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Computer data storage is a technology consisting of
computer A computer is a machine that can be programmed to Execution (computing), carry out sequences of arithmetic or logical operations automatically. Modern computers can perform generic sets of operations known as Computer program, programs. These ...

computer
components and
recording media On a reel-to-reel tape recorder (Sony TC-630), the recorder is data storage equipment and the magnetic tape is a data storage medium. Data Data are units of information Information can be thought of as the resolution of uncertaint ...
that are used to retain digital
data Data (; ) are individual facts A fact is something that is truth, true. The usual test for a statement of fact is verifiability—that is whether it can be demonstrated to correspond to experience. Standard reference works are often used ...
. It is a core function and fundamental component of computers. The
central processing unit A central processing unit (CPU), also called a central processor, main processor or just processor, is the electronic circuit 200px, A circuit built on a printed circuit board (PCB). An electronic circuit is composed of individual electroni ...

central processing unit
(CPU) of a computer is what manipulates data by performing computations. In practice, almost all computers use a
storage hierarchy In computer architecture, the memory hierarchy separates computer storage into a hierarchy based on response time. Since response time, complexity, and capacity are related, the levels may also be distinguished by their performance and controlling ...

storage hierarchy
, which puts fast but expensive and small storage options close to the CPU and slower but less expensive and larger options further away. Generally the fast volatile technologies (which lose data when off power) are referred to as "memory", while slower persistent technologies are referred to as "storage". Even the first computer designs,
Charles Babbage Charles Babbage (; 26 December 1791 – 18 October 1871) was an English polymath A polymath ( el, πολυμαθής, , "having learned much"; la, homo universalis, "universal human") is an individual whose knowledge spans a subs ...

Charles Babbage
's
Analytical Engine The Analytical Engine was a proposed mechanical general-purpose computer designed by English mathematician and computer pioneer Charles Babbage. It was first described in 1837 as the successor to Babbage's difference engine, which was a des ...
and
Percy Ludgate Percy Edwin Ludgate (2 August 1883 – 16 October 1922) was an Irish amateur scientist who designed the second analytical engine The Analytical Engine was a proposed mechanical general-purpose computer designed by English mathematician and ...

Percy Ludgate
's Analytical Machine, clearly distinguished between processing and memory (Babbage stored numbers as rotations of gears, while Ludgate stored numbers as displacements of rods in shuttles). This distinction was extended in the
Von Neumann architecture The von Neumann architecture — also known as the von Neumann model or Princeton architecture — is a computer architecture In computer engineering Computer engineering (CoE or CpE) is a branch of engineering Engineering is ...

Von Neumann architecture
, where the CPU consists of two main parts: The
control unit The control unit (CU) is a component of a computer's central processing unit A central processing unit (CPU), also called a central processor, main processor or just Processor (computing), processor, is the electronic circuitry that executes In ...
and the
arithmetic logic unit In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and sof ...
(ALU). The former controls the flow of data between the CPU and memory, while the latter performs arithmetic and
logical operations Logic (from Greek: grc, λογική, label=none, lit=possessed of reason Reason is the capacity of consciously making sense of things, applying logic Logic (from Ancient Greek, Greek: grc, wikt:λογική, λογική, label=n ...
on data.


Functionality

Without a significant amount of memory, a computer would merely be able to perform fixed operations and immediately output the result. It would have to be reconfigured to change its behavior. This is acceptable for devices such as desk
calculator An electronic calculator is typically a portable device used to perform s, ranging from basic to complex . The first calculator was created in the early 1960s. Pocket-sized devices became available in the 1970s, especially after the , the f ...

calculator
s,
digital signal processors A digital signal processor (DSP) is a specialized microprocessor A microprocessor is a computer processor where the data processing logic and control is included on a single integrated circuit, or a small number of integrated circuits. The ...
, and other specialized devices.
Von Neumann Von Neumann may refer to: * John von Neumann (1903–1957), a Hungarian American mathematician * Von Neumann family * Von Neumann (surname), a German surname * Von Neumann (crater), a lunar impact crater See also

* Von Neumann algebra * Von Ne ...

Von Neumann
machines differ in having a memory in which they store their operating instructions and data. Such computers are more versatile in that they do not need to have their hardware reconfigured for each new program, but can simply be reprogrammed with new in-memory instructions; they also tend to be simpler to design, in that a relatively simple processor may keep
state State may refer to: Arts, entertainment, and media Literature * ''State Magazine'', a monthly magazine published by the U.S. Department of State * The State (newspaper), ''The State'' (newspaper), a daily newspaper in Columbia, South Carolina, Un ...
between successive computations to build up complex procedural results. Most modern computers are von Neumann machines.


Data organization and representation

A modern
digital computer A computer is a machine that can be programmed to Execution (computing), carry out sequences of arithmetic or logical operations automatically. Modern computers can perform generic sets of operations known as Computer program, programs. These ...

digital computer
represents
data Data (; ) are individual facts A fact is something that is truth, true. The usual test for a statement of fact is verifiability—that is whether it can be demonstrated to correspond to experience. Standard reference works are often used ...

data
using the
binary numeral system In mathematics and digital electronics, a binary number is a number expressed in the base-2 numeral system or binary numeral system, which uses only two symbols: typically "0" (zero) and "1" (one). The base-2 numeral system is a positional notat ...
. Text, numbers, pictures, audio, and nearly any other form of information can be converted into a string of
bit The bit is a basic unit of information in computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithm of an algorithm (Euclid's algo ...
s, or binary digits, each of which has a value of 0 or 1. The most common unit of storage is the
byte The byte is a unit of digital information that most commonly consists of eight bit The bit is a basic unit of information in computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It ...
, equal to 8 bits. A piece of information can be handled by any computer or device whose storage space is large enough to accommodate ''the binary representation of the piece of information'', or simply
data Data (; ) are individual facts A fact is something that is truth, true. The usual test for a statement of fact is verifiability—that is whether it can be demonstrated to correspond to experience. Standard reference works are often used ...
. For example, the
complete works of Shakespeare ''Complete Works of William Shakespeare'' is the standard name given to any volume containing all the plays and poems of William Shakespeare William Shakespeare (bapt. 26 April 1564 – 23 April 1616) was an English playwright, poet and ...

complete works of Shakespeare
, about 1250 pages in print, can be stored in about five
megabyte The megabyte is a multiple of the unit byte for digital information. Its recommended unit symbol is MB. The unit prefix ''mega'' is a multiplier of (106) in the International System of Units (SI). Therefore, one megabyte is one million bytes o ...
s (40 million bits) with one byte per character. Data are
encoded In communication Communication (from Latin ''communicare'', meaning "to share" or "to be in relation with") is "an apparent answer to the painful divisions between self and other, private and public, and inner thought and outer world." As ...

encoded
by assigning a bit pattern to each
character Character(s) may refer to: Arts, entertainment, and media Literature * ''Character'' (novel), a 1936 Dutch novel by Ferdinand Bordewijk * ''Characters'' (Theophrastus), a classical Greek set of character sketches attributed to Theophrastus M ...
,
digit Digit may refer to: Mathematics and science * Numerical digit, as used in mathematics or computer science ** Arabic numerals, the most common modern representation of numerical digits * Digit (anatomy), one of several most distal parts of a limb ...
, or
multimedia Multimedia is a form of communication that combines different such as , , , , or into a single interactive presentation, in contrast to traditional mass media which featured little to no interaction fr ...

multimedia
object. Many standards exist for encoding (e.g.
character encoding Character encoding is the process of assigning numbers to graphical Graphics (from Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country ...
s like
ASCII ASCII ( ), abbreviated from American Standard Code for Information Interchange, is a character encoding Character encoding is the process of assigning numbers to graphical Graphics (from Greek Greek may refer to: Greece Anything of, ...
, image encodings like
JPEG JPEG ( ) is a commonly used method of lossy compression In information technology, lossy compression or irreversible compression is the class of data encoding methods that uses inexact approximations and partial data discarding to represe ...

JPEG
, video encodings like
MPEG-4 MPEG-4 is a method of defining compression Compression may refer to: Physical science *Compression (physics), size reduction due to forces *Compression member, a structural element such as a column *Compressibility, susceptibility to compression ...
). By adding bits to each encoded unit, redundancy allows the computer to both detect errors in coded data and correct them based on mathematical algorithms. Errors generally occur in low probabilities due to
random In common parlance, randomness is the apparent or actual lack of pattern A pattern is a regularity in the world, in human-made design, or in abstract ideas. As such, the elements of a pattern repeat in a predictable manner. A geometric p ...

random
bit value flipping, or "physical bit fatigue", loss of the physical bit in storage of its ability to maintain a distinguishable value (0 or 1), or due to errors in inter or intra-computer communication. A random bit flip (e.g. due to random
radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and f ...

radiation
) is typically corrected upon detection. A bit, or a group of malfunctioning physical bits (not always the specific defective bit is known; group definition depends on specific storage device) is typically automatically fenced-out, taken out of use by the device, and replaced with another functioning equivalent group in the device, where the corrected bit values are restored (if possible). The
cyclic redundancy check A cyclic redundancy check (CRC) is an error-detecting code In information theory Information theory is the scientific study of the quantification, storage, and communication Communication (from Latin ''communicare'', meaning "to sha ...
(CRC) method is typically used in communications and storage for
error detection In information theory and coding theory with applications in computer science and telecommunication, error detection and correction or error control are techniques that enable Reliability (computer networking), reliable delivery of digital da ...
. A detected error is then retried.
Data compression In signal processing Signal processing is an electrical engineering Electrical engineering is an engineering discipline concerned with the study, design, and application of equipment, devices, and systems which use electricity, electro ...
methods allow in many cases (such as a database) to represent a string of bits by a shorter bit string ("compress") and reconstruct the original string ("decompress") when needed. This utilizes substantially less storage (tens of percents) for many types of data at the cost of more computation (compress and decompress when needed). Analysis of trade-off between storage cost saving and costs of related computations and possible delays in data availability is done before deciding whether to keep certain data compressed or not. For security reasons, certain types of data (e.g. credit-card information) may be kept
encrypted In cryptography, encryption is the process of Code, encoding information. This process converts the original representation of the information, known as plaintext, into an alternative form known as ciphertext. Ideally, only authorized parties can ...

encrypted
in storage to prevent the possibility of unauthorized information reconstruction from chunks of storage snapshots.


Hierarchy of storage

Generally, the lower a storage is in the hierarchy, the lesser its
bandwidth Bandwidth commonly refers to: * Bandwidth (signal processing) or ''analog bandwidth'', ''frequency bandwidth'', or ''radio bandwidth'', a measure of the width of a frequency range * Bandwidth (computing), the rate of data transfer, bit rate or thr ...
and the greater its access
latency Latency or latent may refer to: Science and technology * Latent heat, energy released or absorbed, by a body or a thermodynamic system, during a constant-temperature process * Latent variable, a variable that is not directly observed but inferred i ...
is from the CPU. This traditional division of storage to primary, secondary, tertiary, and off-line storage is also guided by cost per bit. In contemporary usage, ''memory'' is usually
semiconductor A semiconductor material has an electrical conductivity Electrical resistivity (also called specific electrical resistance or volume resistivity) is a fundamental property of a material that measures how strongly it resists electric curre ...
storage read-write
random-access memory Random-access memory (RAM; ) is a form of computer memory In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic proces ...
, typically
DRAM Dynamic random-access memory (dynamic RAM or DRAM) is a type of random-access Random access (more precisely and more generally called direct access) is the ability to access an arbitrary element of a sequence in equal time or any datum fr ...

DRAM
(dynamic RAM) or other forms of fast but temporary storage. ''Storage'' consists of storage devices and their media not directly accessible by the
CPU A central processing unit (CPU), also called a central processor, main processor or just processor, is the electronic circuit File:PExdcr01CJC.jpg, 200px, A circuit built on a printed circuit board (PCB). An electronic circuit is composed of ...

CPU
(
secondary Secondary is an adjective meaning "second" or "second hand". It may refer to: * Secondary (chemistry), term used in organic chemistry to classify various types of compounds * The group of (usually at least four) defensive backs in gridiron football ...
or
tertiary storage A spindle of DVD-RW's. Computer data storage is a technology consisting of computer A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations automatically. Modern computers can perfor ...
), typically
hard disk drive A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device On a reel-to-reel tape recorder (Sony TC-630), the recorder is data storage equipment and the magnetic tape is a data stora ...

hard disk drive
s,
optical disc In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithm of an algorithm (Euclid's algorithm) for calculating the greatest ...
drives, and other devices slower than RAM but
non-volatile Non-volatile memory (NVM) or non-volatile storage is a type of computer memory In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of ...
(retaining contents when powered down). Historically, ''memory'' has been called ''core memory'', ''main memory'', ''real storage'', or ''internal memory''. Meanwhile, non-volatile storage devices have been referred to as ''secondary storage'', ''external memory'', or ''auxiliary/peripheral storage''.


Primary storage

''Primary storage'' (also known as ''main memory'', ''internal memory'', or ''prime memory''), often referred to simply as ''memory'', is the only one directly accessible to the CPU. The CPU continuously reads instructions stored there and executes them as required. Any data actively operated on is also stored there in uniform manner. Historically,
early computers Early may refer to: History * The beginning or oldest part of a defined historical periodHuman history is commonly divided into three main Era, eras — Ancient history, Ancient, Post-classical history, Post-classical, and Modern history, Modern. ...
used delay lines,
Williams tube The Williams tube, or the Williams–Kilburn tube after inventors Freddie Williams and Tom Kilburn, is an early form of computer memory In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing ...
s, or rotating magnetic drums as primary storage. By 1954, those unreliable methods were mostly replaced by
magnetic core memory Magnetic-core memory was the predominant form of random-access Random access (more precisely and more generally called direct access) is the ability to access an arbitrary element of a sequence in equal time or any datum from a population of ...
. Core memory remained dominant until the 1970s, when advances in
integrated circuit An integrated circuit or monolithic integrated circuit (also referred to as an IC, a chip, or a microchip) is a set of electronic circuit 200px, A circuit built on a printed circuit board (PCB). An electronic circuit is composed of indiv ...

integrated circuit
technology allowed
semiconductor memory Semiconductor memory is a used for , such as . It typically refers to MOS memory, where data is stored within (MOS) on a memory chip. There are numerous different types using different semiconductor technologies. The two main types of (RA ...
to become economically competitive. This led to modern
random-access memory Random-access memory (RAM; ) is a form of computer memory In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic proces ...
(RAM). It is small-sized, light, but quite expensive at the same time. The particular types of RAM used for primary storage are volatile, meaning that they lose the information when not powered. Besides storing opened programs, it serves as disk cache to improve both reading and writing performance. Operating systems borrow RAM capacity for caching so long as not needed by running software. As shown in the diagram, traditionally there are two more sub-layers of the primary storage, besides main large-capacity RAM: *
Processor register Processor may refer to: Computing Hardware * Processor (computing) In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic p ...
s are located inside the processor. Each register typically holds a
word In linguistics Linguistics is the scientific study of language A language is a structured system of communication used by humans, including speech (spoken language), gestures (Signed language, sign language) and writing. Most lang ...
of data (often 32 or 64 bits). CPU instructions instruct the
arithmetic logic unit In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and sof ...
to perform various calculations or other operations on this data (or with the help of it). Registers are the fastest of all forms of computer data storage. * Processor cache is an intermediate stage between ultra-fast registers and much slower main memory. It was introduced solely to improve the performance of computers. Most actively used information in the main memory is just duplicated in the cache memory, which is faster, but of much lesser capacity. On the other hand, main memory is much slower, but has a much greater storage capacity than processor registers. Multi-level setup is also commonly used—''primary cache'' being smallest, fastest and located inside the processor; ''secondary cache'' being somewhat larger and slower. Main memory is directly or indirectly connected to the central processing unit via a ''memory bus''. It is actually two buses (not on the diagram): an
address bus In computer architecture In computer engineering Computer engineering (CoE or CpE) is a branch of engineering Engineering is the use of scientific method, scientific principles to design and build machines, structures, and other ...
and a
data bus In computer architecture In computer engineering, computer architecture is a set of rules and methods that describe the functionality, organization, and implementation of computer A computer is a machine that can be programmed to carry ...
. The CPU firstly sends a number through an address bus, a number called
memory address In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and softwa ...
, that indicates the desired location of data. Then it reads or writes the data in the memory cells using the data bus. Additionally, a
memory management unit MMU could be used with the Motorola 68010 The Motorola MC68010 processor is a 16/32-bit microprocessor A microprocessor is a computer processor where the data processing logic and control is included on a single integrated circuit, or a s ...
(MMU) is a small device between CPU and RAM recalculating the actual memory address, for example to provide an abstraction of
virtual memory In computing, virtual memory, or virtual storage is a Memory management (operating systems), memory management technique that provides an "idealized abstraction of the storage resources that are actually available on a given machine" which "cre ...

virtual memory
or other tasks. As the RAM types used for primary storage are volatile (uninitialized at start up), a computer containing only such storage would not have a source to read instructions from, in order to start the computer. Hence, non-volatile primary storage containing a small startup program (
BIOS In computing, BIOS (, ); an acronym for Basic Input/Output System and also known as the System BIOS, ROM BIOS or PC BIOS) is firmware used to perform Computer hardware, hardware initialization during the booting process (power-on startup), and t ...

BIOS
) is used to
bootstrap Bootstrapping is a self-starting process that is supposed to proceed without external input. Bootstrapping, bootstrap, or bootstraps may also refer to: * Bootstrap (front-end framework), a free collection of tools for creating websites and web app ...
the computer, that is, to read a larger program from non-volatile ''secondary'' storage to RAM and start to execute it. A non-volatile technology used for this purpose is called ROM, for
read-only memory Read-only memory (ROM) is a type of non-volatile memory used in computers and other electronic devices. Data stored in ROM cannot be electronically modified after the manufacture of the memory device. Read-only memory is useful for storing sof ...
(the terminology may be somewhat confusing as most ROM types are also capable of ''
random access Random access (more precisely and more generally called direct access) is the ability to access an arbitrary element of a sequence in equal time or any datum from a population of addressable elements roughly as easily and efficiently as any othe ...
''). Many types of "ROM" are not literally ''read only'', as updates to them are possible; however it is slow and memory must be erased in large portions before it can be re-written. Some
embedded system An embedded system is a computer system A computer is a machine that can be programmed to carry out Sequence, sequences of arithmetic or logical operations automatically. Modern computers can perform generic sets of operations known as Co ...
s run programs directly from ROM (or similar), because such programs are rarely changed. Standard computers do not store non-rudimentary programs in ROM, and rather, use large capacities of secondary storage, which is non-volatile as well, and not as costly. Recently, ''primary storage'' and ''secondary storage'' in some uses refer to what was historically called, respectively, ''secondary storage'' and ''tertiary storage''.


Secondary storage

''Secondary storage'' (also known as ''external memory'' or ''auxiliary storage'') differs from primary storage in that it is not directly accessible by the CPU. The computer usually uses its input/output channels to access secondary storage and transfer the desired data to primary storage. Secondary storage is non-volatile (retaining data when its power is shut off). Modern computer systems typically have two orders of magnitude more secondary storage than primary storage because secondary storage is less expensive. In modern computers,
hard disk drive A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device On a reel-to-reel tape recorder (Sony TC-630), the recorder is data storage equipment and the magnetic tape is a data stora ...

hard disk drive
s (HDDs) or
solid-state drive A solid-state drive (SSD) is a solid-state storage Solid-state storage (SSS) is a type of non-volatile computer storage that stores and retrieves digital information Digital usually refers to something using digits, particularly binary di ...
s (SSDs) are usually used as secondary storage. The
access time Access time is the time delay or latency Latency or latent may refer to: Science and technology * Latent heat, energy released or absorbed, by a body or a thermodynamic system, during a constant-temperature process * Latent variable, a variable ...
per byte for HDDs or SSDs is typically measured in
millisecond A millisecond (from ''milli- ''Milli-'' (symbol m) is a unit prefixA unit prefix is a specifier or mnemonic that is prepended to units of measurement to indicate multiples or fractions of the units. Units of various order of magnitude, sizes are ...
s (one thousandth seconds), while the access time per byte for primary storage is measured in
nanosecond A nanosecond (ns) is an SI unit of time equal to one billionth of a second The second (symbol: s, abbreviation: sec) is the SI base unit, base unit of time in the International System of Units (SI) (French: Système International d’unités) ...
s (one billionth seconds). Thus, secondary storage is significantly slower than primary storage. Rotating
optical storage IBM International Business Machines Corporation (IBM) is an American multinational technology company headquartered in Armonk, New York, with operations in over 170 countries. The company began in 1911, founded in Endicott, New York, as ...
devices, such as and
DVD The DVD (common abbreviation for Digital Video Disc or Digital Versatile Disc) is a digital Digital usually refers to something using digits, particularly binary digits. Technology and computing Hardware *Digital electronics Digital electr ...

DVD
drives, have even longer access times. Other examples of secondary storage technologies include
USB flash drive A USB flash drive (i.e. thumb drive) is a that includes with an integrated interface. It is typically removable, rewritable and much smaller than an . Most weigh less than . Since first appearing on the market in late 2000, as with virtuall ...

USB flash drive
s,
floppy disk A floppy disk or floppy diskette (sometimes casually referred to as a floppy or diskette) is a type of disk storage Disk storage (also sometimes called drive storage) is a general category of storage mechanisms where data is recorded by vario ...

floppy disk
s,
magnetic tape Magnetic tape is a medium for , made of a thin, magnetizable coating on a long, narrow strip of . It was developed in Germany in 1928, based on . Devices that record and playback audio and video using magnetic tape are s and s respectively. A ...
,
paper tape Five- and eight-hole punched paper tape file:Harwell-dekatron-witch-10.jpg, Paper tape reader on the Harwell computer with a small piece of five-hole tape connected in a circle – creating a physical program loop Punched tape or perforated pape ...

paper tape
,
punched card A punched card (also punch cardSteven Pinker, in ''The Stuff of Thought'', Viking, 2007, p.362, notes the loss of ''-ed'' in pronunciation ''as it did in ice cream, mincemeat, and box set, formerly iced cream, minced meat, and boxed set.'' or p ...

punched card
s, and
RAM disk A RAM drive (also called a RAM disk) is a block of random-access memory (primary storage or volatile memory) that a computer's software is treating as if the memory were a disk drive (secondary storage). It is sometimes referred to as a virtual RA ...
s. Once the disk read/write head on HDDs reaches the proper placement and the data, subsequent data on the track are very fast to access. To reduce the seek time and rotational latency, data are transferred to and from disks in large contiguous blocks. Sequential or block access on disks is orders of magnitude faster than random access, and many sophisticated paradigms have been developed to design efficient algorithms based upon sequential and block access. Another way to reduce the I/O bottleneck is to use multiple disks in parallel in order to increase the bandwidth between primary and secondary memory. Secondary storage is often formatted according to a
file system In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and soft ...
format, which provides the abstraction necessary to organize data into
files File or filing may refer to: Mechanical tools and processes * File (tool), used to remove fine amounts of material from a workpiece **Filing (metalworking), a material removal process in manufacturing ** Nail file, a tool used to gently abrade aw ...
and
directories Directory may refer to: * Directory (computing), or folder, a file system structure in which to store computer files * Directory (OpenVMS command) * Directory service, a software application for organizing information about a computer network's us ...
, while also providing
metadata Metadata is "data Data (; ) are individual facts, statistics, or items of information, often numeric. In a more technical sense, data are a set of values of qualitative property, qualitative or quantity, quantitative variable (research), v ...

metadata
describing the owner of a certain file, the access time, the access permissions, and other information. Most computer
operating system An operating system (OS) is system software System software is software designed to provide a platform for other software. Examples of system software include operating systems (OS) like macOS, Linux, Android (operating system), Android and Mi ...

operating system
s use the concept of
virtual memory In computing, virtual memory, or virtual storage is a Memory management (operating systems), memory management technique that provides an "idealized abstraction of the storage resources that are actually available on a given machine" which "cre ...

virtual memory
, allowing utilization of more primary storage capacity than is physically available in the system. As the primary memory fills up, the system moves the least-used chunks (
pages Page most commonly refers to: * Page (paper) A page is one side of a leaf A leaf (plural leaves) is the principal lateral appendage of the vascular plant plant stem, stem, usually borne above ground and specialized for photosynthesis. T ...
) to a swap file or page file on secondary storage, retrieving them later when needed. If a lot of pages are moved to slower secondary storage, the system performance is degraded.


Tertiary storage

''Tertiary storage'' or ''tertiary memory'' is a level below secondary storage. Typically, it involves a robotic mechanism which will ''mount'' (insert) and ''dismount'' removable mass storage media into a storage device according to the system's demands; such data are often copied to secondary storage before use. It is primarily used for archiving rarely accessed information since it is much slower than secondary storage (e.g. 5–60 seconds vs. 1–10 milliseconds). This is primarily useful for extraordinarily large data stores, accessed without human operators. Typical examples include
tape libraries In computer storage, a tape library, sometimes called a tape silo, tape robot or tape jukebox, is a storage device that contains one or more tape drives, a number of slots to hold magnetic tape data storage, tape Magnetic tape data storage#Cartri ...
and optical jukeboxes. When a computer needs to read information from the tertiary storage, it will first consult a catalog
database In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and sof ...

database
to determine which tape or disc contains the information. Next, the computer will instruct a
robotic arm Image:STS-3 Canadarm captures PDP.jpg, 250px, The Canadarm while deploying a payload from the cargo bay of the Space Shuttle A robotic arm is a type of mechanical arm, usually Program (machine), programmable, with similar functions to a human arm; ...

robotic arm
to fetch the medium and place it in a drive. When the computer has finished reading the information, the robotic arm will return the medium to its place in the library. Tertiary storage is also known as ''
nearline storageNearline storage (a portmanteau A portmanteau (, ) or portmanteau word (from "Portmanteau (luggage), portmanteau") is a Blend word, blend of words
'' because it is "near to online". The formal distinction between online, nearline, and offline storage is: * Online storage is immediately available for I/O. * Nearline storage is not immediately available, but can be made online quickly without human intervention. * Offline storage is not immediately available, and requires some human intervention to become online. For example, always-on spinning hard disk drives are online storage, while spinning drives that spin down automatically, such as in massive arrays of idle disks (
MAID A maid, or housemaid or maidservant, is a female domestic worker A domestic worker is a person who works within the scope of a residence. The term "domestic service" applies to the equivalent occupational category. In traditional English ...

MAID
), are nearline storage. Removable media such as tape cartridges that can be automatically loaded, as in
tape libraries In computer storage, a tape library, sometimes called a tape silo, tape robot or tape jukebox, is a storage device that contains one or more tape drives, a number of slots to hold magnetic tape data storage, tape Magnetic tape data storage#Cartri ...
, are nearline storage, while tape cartridges that must be manually loaded are offline storage.


Off-line storage

''Off-line storage'' is a computer data storage on a medium or a device that is not under the control of a . The medium is recorded, usually in a secondary or tertiary storage device, and then physically removed or disconnected. It must be inserted or connected by a human operator before a computer can access it again. Unlike tertiary storage, it cannot be accessed without human interaction.
Off-line In computer technology and telecommunications, online indicates a state of connectivity, and offline indicates a disconnected state. In modern terminology this usually refers to an Internet connection, but (especially when expressed "on line" or ...
storage is used to , since the detached medium can easily be physically transported. Additionally, its useful for cases of disaster, where, for example, a fire destroys the original data, a medium in a remote location will be unaffected, enabling
disaster recovery Disaster recovery involves a set of policies, tools and procedures to enable the recovery or continuation of vital technology infrastructure and systems following a natural Nature, in the broadest sense, is the natural, physical, material ...
. Off-line storage increases general
information security #REDIRECT Information security #REDIRECT Information security Information security, sometimes shortened to infosec, is the practice of protecting information by mitigating information risks. It is part of Risk management information systems, inform ...

information security
, since it is physically inaccessible from a computer, and data confidentiality or integrity cannot be affected by computer-based attack techniques. Also, if the information stored for archival purposes is rarely accessed, off-line storage is less expensive than tertiary storage. In modern personal computers, most secondary and tertiary storage media are also used for off-line storage. Optical discs and flash memory devices are most popular, and to much lesser extent removable hard disk drives. In enterprise uses, magnetic tape is predominant. Older examples are floppy disks, Zip disks, or punched cards.


Characteristics of storage

Storage technologies at all levels of the storage hierarchy can be differentiated by evaluating certain core characteristics as well as measuring characteristics specific to a particular implementation. These core characteristics are volatility, mutability, accessibility, and addressability. For any particular implementation of any storage technology, the characteristics worth measuring are capacity and performance.


Volatility

Non-volatile memory Non-volatile memory (NVM) or non-volatile storage is a type of computer memory that can retain stored information even after power is removed. In contrast, volatile memory needs constant power in order to retain data. Examples of non-volatile ...
retains the stored information even if not constantly supplied with electric power. It is suitable for long-term storage of information.
Volatile memory Volatile memory, in contrast to non-volatile memory Non-volatile memory (NVM) or non-volatile storage is a type of computer memory that can retain stored information even after power is removed. In contrast, volatile memory needs constant pow ...
requires constant power to maintain the stored information. The fastest memory technologies are volatile ones, although that is not a universal rule. Since the primary storage is required to be very fast, it predominantly uses volatile memory.
Dynamic random-access memory Dynamic random-access memory (dynamic RAM or DRAM) is a type of random-access memory, random-access semiconductor memory that stores each bit of data in a memory cell (computing), memory cell, usually consisting of a tiny capacitor and a tr ...
is a form of volatile memory that also requires the stored information to be periodically reread and rewritten, or refreshed, otherwise it would vanish.
Static random-access memory clone (2K × 8 bits) Static random-access memory (static RAM or SRAM) is a type of random-access memory File:Random Access Memory HyperX.jpg, 8GB DDR3 RAM stick with a white Heat sink, heatsink Random-access memory (RAM; ) is a f ...
is a form of volatile memory similar to DRAM with the exception that it never needs to be refreshed as long as power is applied; it loses its content when the power supply is lost. An
uninterruptible power supply An uninterruptible power supply or uninterruptible power source (UPS) is an electrical apparatus that provides emergency power to a load when the input power source or mains power fails. A UPS differs from an auxiliary or emergency power syste ...

uninterruptible power supply
(UPS) can be used to give a computer a brief window of time to move information from primary volatile storage into non-volatile storage before the batteries are exhausted. Some systems, for example
EMC Symmetrix The Symmetrix system is Dell EMC's enterprise storage array ARRAY, also known as ARRAY Now, is an independent distribution company launched by film maker and former publicist Ava DuVernay in 2010 under the name African-American Film Festival Re ...
, have integrated batteries that maintain volatile storage for several minutes.


Mutability

; 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. ; Slow write, fast read storage : 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 CD-RW (Compact Disc-Rewritable) is a digital Digital usually refers to something using digits, particularly binary digits. Technology and computing Hardware *Digital electronics Digital electronics is a field of electronics Electron ...
and
SSD A solid-state drive (SSD) is a solid-state storage Solid-state storage (SSS) is a type of non-volatile computer storage that stores and retrieves digital information Digital usually refers to something using digits, particularly binary dig ...
. ; Write once storage :
Write Once Read ManyWrite once read many (WORM) describes a data storage device in which information, once written, cannot be modified. This write protection affords the assurance that the data cannot be tampered with once it is written to the device. On ordinary (no ...
(WORM) allows the information to be written only once at some point after manufacture. Examples include semiconductor
programmable read-only memory A programmable read-only memory (PROM) is a form of digital memory where the contents can be changed once after manufacture of the device. The data is then permanent and cannot be changed. It is one type of read-only memory Read-only memory (RO ...
and
CD-R CD-R (Compact disc-recordable) is a digital media, digital optical disc data storage device, storage format. A CD-R disc is a compact disc that can be Write once read many, written once and read arbitrarily many times. CD-R discs (CD-Rs) are r ...

CD-R
. ; Read only storage : Retains the information stored at the time of manufacture. Examples include mask ROM ICs and
CD-ROM A CD-ROM (, compact disc read-only memory) is a pre-pressed optical compact disc The compact disc (CD) is a digital Digital usually refers to something using digits, particularly binary digits. Technology and computing Hardware *Digital ...

CD-ROM
.


Accessibility

;
Random access Random access (more precisely and more generally called direct access) is the ability to access an arbitrary element of a sequence in equal time or any datum from a population of addressable elements roughly as easily and efficiently as any othe ...
: Any location in storage can be accessed at any moment in approximately the same amount of time. Such characteristic is well suited for primary and secondary storage. Most semiconductor memories and disk drives provide random access, though only
flash memory Flash memory is an electronic Electronic may refer to: *Electronics Electronics comprises the physics, engineering, technology and applications that deal with the emission, flow and control of electrons in vacuum and matter. It uses act ...
supports random access without
latency Latency or latent may refer to: Science and technology * Latent heat, energy released or absorbed, by a body or a thermodynamic system, during a constant-temperature process * Latent variable, a variable that is not directly observed but inferred i ...
, as no mechanical parts need to be moved. ;
Sequential access Sequential access is a term describing a group of elements (such as data in a memory array or a Hard disk drive, disk file or on magnetic tape data storage) being accessed in a predetermined, ordered sequence. It is the opposite of random access, t ...
: The accessing of pieces of information will be in a serial order, one after the other; therefore the time to access a particular piece of information depends upon which piece of information was last accessed. Such characteristic is typical of off-line storage.


Addressability

; Location-addressable : Each individually accessible unit of information in storage is selected with its numerical
memory address In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and softwa ...
. 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. ; File addressable: Information is divided into ''
files File or filing may refer to: Mechanical tools and processes * File (tool), used to remove fine amounts of material from a workpiece **Filing (metalworking), a material removal process in manufacturing ** Nail file, a tool used to gently abrade aw ...
'' of variable length, and a particular file is selected with
human-readable 220px, ISBN represented as EAN-13 bar code showing both human-readable and machine-readable data A human-readable medium or human-readable format is any encoding of data Data are units of information Information can be thought of as ...
directory and file names. The underlying device is still location-addressable, but the
operating system An operating system (OS) is system software System software is software designed to provide a platform for other software. Examples of system software include operating systems (OS) like macOS, Linux, Android (operating system), Android and Mi ...

operating system
of a computer provides the file system
abstraction Abstraction in its main sense is a conceptual process where general rules Rule or ruling may refer to: Human activity * The exercise of political Politics (from , ) is the set of activities that are associated with Decision-making, mak ...
to make the operation more understandable. In modern computers, secondary, tertiary and off-line storage use file systems. ; Content-addressable: Each individually accessible unit of information is selected based on the basis of (part of) the contents stored there. Content-addressable storage can be implemented using
software Software is a collection of instructions Instruction or instructions may refer to: Computing * Instruction, one operation of a processor within a computer architecture instruction set * Computer program, a collection of instructions Music * I ...

software
(computer program) or
hardware Hardware may refer to: Technology Computing and electronics * Computer hardware, physical parts of a computer * Digital electronics, electronics that operate on digital signals * Electronic component, device in an electronic system used to affect e ...
(computer device), with hardware being faster but more expensive option. Hardware content addressable memory is often used in a computer's
CPU cache A CPU cache is a hardware cache In computing, a cache ( , or in Australian English) is a hardware or software component that stores data so that future requests for that data can be served faster; the data stored in a cache might be the resu ...
.


Capacity

; Raw capacity: The total amount of stored information that a storage device or medium can hold. It is expressed as a quantity of
bit The bit is a basic unit of information in computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithm of an algorithm (Euclid's algo ...
s or
byte The byte is a unit of digital information that most commonly consists of eight bit The bit is a basic unit of information in computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It ...
s (e.g. 10.4
megabyte The megabyte is a multiple of the unit byte for digital information. Its recommended unit symbol is MB. The unit prefix ''mega'' is a multiplier of (106) in the International System of Units (SI). Therefore, one megabyte is one million bytes o ...
s). ; Memory storage density: 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 inch).


Performance

;
Latency Latency or latent may refer to: Science and technology * Latent heat, energy released or absorbed, by a body or a thermodynamic system, during a constant-temperature process * Latent variable, a variable that is not directly observed but inferred i ...
: The time it takes to access a particular location in storage. The relevant
unit of measurement A unit of measurement is a definite magnitude Magnitude may refer to: Mathematics *Euclidean vector, a quantity defined by both its magnitude and its direction *Magnitude (mathematics), the relative size of an object *Norm (mathematic ...
is typically
nanosecond A nanosecond (ns) is an SI unit of time equal to one billionth of a second The second (symbol: s, abbreviation: sec) is the SI base unit, base unit of time in the International System of Units (SI) (French: Système International d’unités) ...
for primary storage,
millisecond A millisecond (from ''milli- ''Milli-'' (symbol m) is a unit prefixA unit prefix is a specifier or mnemonic that is prepended to units of measurement to indicate multiples or fractions of the units. Units of various order of magnitude, sizes are ...
for secondary storage, and second for tertiary storage. It may make sense to separate read latency and write latency (especially for non-volatile memory) and in case of sequential access storage, minimum, maximum and average latency. ; Throughput (disk drive), Throughput: The rate at which information can be read from or written to the storage. In computer data storage, throughput is usually expressed in terms of megabytes per second (MB/s), though bit rate may also be used. As with latency, read rate and write rate may need to be differentiated. Also accessing media sequentially, as opposed to randomly, typically yields maximum throughput. ; Granularity: The size of the largest "chunk" of data that can be efficiently accessed as a single unit, e.g. without introducing additional latency. ;Reliability: The probability of spontaneous bit value change under various conditions, or overall failure rate. Utilities such as hdparm and sar (Unix), sar can be used to measure IO performance in Linux.


Energy use

* Storage devices that reduce fan usage automatically shut-down during inactivity, and low power hard drives can reduce energy consumption by 90 percent. * 2.5-inch hard disk drives often consume less power than larger ones. Low capacity
solid-state drive A solid-state drive (SSD) is a solid-state storage Solid-state storage (SSS) is a type of non-volatile computer storage that stores and retrieves digital information Digital usually refers to something using digits, particularly binary di ...
s have no moving parts and consume less power than hard disks. Also, memory may use more power than hard disks. Large caches, which are used to avoid hitting the memory wall, may also consume a large amount of power.


Security

Hardware-based full disk encryption, Full disk encryption, Disk encryption software, volume and virtual disk encryption, andor file/folder encryption is readily available for most storage devices. Hardware memory encryption is available in Intel Architecture, supporting Total Memory Encryption (TME) and page granular memory encryption with multiple keys (MKTME). and in SPARC M7 generation since October 2015.


Vulnerability and reliability

Distinct types of data storage have different points of failure and various methods of predictive failure analysis. Vulnerabilities that can instantly lead to total loss are head crashing on mechanical hard drives and failure of electronic components on flash storage.


Error detection

Impending failure on hard disk drives is estimable using S.M.A.R.T. diagnostic data that includes the power-on hours, hours of operation and the count of spin-ups, though its reliability is disputed. Flash storage may experience downspiking transfer rates as a result of accumulating errors, which the flash memory controller attempts to correct. The health of optical media can be determined by Optical_disc#Surface_error_scanning, measuring correctable minor errors, of which high counts signify deteriorating and/or low-quality media. Too many consecutive minor errors can lead to data corruption. Not all vendors and models of optical drives support error scanning.


Storage media

, the most commonly used data storage media are semiconductor, magnetic, and optical, while paper still sees some limited usage. Some other fundamental storage technologies, such as all-flash arrays (AFAs) are proposed for development.


Semiconductor

Semiconductor memory uses
semiconductor A semiconductor material has an electrical conductivity Electrical resistivity (also called specific electrical resistance or volume resistivity) is a fundamental property of a material that measures how strongly it resists electric curre ...
-based
integrated circuit An integrated circuit or monolithic integrated circuit (also referred to as an IC, a chip, or a microchip) is a set of electronic circuit 200px, A circuit built on a printed circuit board (PCB). An electronic circuit is composed of indiv ...

integrated circuit
(IC) chips to store information. Data are typically stored in metal–oxide–semiconductor (MOS) memory cell (computing), memory cells. A semiconductor memory chip may contain millions of memory cells, consisting of tiny MOSFET, MOS field-effect transistors (MOSFETs) and/or MOS capacitors. Both ''volatile'' and ''non-volatile'' forms of semiconductor memory exist, the former using standard MOSFETs and the latter using floating-gate MOSFETs. In modern computers, primary storage almost exclusively consists of dynamic volatile semiconductor
random-access memory Random-access memory (RAM; ) is a form of computer memory In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic proces ...
(RAM), particularly dynamic random-access memory (DRAM). Since the turn of the century, a type of non-volatile floating-gate semiconductor memory known as
flash memory Flash memory is an electronic Electronic may refer to: *Electronics Electronics comprises the physics, engineering, technology and applications that deal with the emission, flow and control of electrons in vacuum and matter. It uses act ...
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 that are designed for them. As early as 2006, Laptop, notebook and desktop computer manufacturers started using flash-based
solid-state drive A solid-state drive (SSD) is a solid-state storage Solid-state storage (SSS) is a type of non-volatile computer storage that stores and retrieves digital information Digital usually refers to something using digits, particularly binary di ...
s (SSDs) as default configuration options for the secondary storage either in addition to or instead of the more traditional HDD.


Magnetic

Magnetic storage uses different patterns of magnetization on a magnetism, magnetically coated surface to store information. Magnetic storage is ''non-volatile''. The information is accessed using one or more read/write heads which may contain one or more recording transducers. A read/write head only covers a part of the surface so that the head or medium or both must be moved relative to another in order to access data. In modern computers, magnetic storage will take these forms: * Disk storage, Magnetic disk ** Floppy disk, used for off-line storage ** Hard disk drive, used for secondary storage * Magnetic tape data storage, Magnetic tape, used for tertiary and off-line storage * Carousel memory (magnetic rolls) In early computers, magnetic storage was also used as: * Primary storage in a form of Drum memory, magnetic memory, or Magnetic core memory, core memory, core rope memory, thin-film memory and/or twistor memory. * Tertiary (e.g. NCR CRAM) or off line storage in the form of magnetic cards. * Magnetic tape was then often used for secondary storage. Magnetic storage does not have a definite limit of rewriting cycles like flash storage and re-writeable optical media, as altering magnetic fields causes no physical wear. Rather, their life span is limited by mechanical parts.


Optical

Optical storage, the typical
optical disc In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithm of an algorithm (Euclid's algorithm) for calculating the greatest ...
, stores information in deformities on the surface of a circular disc and reads this information by illuminating the surface with a laser diode and observing the reflection. Optical disc storage is ''non-volatile''. The deformities may be permanent (read only media), formed once (write once media) or reversible (recordable or read/write media). The following forms are currently in common use: * ,
CD-ROM A CD-ROM (, compact disc read-only memory) is a pre-pressed optical compact disc The compact disc (CD) is a digital Digital usually refers to something using digits, particularly binary digits. Technology and computing Hardware *Digital ...

CD-ROM
,
DVD The DVD (common abbreviation for Digital Video Disc or Digital Versatile Disc) is a digital Digital usually refers to something using digits, particularly binary digits. Technology and computing Hardware *Digital electronics Digital electr ...

DVD
, Blu-ray Disc, BD-ROM: Read only storage, used for mass distribution of digital information (music, video, computer programs) *
CD-R CD-R (Compact disc-recordable) is a digital media, digital optical disc data storage device, storage format. A CD-R disc is a compact disc that can be Write once read many, written once and read arbitrarily many times. CD-R discs (CD-Rs) are r ...

CD-R
, DVD-R, DVD+R, Blu-ray Disc recordable, BD-R: Write once storage, used for tertiary and off-line storage *
CD-RW CD-RW (Compact Disc-Rewritable) is a digital Digital usually refers to something using digits, particularly binary digits. Technology and computing Hardware *Digital electronics Digital electronics is a field of electronics Electron ...
, DVD-RW, DVD+RW, DVD-RAM, Blu-ray Disc recordable, BD-RE: Slow write, fast read storage, used for tertiary and off-line storage * Ultra Density Optical or UDO is similar in capacity to Blu-ray Disc recordable, BD-R or BD-RE and is slow write, fast read storage used for tertiary and off-line storage. Magneto-optical drive, Magneto-optical disc storage is optical disc storage where the magnetic state on a ferromagnetism, ferromagnetic surface stores 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. 3D optical data storage has also been proposed. Light induced magnetization melting in magnetic photoconductors has also been proposed for high-speed low-energy consumption magneto-optical storage.


Paper

Paper data storage, typically in the form of punched tape, paper tape or
punched card A punched card (also punch cardSteven Pinker, in ''The Stuff of Thought'', Viking, 2007, p.362, notes the loss of ''-ed'' in pronunciation ''as it did in ice cream, mincemeat, and box set, formerly iced cream, minced meat, and boxed set.'' or p ...

punched card
s, has long been used to store information for automatic processing, particularly before general-purpose computers existed. Information was recorded by punching holes into the paper or cardboard medium and was read mechanically (or later optically) to determine whether a particular location on the medium was solid or contained a hole. Barcodes make it possible for objects that are sold or transported to have some computer-readable information securely attached. Relatively small amounts of digital data (compared to other digital data storage) may be backed up on paper as a matrix barcode for very long-term storage, as the longevity of paper typically exceeds even magnetic data storage.


Other storage media or substrates

; Vacuum-tube memory: A
Williams tube The Williams tube, or the Williams–Kilburn tube after inventors Freddie Williams and Tom Kilburn, is an early form of computer memory In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing ...
used a cathode ray tube, and a Selectron tube used a large vacuum tube to store information. These primary storage devices were short-lived in the market, since the Williams tube was unreliable, and the Selectron tube was expensive. ; Electro-acoustic memory: Delay-line memory used Longitudinal wave, sound waves in a substance such as mercury (element), mercury to store information. Delay-line memory was dynamic volatile, cycle sequential read/write storage, and was used for primary storage. ; Optical tape: is a medium for optical storage, generally consisting of a long and narrow strip of plastic, onto which patterns can be written and from which the patterns can be read back. It shares some technologies with cinema film stock and optical discs, but is compatible with neither. The motivation behind developing this technology was the possibility of far greater storage capacities than either magnetic tape or optical discs. ; Phase-change memory: uses different mechanical phases of phase-change material to store information in an X–Y addressable matrix and reads the information by observing the varying electrical 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. Most rewritable and many write-once optical disks already use phase-change material to store information. ; Holographic data storage: stores information optically inside crystals or photopolymers. 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. See Holographic Versatile Disc (HVD). ; Molecular memory: stores information in polymer that can store electric charge. Molecular memory might be especially suited for primary storage. The theoretical storage capacity of molecular memory is 10 terabits per square inch (16 Gbit/mm2). ; Magnetic photoconductors: store magnetic information, which can be modified by low-light illumination. ; DNA digital data storage, DNA: stores information in DNA nucleotides. It was first done in 2012, when researchers achieved a ratio of 1.28 petabytes per gram of DNA. In March 2017 scientists reported that a new algorithm called a DNA fountain achieved 85% of the theoretical limit, at 215 petabytes per gram of DNA.


Related technologies


Redundancy

While a group of bits malfunction may be resolved by error detection and correction mechanisms (see above), storage device malfunction requires different solutions. The following solutions are commonly used and valid for most storage devices: * Device Disk mirroring, mirroring (replication) – A common solution to the problem is constantly maintaining an identical copy of device content on another device (typically of a same type). The downside is that this doubles the storage, and both devices (copies) need to be updated simultaneously with some overhead and possibly some delays. The upside is possible concurrent read of a same data group by two independent processes, which increases performance. When one of the replicated devices is detected to be defective, the other copy is still operational, and is being utilized to generate a new copy on another device (usually available operational in a pool of stand-by devices for this purpose). * Redundant array of independent disks (RAID) – This method generalizes the device mirroring above by allowing one device in a group of N devices to fail and be replaced with the content restored (Device mirroring is RAID with N=2). RAID groups of N=5 or N=6 are common. N>2 saves storage, when comparing with N=2, at the cost of more processing during both regular operation (with often reduced performance) and defective device replacement. Device mirroring and typical RAID are designed to handle a single device failure in the RAID group of devices. However, if a second failure occurs before the RAID group is completely repaired from the first failure, then data can be lost. The probability of a single failure is typically small. Thus the probability of two failures in a same RAID group in time proximity is much smaller (approximately the probability squared, i.e., multiplied by itself). If a database cannot tolerate even such smaller probability of data loss, then the RAID group itself is replicated (mirrored). In many cases such mirroring is done geographically remotely, in a different storage array, to handle also recovery from disasters (see disaster recovery above).


Network connectivity

A secondary or tertiary storage may connect to a computer utilizing computer networks. This concept does not pertain to the primary storage, which is shared between multiple processors to a lesser degree. * Direct-attached storage (DAS) is a traditional mass storage, that does not use any network. This is still a most popular approach. This retronym was coined recently, together with NAS and SAN. * Network-attached storage (NAS) is mass storage attached to a computer which another computer can access at file level over a local area network, a private wide area network, or in the case of File hosting service, online file storage, over the Internet. NAS is commonly associated with the Network File System (protocol), NFS and Server Message Block, CIFS/SMB protocols. * Storage area network (SAN) is a specialized network, that provides other computers with storage capacity. The crucial difference between NAS and SAN, is that NAS presents and manages file systems to client computers, while SAN provides access at block-addressing (raw) level, leaving it to attaching systems to manage data or file systems within the provided capacity. SAN is commonly associated with Fibre Channel networks.


Robotic storage

Large quantities of individual magnetic tapes, and optical or magneto-optical discs may be stored in robotic tertiary storage devices. In tape storage field they are known as
tape libraries In computer storage, a tape library, sometimes called a tape silo, tape robot or tape jukebox, is a storage device that contains one or more tape drives, a number of slots to hold magnetic tape data storage, tape Magnetic tape data storage#Cartri ...
, and in optical storage field optical jukeboxes, or optical disk libraries per analogy. The smallest forms of either technology containing just one drive device are referred to as autoloader (data storage device), autoloaders or autochangers. Robotic-access storage devices may have a number of slots, each holding individual media, and usually one or more picking robots that traverse the slots and load media to built-in drives. The arrangement of the slots and picking devices affects performance. Important characteristics of such storage are possible expansion options: adding slots, modules, drives, robots. Tape libraries may have from 10 to more than 100,000 slots, and provide terabytes or petabytes of near-line information. Optical jukeboxes are somewhat smaller solutions, up to 1,000 slots. Robotic storage is used for backups, and for high-capacity archives in imaging, medical, and video industries. Hierarchical storage management is a most known archiving strategy of automatically ''migrating'' long-unused files from fast hard disk storage to libraries or jukeboxes. If the files are needed, they are ''retrieved'' back to disk.


See also


Primary storage topics

* Aperture (computer memory) *
Dynamic random-access memory Dynamic random-access memory (dynamic RAM or DRAM) is a type of random-access memory, random-access semiconductor memory that stores each bit of data in a memory cell (computing), memory cell, usually consisting of a tiny capacitor and a tr ...
(DRAM) * CAS latency, Memory latency * Mass storage * Memory cell (disambiguation) * Memory management ** Dynamic memory allocation *** Memory leak ** Virtual memory * Memory protection * Page address register *
Static random-access memory clone (2K × 8 bits) Static random-access memory (static RAM or SRAM) is a type of random-access memory File:Random Access Memory HyperX.jpg, 8GB DDR3 RAM stick with a white Heat sink, heatsink Random-access memory (RAM; ) is a f ...
(SRAM) * Stable storage


Secondary, tertiary and off-line storage topics

* Cloud storage * Data deduplication * Data proliferation * Data storage tag used for capturing research data * Disk utility * File system ** List of file formats * Flash memory * Geoplexing * Information repository * Removable media * Solid-state drive * Spindle (computer), Spindle * Virtual tape library * Wait state * Write buffer * Write protection * Noise-predictive maximum-likelihood detection *Object(-based) storage


Data storage conferences

* Storage Networking World * Storage World Conference


References


Further reading

*
Memory & Storage
Computer History Museum {{DEFAULTSORT:Computer Data Storage Computer data storage, Computer architecture