Computer data storage: Difference between revisions

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{{short description|Storage of digital data readable by computers}}

{{Merge from|Non-volatileData memoryat rest|discuss=Talk:Computer data storage#Proposed merge of Non-volatileData at memoryrest into Computer data storage|date=AprilMarch 20242025}}

{{Use dmy dates|date=June 2020}}

{{broader|Data storage}}

[[File:DDR2 ram mounted.jpg|thumb|1 [[Gibibyte|GiB]] of [[Synchronous dynamic random-access memory|SDRAM]] mounted in a [[computer]]. An example of ''primary storage''.]]

[[File:IBM DJNA-351520 Hard Disk A.jpg|thumb|15 [[Gigabyte|GB]] [[Parallel ATA|PATA]] hard disk drive (HDD) from 1999. When connected to a computer it serves as ''secondary'' storage.]]

[[File:Super DLTtape I.jpg|thumb|160 [[Gigabyte|GB]] [[Digital Linear Tape|SDLT]] [[tape cartridge]], an example of ''off-line'' storage. When used within a robotic [[tape library]], it is classified as ''tertiary'' storage instead.]]

[[File:Sony_CRX310S-Internal-PC-DVD-Drive-Opened.jpg|thumb|Read/Write DVD drive with cradle for media extended]]

'''Computer data storage''' or '''digital data storage''' is a technology consisting of [[computer]] components and [[Data storage|recording media]] that are used to retain [[digital data]]. It is a core function and fundamental component of computers.<ref name="Patterson">{{Cite book |title=Computer organization and design: The hardware/software interface |last1=Patterson |first1=David A. |last2=Hennessy |first2=John L. |date=2005 |publisher=[[Morgan Kaufmann Publishers]] |isbn=1-55860-604-1 |edition=3rd |___location=[[Amsterdam]] |oclc=56213091 |url-access=registration |url=https://archive.org/details/isbn_9781558606043 }}</ref>{{rp|15–16}}

The [[central processing unit]] (CPU) of a computer is what manipulates data by performing computations. In practice, almost all computers use a [[memory hierarchy|storage hierarchy]],<ref name="Patterson"/>{{rp|468–473}} 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{{efn|Most contemporary computers use volatile technologies (which lose data when power is off); early computers used both volatile and persistent technologies.}} technologies are referred to as "memory", while slower persistent technologies are referred to as "storage".

Even the first computer designs, [[Charles Babbage]]'s [[Analytical Engine]] 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]], where the CPU consists of two main parts: The [[control unit]] and the [[arithmetic logic unit]] (ALU). The former controls the flow of data between the CPU and memory, while the latter performs arithmetic and [[Bitwise operation|logical operations]] 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]]s, [[digital signal processing|digital signal processors]], and other specialized devices. [[von Neumann architecture|Von Neumann]] machines differ in having a memory in which they store their operating [[Instruction set architecture#Instructions|instructions]] and data.<ref name="Patterson"/>{{rp|20}} Such computers are more versatile in that they do not need to have their hardware reconfigured for each new program, but can simply be [[computer programming|reprogrammed]] with new in-memory instructions; they also tend to be simpler to design, in that a relatively simple processor may keep [[State (computer science)|state]] between successive computations to build up complex procedural results. Most modern computers are von Neumann machines.

== Data organization and representation ==

A modern [[Computer|digital computer]] represents [[data]] using the [[Binary number|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 0&nbsp;or&nbsp;1. The most common unit of storage is the [[byte]], 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 (computing)|data]]. For example, the [[Complete Works of Shakespeare|complete works of Shakespeare]], about 1250&nbsp;pages in print, can be stored in about five [[megabyte]]s (40&nbsp;million bits) with one byte per character.

Data are [[Code|encoded]] by assigning a bit pattern to each [[Character (computing)|character]], [[Numerical digit|digit]], or [[multimedia]] object. Many standards exist for encoding (e.g. [[character encoding]]s like [[ASCII]], image encodings like [[JPEG]], and video encodings like [[MPEG-4]]).

By adding bits to each encoded unit, redundancy allows the computer to detect errors in coded data and correct them based on mathematical algorithms. Errors generally occur in low probabilities due to [[Randomness|random]] bit value flipping, or "physical bit fatigue", loss of the physical bit in the storage of its ability to maintain a distinguishable value (0&nbsp;or&nbsp;1), or due to errors in inter or intra-computer communication. A random [[RAM parity|bit flip]] (e.g. due to random [[radiation]]) is typically corrected upon detection. A bit or a group of malfunctioning physical bits (the specific defective bit is not always known; group definition depends on the 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]] (CRC) method is typically used in communications and storage for [[error detection and correction|error detection]]. A detected error is then retried.

[[Data compression]] 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 percent) for many types of data at the cost of more computation (compress and decompress when needed). Analysis of the 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 [[data security|security reasons]], certain types of data (e.g. [[credit card]] information) may be kept [[encryption|encrypted]] in storage to prevent the possibility of unauthorized information reconstruction from chunks of storage snapshots.

== Hierarchy of storage ==

{{Main|Memory hierarchy}}

[[File:Computer storage types.svg|thumb|right|350px|Various forms of storage, divided according to their distance from the [[central processing unit]]. The fundamental components of a general-purpose computer are [[arithmetic logic unit|arithmetic and logic unit]], [[control unit|control circuitry]], storage space, and [[input/output]] devices. Technology and capacity as in common [[home computer]]s around 2005.]]

Generally, the lower a storage is in the hierarchy, the lesser its [[Bandwidth (computing)|bandwidth]] and the greater its access [[latency (engineering)|latency]] 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 fast but temporary [[semiconductor memory|semiconductor]] [[random-access memory|read-write memory]], typically [[Dynamic random-access memory|DRAM]] (dynamic RAM) or other such devices. ''Storage'' consists of storage devices and their media not directly accessible by the [[CPU]] ([[Secondary storage|secondary]] or [[tertiary storage]]), typically [[hard disk drive]]s, [[optical disc]] drives, and other devices slower than RAM but [[non-volatile memory|non-volatile]] (retaining contents when powered down).<ref>''Storage'' as defined in Microsoft Computing Dictionary, 4th Ed. (c)1999 or in The Authoritative Dictionary of IEEE Standard Terms, 7th Ed., (c) 2000.</ref>

Historically, ''memory'' has, depending on technology, been called ''central memory'', ''core memory'', ''core storage'', ''drum'', ''main memory'', ''real storage'', or ''internal memory''. Meanwhile, slower persistent storage devices have been referred to as ''secondary storage'', ''external memory'', or ''auxiliary/peripheral storage''.

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As shown in the diagram, traditionally there are two more sub-layers of the primary storage, besides main large-capacity RAM:

* [[Processor register]]s are located inside the processor. Each register typically holds a [[Word (computer architecture)|word]] of data (often 32 or 64&nbsp;bits). CPU instructions instruct the [[arithmetic logic unit]] 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.

* [[CPU cache|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 [[Memory hierarchy|hierarchical cache]] setup is also commonly used—''primary cache'' being smallest, fastest and located inside the processor; ''secondary cache'' being somewhat larger and slower. <!-- Please DO NOT EXPAND above text, especially with L1/L2/etc variants - reader can always click the link. -->

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 [[Bus (computing)#Address bus|address bus]] and a [[Bus (computing)|data bus]]. The CPU firstly sends a number through an address bus, a number called [[memory address]], that indicates the desired ___location of data. Then it reads or writes the data in the [[Memory cell (computing)|memory cells]] using the data bus. Additionally, a [[memory management unit]] (MMU) is a small device between CPU and RAM recalculating the actual memory address, for example to provide an abstraction of [[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 memory|non-volatile primary storage]] containing a small startup program ([[BIOS]]) is used to [[Bootstrapping#Computing|bootstrap]] 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]] (the terminology may be somewhat confusing as most ROM types are also capable of ''[[random access]]'').

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Recently, ''primary storage'' and ''secondary storage'' in some uses refer to what was historically called, respectively, ''secondary storage'' and ''tertiary storage''.<ref>{{cite web|url=http://searchstorage.techtarget.com/topics/0,295493,sid5_tax298620,00.html|title=Primary storage or storage hardware (shows usage of term "primary storage" meaning "hard disk storage")|url-status=dead|archive-url=https://web.archive.org/web/20080910151628/http://searchstorage.techtarget.com/topics/0,295493,sid5_tax298620,00.html|archive-date=10 September 2008|website=searchstorage.techtarget.com|access-date=18 June 2011}}</ref>

The primary storage, including [[Read-only memory|ROM]], [[EEPROM]], [[NOR flash]], and [[Random access memory|RAM]],<ref>{{cite book | url=https://books.google.com/books?id=QGPHAl9GE-IC&dq=size+of+a+memory+address&pg=PA321 | isbn=978-0-7637-3769-6 | title=The Essentials of Computer Organization and Architecture | date=2006 | publisher=Jones & Bartlett Learning }}</ref> are usually [[byte-addressable]].

=== Secondary storage ===

<!-- Note that additional direct links point to this section by its name. -->''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]]s (HDDs) or [[solid-state drive]]s (SSDs) are usually used as secondary storage. The [[access time]] per byte for HDDs or SSDs is typically measured in [[millisecond]]s (thousandths of a second), while the access time per byte for primary storage is measured in [[nanosecond]]s (billionths of a second). Thus, secondary storage is significantly slower than primary storage. Rotating [[Optical disc drive|optical storage]] devices, such as [[Compact disc|CD]] and [[DVD]] drives, have even longer access times. Other examples of secondary storage technologies include [[USB flash drive]]s, [[floppy disk]]s, [[magnetic-tape data storage|magnetic tape]], [[punched tape|paper tape]], [[punched card]]s, and [[RAM drive|RAM disks]].

Once the [[Disk read-and-write head|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 on sequential and block access. Another way to reduce the I/O bottleneck is to use multiple disks in parallel to increase the bandwidth between primary and secondary memory, for example, using [[RAID]].<ref>{{cite book|author=[[Jeffrey Vitter|J. S. Vitter]]|url=http://www.ittc.ku.edu/~jsv/Papers/Vit.IO_book.pdf|url-status=live|title=Algorithms and data structures for external memory|archive-url=https://web.archive.org/web/20110104233254/http://www.ittc.ku.edu/~jsv/Papers/Vit.IO_book.pdf |archive-date=4 January 2011|series=Series on foundations and trends in theoretical computer science|publisher=now Publishers|___location=Hanover, MA|year=2008|isbn=978-1-60198-106-6}}</ref>

Secondary storage is often formatted according to a [[file system]] format, which provides the abstraction necessary to organize data into [[Computer file|files]] and [[Directory (computing)|directories]], while also providing [[metadata]] describing the owner of a certain file, the access time, the access permissions, and other information.

Most computer [[operating system]]s use the concept of [[virtual memory]], allowing the 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 ([[Page (computer memory)|pages]]) 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.

The secondary storage, including [[Hard disk drive|HDD]], [[optical disc drive|ODD]] and [[Solid state drive|SSD]], are usually block-addressable.

=== Tertiary storage ===

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[[File:StorageTek Powderhorn tape library.jpg|thumb|A large [[tape library]], with tape cartridges placed on shelves in the front, and a robotic arm moving in the back. The visible height of the library is about 180&nbsp;cm.]]

''Tertiary storage'' or ''tertiary memory''<ref>{{cite web|url=http://www.eecs.berkeley.edu/Pubs/TechRpts/1994/CSD-94-847.pdf|url-status=live|title=A thesis on tertiary storage|archive-url=https://web.archive.org/web/20070927233543/http://www.eecs.berkeley.edu/Pubs/TechRpts/1994/CSD-94-847.pdf |archive-date=27 September 2007|access-date=18 June 2011}}</ref> 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 library|tape libraries]] and [[optical jukebox]]es.

When a computer needs to read information from the tertiary storage, it will first consult a catalog [[database]] to determine which tape or disc contains the information. Next, the computer will instruct a [[industrial robot|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.

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* 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 ([[Non-RAID drive architectures#MAID|MAID]]), are nearline storage. Removable media such as [[tape cartridgescartridge]]s that can be automatically loaded, as in [[Tape library|tape libraries]], are nearline storage, while tape cartridges that must be manually loaded are offline storage.

=== Off-line storage ===

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|url-status = dead

|archive-url = https://web.archive.org/web/20090302235918/http://www.its.bldrdoc.gov/fs-1037/fs-1037c.htm

|archive-date = 2 March 2009}} See also article [[Federal Standard 1037C|Federal standard 1037C]].</ref> 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.

[[Online and offline|Off-line]] storage is used to [[Data communication|transfer information]] since the detached medium can easily be physically transported. Additionally, it is useful for cases of disaster, where, for example, a fire destroys the original data, a medium in a remote ___location will be unaffected, enabling [[IT disaster recovery|disaster recovery]]. Off-line storage increases general [[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 the most popular, and to a much lesser extent removable hard disk drives; older examples include floppy disks and Zip disks. In enterprise uses, magnetic tape cartridges are predominant; older examples include open-reel magnetic tape and punched cards.

== Characteristics of storage ==

[[File:Samsung-1GB-DDR2-Laptop-RAM.jpg|thumb|250px|A 1&nbsp;GiB module of [[SO-DIMM|laptop]] [[DDR2 SDRAM|DDR2]] [[Random-access memory|RAM]]]]

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.

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=== 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]] and [[Solid-state drive|SSD]].

; Write once storage : [[Write once read many]] (WORM) allows the information to be written only once at some point after manufacture. Examples include semiconductor [[Programmable ROM|programmable read-only memory]] and [[CD-R]].

; Read only storage : Retains the information stored at the time of manufacture. Examples include [[Read-only memory#Factory programmed|mask ROM ICs]] and [[CD-ROM]].

=== Accessibility ===

; [[Random access]]: 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 memory|semiconductor memories]], [[Flash_memory|flash memories]] and [[hard disk drive]]s provide random access, though both semiconductor and flash memories have minimal [[latency_(engineering)|latency]] when compared to hard disk drives, as no mechanical parts need to be moved.

; [[Sequential access]]: 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 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 system|File addressable]]: Information is divided into ''[[computer file|files]]'' of variable length, and a particular file is selected with [[Human-readable medium|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 (computer science)|abstraction]] to make the operation more understandable. In modern computers, secondary, tertiary and off-line storage use file systems.

; [[content-addressable memory|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]] (computer program) or [[Computer hardware|hardware]] (computer device), with hardware being faster but more expensive option. Hardware content addressable memory is often used in a computer's [[CPU cache]].

=== 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]]s or [[byte]]s (e.g. 10.4 [[megabyte]]s).

; [[Areal density (computer storage)|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 ===

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=== 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.<ref>{{cite web|url=http://www.springlightcfl.com/consumer/energy_savings_calculator.aspx|title=Energy savings calculator|url-status=dead|archive-url=https://web.archive.org/web/20081221131054/http://springlightcfl.com/consumer/energy_savings_calculator.aspx|archive-date=21 December 2008}}</ref><ref>{{Cite web|url=http://www.simpletech.com/content/eco-friendly-redrive|url-status=dead|archive-url=https://web.archive.org/web/20080805092907/http://www.simpletech.com/content/eco-friendly-redrive|archive-date=5 August 2008|title=How much of the [re]drive is actually eco-friendly?|website=Simple tech}}</ref>

* 2.5-inch hard disk drives often consume less power than larger ones.<ref>{{cite web|title=IS the Silent PC Future 2.5-inches wide?|url=http://www.silentpcreview.com/article145-page1.html|access-date=2 August 2008|author=Mike Chin|date=8 March 2004|url-status=live|archive-url=https://web.archive.org/web/20080720000101/http://www.silentpcreview.com/article145-page1.html|archive-date=20 July 2008}}</ref><ref>{{cite web|url=http://www.silentpcreview.com/article29-page2.html|title=Recommended hard drives|access-date=2 August 2008|author=Mike Chin|date=18 September 2002|url-status=live|archive-url=https://web.archive.org/web/20080905085853/http://www.silentpcreview.com/article29-page2.html|archive-date=5 September 2008}}</ref> Low capacity [[solid-state drive]]s have no moving parts and consume less power than hard disks.<ref>{{Cite web|url=http://techreport.com/articles.x/10334/13|title=Super Talent's 2.5" IDE flash hard drive|website=The tech report|date=12 July 2006|page=13|archive-url=https://web.archive.org/web/20120126045422/http://techreport.com/articles.x/10334/13|archive-date=26 January 2012|access-date=18 June 2011}}</ref><ref>{{Cite web|url=httphttps://www.tomshardware.com/reviews/conventional-hard-drive-obsoletism,1324-5.html|title=Power consumption – Tom's hardware : Conventional hard drive obsoletism? Samsung's 32 GB flash drive previewed|website=tomshardware.com|date=20 September 2006|access-date=18 June 2011}}</ref><ref name=xbitSSDvsHD/> Also, memory may use more power than hard disks.<ref name=xbitSSDvsHD>{{cite web|title=SSD, i-RAM and traditional hard disk drives|date=23 April 2008|url=http://www.xbitlabs.com/articles/storage/display/ssd-iram.html|author=Aleksey Meyev|publisher=X-bit labs|url-status=dead|archive-url=https://web.archive.org/web/20081218134355/http://www.xbitlabs.com/articles/storage/display/ssd-iram.html|archive-date=18 December 2008}}</ref> Large caches, which are used to avoid hitting the [[memory wall]], may also consume a large amount of power.

=== Security ===

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==== Error detection ====

[[File:QPxTool DVD error rate graph.png|thumb|[[Optical_disc#Surface_error_scanning|Error rate measurement]] on a [[DVD recordable#DVD+R and DVD+RW (DVD "plus")|DVD+R]]. The minor errors are correctable and within a healthy range.]]

Impending failure on [[hard disk drive]]s 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.<ref>{{cite web|title=What S.M.A.R.T. hard disk errors actually tell us |url=https://www.backblaze.com/blog/what-smart-stats-indicate-hard-drive-failures/|website=Backblaze|date=6 October 2016}}</ref>

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 disc|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 disc drive|optical drives]] support error scanning.<ref>{{cite web |title=QPxTool - check the quality|url=https://qpxtool.sourceforge.io/faq.html|website=qpxtool.sourceforge.io}}</ref>

== {{anchor|media}}Storage media ==

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{{Main|Semiconductor memory}}

[[Semiconductor memory]] uses [[semiconductor]]-based [[integrated circuit]] (IC) chips to store information. Data are typically stored in [[MOSFET|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 [[MOSFET#MOS capacitor|MOS capacitors]]. Both ''volatile'' and ''non-volatile'' forms of semiconductor memory exist, the former using standard MOSFETs and the latter using [[floating-gate MOSFET]]s.

In modern computers, primary storage almost exclusively consists of dynamic volatile semiconductor [[random-access memory]] (RAM), particularly [[dynamic random-access memory]] (DRAM). Since the turn of the century, a type of non-volatile [[Floating-gate MOSFET|floating-gate]] 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 that are designed for them.

As early as 2006, [[Laptop|notebook]] and [[desktop computer]] manufacturers started using flash-based [[solid-state drive]]s (SSDs) as default configuration options for the secondary storage either in addition to or instead of the more traditional HDD.<ref>{{Cite web|url=http://www.extremetech.com/article2/0,1558,1966644,00.asp|title=New Samsung notebook replaces hard drive with flash|archive-url=https://web.archive.org/web/20101230081543/http://www.extremetech.com/article2/0,1558,1966644,00.asp|archive-date=30 December 2010|website=Extreme tech|date=23 May 2006|access-date=18 June 2011}}</ref><ref>{{Cite web|url=http://www.technewsworld.com/rsstory/60700.html?wlc=1308338527|title=Toshiba tosses hat into notebook flash storage ring|archive-url=https://web.archive.org/web/20120318100815/http://www.technewsworld.com/rsstory/60700.html?wlc=1308338527|archive-date=18 March 2012|website=technewsworld.com|access-date=18 June 2011}}</ref><ref>{{Cite web|url=https://www.apple.com/macpro/features/storage.html|url-status=dead|title=Mac Pro – Storage and RAID options for your Mac Pro|archive-url=https://web.archive.org/web/20130606070652/http://www.apple.com/macpro/features/storage.html|archive-date=6 June 2013|publisher=Apple|date=27 July 2006|access-date=18 June 2011}}</ref><ref>{{Cite web|url=https://www.apple.com/macbookair/design.html|url-status=dead|title=MacBook Air – The best of iPad meets the best of Mac|archive-url=https://web.archive.org/web/20130527205906/http://www.apple.com/macbookair/design.html|archive-date=27 May 2013|publisher=Apple|access-date=18 June 2011}}</ref><ref>{{Cite web|url=http://news.inventhelp.com/Articles/Computer/Inventions/apple-macbook-air-12512.aspx|url-status=dead|title=MacBook Air replaces the standard notebook hard disk for solid state flash storage|archive-url=https://web.archive.org/web/20110823052120/http://news.inventhelp.com/Articles/Computer/Inventions/apple-macbook-air-12512.aspx|archive-date=23 August 2011|website=news.inventhelp.com|date=15 November 2010|access-date=18 June 2011}}</ref>

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{{Magnetic storage media}}

[[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]];

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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.<ref>{{cite web |title=Comparing SSD and HDD endurance in the age of QLC SSDs|url=https://www.micron.com/-/media/client/global/documents/products/white-paper/5210_ssd_vs_hdd_endurance_white_paper.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://www.micron.com/-/media/client/global/documents/products/white-paper/5210_ssd_vs_hdd_endurance_white_paper.pdf |archive-date=2022-10-09 |url-status=live|publisher=Micron technology}}</ref><ref>{{cite web|title=Comparing SSD and HDD - A comprehensive comparison of the storage drives|url=https://www.stellarinfo.co.in/kb/ssd-vs-hdd.php |website=www.stellarinfo.co.in|date=28 February 2025 |language=en}}</ref>

=== Optical ===

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[[Optical storage]], the typical [[optical disc]], 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 in common use {{As of|2009|lc=true}}:<ref>{{Cite web|url=http://www.dvddemystified.com/dvdfaq.html|title=The DVD FAQ - A comprehensive reference of DVD technologies|archive-url=https://web.archive.org/web/20090822172353/http://www.dvddemystified.com/dvdfaq.html|archive-date=22 August 2009}}</ref>

* [[Compact disc|CD]], [[CD-ROM]], [[DVD]], [[Blu-ray|BD-ROM]]: Read only storage, used for mass distribution of digital information (music, video, computer programs);

* [[CD-R]], [[DVD recordable#DVD-R and DVD-RW (DVD "dash")|DVD-R]], [[DVD recordable#DVD+R and DVD+RW (DVD "plus")|DVD+R]], [[Blu-ray Disc recordable|BD-R]]: Write once storage, used for tertiary and off-line storage;

* [[CD-RW]], [[DVD recordable#DVD-R and DVD-RW (DVD "dash")|DVD-RW]], [[DVD recordable#DVD+R and DVD+RW (DVD "plus")|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.

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{{Paper data storage media}}

[[Paper data storage]], typically in the form of [[punched tape|paper tape]] or [[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. [[Barcode]]s 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 [[Barcode#Matrix (2D) barcodes|matrix barcode]] for very long-term storage, as the longevity of paper typically exceeds even magnetic data storage.<ref>{{Cite web|url=https://www.extremetech.com/extreme/134427-a-paper-based-backup-solution-not-as-stupid-as-it-sounds|date=14 August 2012|title=A paper-based backup solution (not as stupid as it sounds)}}</ref><ref>{{Cite magazine|url=https://www.wired.com/2012/08/paperback-paper-backup/|date=16 August 2012|title=PaperBack paper backup|magazine=Wired|last1=Sterling|first1=Bruce}}</ref>

=== Other storage media or substrates ===

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; [[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 and conductance|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 [[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. See [[Holographic Versatile Disc]] (HVD).

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* [[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|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 library|tape libraries]], and in optical storage field [[optical jukebox]]es, or optical disk libraries per analogy. The smallest forms of either technology containing just one drive device are referred to as [[Tape library#Autoloaders|autoloaders]] or [[Record changer|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 [[Byte#Multiple-byte units|terabytesterabyte]]s or [[Byte#Multiple-byte units|petabytespetabyte]]s of near-line information. Optical jukeboxes are somewhat smaller solutions, up to 1,000 slots.

Robotic storage is used for [[backup]]s, 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.

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== Further reading ==

* {{cite journal|title=The history of storage systems|journal=[[Proceedings of the IEEE]]|author1=Goda, K. |author2=Kitsuregawa, M. |year=2012|pages=1433–1440|volume=100|doi=10.1109/JPROC.2012.2189787|doi-access=free}}

* [httphttps://www.computerhistory.org/revolution/memory-storage/8 Memory & storage], Computer history museum

{{Basic computer components}}