Recording Media that retain Binary Information for some interval of time.
In casual language, ''memory'', or ''active memory'' usually refers to Random-access Memory (RAM), or other forms of fast but temporary (non-persistent) storage, while ''storage'' or ''long-term memory'' typically refers to Hard Disk s (HD) and other forms of storage which are slower to Access but persistent when power is switched off.

A Home Computer operates very much the same way —active memory stores any running Programs and documents, and Hard Disk to store the programs that are not running and the documents that readily available, but are not active and running. Removable Media refers to long-term storage which is typically not readily available until the media is physically connected to the system.

PURPOSES OF STORAGE

The fundamental components of a general-purpose computer are Arithmetic And Logic Unit , Control Circuitry , storage space, and input/output devices. If storage was removed, the device we had would be a simple Calculator instead of a computer. The ability to store Instruction s that form a computer program, and the information that the instructions manipulate is what makes Stored Program Architecture computers versatile.

Digital Computer represents all information using the Binary Numeral System . Text, numbers, pictures, audio, and nearly any other form of information can be converted into a string of Bit s, or binary digits, each of which has a value of 1 or 0. A piece of information can be manipulated by any computer whose storage space is large enough to accommodate the corresponding Data , or ''the binary representation of the piece of information''. For example, a computer with a storage space of eight million bits, or one Megabyte , could be used to edit a small Novel .

.]]Various forms of storage, based on various natural phenomenon, have been invented. So far, no practical universal storage medium exists, and all forms of storage have some drawbacks. Therefore a computer system usually contains several kinds of storage, each with an individual purpose.

Primary storage

Primary Storage can be directly accessed by the Central Processing Unit of the computer. Primary storage typically consists of three kinds of storage:

Processor Register s are internal to the central processing unit. Registers contain information that the arithmetic and logic unit needs to carry out current instruction.

Main Storage contains the programs that are currently being run and the data the programs are operating on. The arithmetic and logic unit can quickly transfer information between a processor register and a location in main storage. In modern computers, Random Access Memory is used for main storage. When people refer to ''computer memory'', they usually mean main storage.

Processor Cache is a special class of storage used by some central processing units. Some of the information in the main storage is duplicated in the processor cache, which is slightly slower but of much higher capacity than processor registers, and significantly faster than main storage.

Secondary, tertiary and off-line storage

Secondary Storage requires the computer to use its Input/output channels to access the information, and is used for long-term storage of persistent information. Secondary storage is typically of higher capacity than primary storage, but it is usually also much slower. In modern computers, Hard Disk s are usually used for this purpose.

Tertiary Storage is a system where a Robotic Arm will handle off-line storage media (see next item) according to computer's commands. Tertiary storage is used in the realms of Enterprise Storage and Scientific Computing , and is something a typical computer user never sees firsthand.

Off-line Storage is a system where the storage medium can be easily removed from the storage device. Off-line storage is used for Data Transfer and archival purposes. In modern computers, Floppy Disk s and Optical Disc s are often used for off-line storage.

Network storage

Network Storage is any type of computer storage that involves accessing information over a Computer Network . Network storage arguably allows to centralize the Information Management in an organization, and to reduce the duplication of information. Network storage includes:

Network-attached Storage is secondary or tertiary storage attached to a computer which an another computer can access over a Local-area Network , a private Wide-area Network , or in the case of Online File Storage , over the Internet .

Network Computer s are computers that do not contain internal secondary storage devices. Instead, documents and other data are stored on a network-attached storage.

Confusingly, these terms are sometimes used differently. Primary storage can be used to refer to local random-access disk storage, which should properly be called secondary storage. If this type of storage is called primary storage, then the term '''secondary storage''' would refer to offline, sequential-access storage like tape media.

CHARACTERISTICS OF STORAGE

The division to primary, secondary, tertiary and off-line storage is based on Memory Hierarchy , or ''distance from the central processing unit''. There are also other ways to characterize various types of storage.

Volatility of information

Volatile Memory requires constant power to maintain the stored information. Volatile memory is typically used only for primary storage.

Non-volatile Memory will retain the stored information even if it is not constantly supplied with electric power. It is suitable for long-term storage of information, and therefore used for secondary, tertiary, and off-line storage.

Dynamic memory is volatile memory which also requires that stored information is periodically '' Refreshed '', or read and rewritten without modifications.

Ability to access non-contiguous information

Random Access means that any location in storage can be accessed at any moment in the same, usually small, amount of time. This makes Random Access Memory well suited for primary storage.

Sequential Access means that the accessing a piece of information will take a varying amount of time time, depending on which piece of information was accessed last. The device may need to ''seek'' (e.g. to position the Read/write Head correctly), or ''cycle'' (e.g. to wait for the correct location in a constantly revolving medium to appear below the read/write head).

Ability to change information

Read/write storage, or '''mutable storage''', allows information to be overwritten at any time. A computer without some amount of read/write storage for primary storage purposes would be useless for many tasks. Modern computers typically use read/write storage also for secondary storage.

Read only storage retains the information stored at the time of manufacture, and '''write once storage''' ( WORM ) allows the information to be written only once at some point after manufacture. These are called '''immutable storage'''. Immutable storage is used for tertiary and off-line storage. Examples include CD-R .

Slow write, fast read storage is read/write storage which allows information to be overwritten multiple times, but with the write operation being much slower than the read operation. Examples include CD-RW .

Addressability of information

In location-addressable storage, each individually accessible unit of information in storage is selected with its numerical Memory Address . In modern computers, location-addressable storage usually limits to primary storage, accessed internally by computer programs, since location-addressability is very efficient, but burdensome for humans.

In File System Storage , information is divided into '' Files '' of variable length, and a particular file is selected with Human-readable directory and file names. The underlying device is still location-addressable, but the Operating System of a computer provides the file system Abstraction to make the operation more understandable. In modern computers, secondary, tertiary and off-line storage use file systems.

In Content-addressable Storage , each individually accessible unit of information is selected with a Hash Value , or a short identifier with no pertaining to the memory address the information is stored on. Content-addressable storage can be implemented using Software (computer program) or Hardware (computer device), with hardware being faster but more expensive option.

Capacity and performance

Storage Capacity is the total amount of stored information that a storage device or medium can hold. It is expressed as a quantity of Bit s or Byte s (e.g. 10.4 Megabyte s).

Storage Density refers to the compactness of stored information. It is the storage capacity of a medium divided with a unit of length, area or volume (e.g. 1.2 megabytes per square centimeter).

Latency is the time it takes to access a particular location in storage. The relevant Unit Of Measurement is typically Nanosecond for primary storage, Millisecond for secondary storage, and Second for tertiary storage. It may make sense to separate ''read latency'' and ''write latency'', and in case of sequential access storage, ''minimum'', ''maximum'' and ''average latency''.

Throughput is the rate at which information can read from or written to the storage. In computer storage, throughput is usually expressed in terms of ''megabytes per second'' or ''MB/s'', though Bit Rate may also be used. As with latency, ''read rate'' and ''write rate'' may need to be differentiated.

TECHNOLOGIES, DEVICES AND MEDIA

Magnetic storage

Magnetic storage uses different patterns of Magnetization on a Magnetically coated surface to store information. Magnetic storage is ''non-volatile''. The information is accessed using one or more Read/write Head s. Since the read/write head only covers a part of the surface, magnetic storage is ''sequential access'' and must seek, cycle or both. In modern computers, the magnetic surface will take these forms:

Magnetic Disk

--- Floppy Disk , used for off-line storage

--- Hard Disk , used for secondary storage

Magnetic Tape , used for tertiary and off-line storage

In early computers, magnetic storage was also used for primary storage in a form of Magnetic Drum , or Core Memory , Core Rope Memory , Thin Film Memory , Twistor Memory or Bubble Memory . Also unlike today, magnetic tape was often used for secondary storage.

Semiconductor storage

Semiconductor Memory uses Semiconductor -based Integrated Circuit s to store information. A semiconductor memory chip may contain millions of tiny Transistor s or Capacitor s. Both ''volatile'' and ''non-volatile'' forms of semiconductor memory exist. In modern computers, primary storage almost exclusively consists of dynamic volatile semiconductor memory or Dynamic Random Access Memory . Since the turn of the century, a type of non-volatile semiconductor memory known as Flash Memory has steadily gained share as off-line storage for home computers. Non-volatile semiconductor memory is also used for secondary storage in various advanced electronic devices and specialized computers.

Optical disc storage

Optical Disc Storage uses tiny pits etched on the surface of a circular disc to store information, and reads this information by illuminating the surface with a Laser Diode and observing the reflection. Optical disc storage is ''non-volatile'' and ''sequential access''. The following forms are currently in common use:

CD , CD-ROM , DVD : Read only storage, used for mass distribution of digital information (music, video, computer programs)

CD-R , DVD-R , DVD+R : Write once storage, used for tertiary and off-line storage

CD-RW , DVD-RW , DVD+RW , DVD-RAM : Slow write, fast read storage, used for tertiary and off-line storage

The following form have also been proposed:

Blu-ray

HD DVD

Phase-change Dual

Magneto-optical disc storage

Magneto-optical Disc Storage is optical disc storage where the magnetic state on a Ferromagnetic surface stored information. The information is read optically and written by combining magnetic and optical methods. Magneto-optical disc storage is ''non-volatile'', ''sequential access'', slow write, fast read storage used for tertiary and off-line storage.

Other early methods

Paper Tape and ''' Punch Card s''' have been used to store information for automatic processing since the 1890s , long before general-purpose computers existed. Information was recorded by punching holes into the paper or cardboard medium, and was read by electrically (or, later, optically) sensing whether a particular location on the medium was solid or contained a hole.

Williams Tube used a Cathode Ray Tube , and ''' Selectron Tube ''' used a large Vacuum Tube to store information. These primary storage devices were short-lived in the market, since Williams tube was unreliable and Selectron tube was expensive.

Delay Line Memory used Sound Wave s in a substance such as Mercury to store information. Delay line memory was ''dynamic volatile'', ''cycle sequential'' read/write storage, and was used for primary storage.

Other proposed methods

Phase-change Memory uses different mechanical phases of Phase Change Material to store information, and reads the information by observing the varying Electric Resistance of the material. Phase-change memory would be ''non-volatile'', ''random access'' read/write storage, and might be used for primary, secondary and off-line storage.

Holographic Storage stores information optically inside Crystal s or Photopolymer s. Holographic storage can utilize the whole volume of the storage medium, unlike optical disc storage which is limited to a small number of surface layers. Holographic storage would be ''non-volatile'', ''sequential access'', and either write once or read/write storage. It might be used for secondary and off-line storage.

Molecular Memory stores information in Polymer s that can store electric charge. Molecular memory might be especially suited for primary storage.

SEE ALSO