View source for Computer data storage

{{short description|Storage of digital data readable by computers}}
{{Merge from|Data at rest|discuss=Talk:Computer data storage#Proposed merge of Data at rest into Computer data storage|date=March 2025}}
{{broader|Data storage}}

[[File:DDR2 ram mounted.jpg|thumb|1&nbsp;[[Gibibyte|GiB]] of SDRAM mounted in a [[Computer|computer]]. An example of ''primary storage''.]]
[[File:IBM DJNA-351520 Hard Disk A.jpg|thumb|15&nbsp;[[Gigabyte|GB]] [[Engineering:Parallel ATA|PATA]] hard disk drive (HDD) from 1999. When connected to a computer it serves as ''secondary'' storage.]]{{Memory types}}
'''Computer data storage''' or '''digital data storage''' is the retention of [[Digital data|digital data]] via [[Engineering:Technology|technology]] consisting of [[Computer|computer]] components and [[Data storage|recording media]]. Digital data storage 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=[[Place:Amsterdam|Amsterdam]] |oclc=56213091 |url-access=registration |url=https://archive.org/details/isbn_9781558606043 }}</ref>{{rp|15–16}} 

Generally, the faster and volatile storage components are referred to as "[[Computer memory|memory]]", while slower persistent components are referred to as "storage". This distinction was extended in the [[Von Neumann architecture]], where the [[Central processing unit|central processing unit]] (CPU) consists of two main parts: The [[Control unit|control unit]] and the [[Arithmetic logic unit|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. In practice, almost all computers use a [[Memory hierarchy|memory hierarchy]],<ref name="Patterson" />{{rp|468–473}} which puts memory close to the CPU and storage further away. 

In modern computers, [[Engineering:Hard disk drive|hard disk drive]]s (HDDs) or [[Solid-state drive|solid-state drive]]s (SSDs) are usually used as storage. 

== Data ==
A modern [[Computer|digital computer]] represents [[Data|data]] using the binary numeral system. The [[Memory cell (computing)|memory cell]] is the fundamental building block of [[Computer memory|computer memory]], storing stores one bit of binary information that can be set to store a 1, reset to store a 0, and accessed by reading the cell.<ref>{{cite book |last1=D. Tang |first1=Denny |url=https://books.google.com/books?id=xsMMWOjFXzAC&q=ferrite%20core%20memory%20magnetic%20bubble&pg=PA91 |title=Magnetic memory: Fundamentals and technology |last2=Lee |first2=Yuan-Jen |publisher=Cambridge University Press |year=2010 |isbn=978-1139484497 |page=91 |access-date=13 December 2015}}</ref><ref>{{cite book |last1=Fletcher |first1=William |url=https://archive.org/details/engineeringappro00fle_wur/page/283 |title=An engineering approach to digital design |publisher=Prentice-Hall |year=1980 |isbn=0-13-277699-5 |page=[https://archive.org/details/engineeringappro00fle_wur/page/283 283] |url-access=registration}}</ref> 

Text, numbers, pictures, audio, and nearly any other form of information can be converted into a string of [[Bit|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|byte]], equal to 8 bits. [[Digital data]] comprises the binary representation of a piece of information, often being [[Code|encoded]] by assigning a bit pattern to each [[Character (computing)|character]], [[Numerical digit|digit]], or [[Engineering:Multimedia|multimedia]] object. Many standards exist for encoding (e.g. [[Character encoding|character encoding]]s like [[ASCII]], image encodings like [[JPEG]], and video encodings like [[MPEG-4]]). 

=== Encryption ===
For [[Data security|security reasons]], certain types of data may be encrypted in storage to prevent the possibility of unauthorized information reconstruction from chunks of storage snapshots. Encryption in transit protects data as it is being transmitted.<ref name="MC_1">{{cite web |date=2023 |title=4 Ways: Transfer Files from One OneDrive Account to Another without Downloading |url=https://www.multcloud.com/tutorials/transfer-files-from-one-onedrive-account-to-another.html |url-status=live |archive-url=https://web.archive.org/web/20230616124728/https://www.multcloud.com/tutorials/transfer-files-from-one-onedrive-account-to-another.html |archive-date=16 June 2023 |access-date=5 December 2023 |publisher=MultCloud}}</ref>

=== Compression ===
[[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.

=== 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 crash|head crash]]ing on mechanical hard drives and [[Engineering:Failure of electronic components|failure of electronic components]] on flash storage.

==== Redundancy ====
[[Redundancy (information theory)|Redundancy]] allows the computer to detect errors in coded data (for example, a random [[RAM parity|bit flip]] due to random [[Physics:Radiation|radiation]]) and correct them based on mathematical algorithms. The [[Cyclic redundancy check|cyclic redundancy check]] (CRC) method is typically used in communications and storage for error detection. Redundancy solutions include [[Replication (computing)|storage replication]], [[Disk mirroring|disk mirroring]] and RAID (Redundant Array of Independent Disks).

==== Error detection ====
[[File:QPxTool DVD error rate graph.png|thumb|[[Engineering:Optical disc#Surface error scanning|Error rate measurement]] on a DVD+R. The minor errors are correctable and within a healthy range.]]
Impending failure on [[Engineering:Hard disk drive|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 |date=6 October 2016 |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}}</ref> The health of optical media can be determined by [[Engineering: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 [[Engineering: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>

== Architecture ==
Without a significant amount of memory, a computer would only be able to perform fixed operations and immediately output the result, thus requiring hardware reconfiguration for a new program to be run. This is often used in devices such as desk [[Engineering:Calculator|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 operating [[Instruction set architecture#Instructions|instructions]] and data are stored,<ref name="Patterson"/>{{rp|20}} such 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 [[Computer architecture|design]], in that a relatively simple [[Engineering:Processor (computing)|processor]] may keep [[State (computer science)|state]] between successive computations to build up complex procedural results. Most modern computers are von Neumann machines.

=== Storage and memory ===
In contemporary usage, the term "storage" typically refers to a subset of computer data storage that comprises storage devices and their media not directly accessible by the CPU, that is, secondary or tertiary storage. Common forms of storage include [[Engineering:Hard disk drive|hard disk drive]]s, [[Engineering:Optical disc|optical disc]] drives, and [[Non-volatile memory|non-volatile]] devices (i.e. devices that retain their contents when the computer is 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> On the other hand, the term "[[Random-access memory|memory]]" is used to refer to [[Engineering:Semiconductor memory|semiconductor]] [[Read–write memory|read-write]] data storage, typically [[Dynamic random-access memory|dynamic random-access memory]] (DRAM). [[Dynamic random-access memory]] is a form of volatile memory that also requires the stored information to be periodically reread and rewritten, or [[Memory refresh|refreshed]]; [[Static random-access memory|static RAM]] (SRAM) is similar to DRAM, albeit it never needs to be refreshed as long as power is applied. 

In contemporary usage, the [[Memory hierarchy|memory hierarchy]] of primary storage and secondary storage in some uses refer to what was historically called, respectively, ''secondary storage'' and ''tertiary storage''.<ref>{{cite web |title=Primary storage or storage hardware (shows usage of term "primary storage" meaning "hard disk storage") |url=http://searchstorage.techtarget.com/topics/0,295493,sid5_tax298620,00.html |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 |access-date=18 June 2011 |website=searchstorage.techtarget.com}}</ref>

==== Primary ====<!-- Note that additional direct links point to this section by its name. -->
[[File:Computer storage types.svg|thumb|right|350px|Various forms of storage, divided according to their distance from the [[Central processing unit|central processing unit]]. The fundamental components of a general-purpose computer are arithmetic and logic unit, [[Control unit|control circuitry]], storage space, and input/output devices. Technology and capacity as in common [[Engineering:Home computer|home computer]]s around 2005.]]

'''Primary storage''' (also known as ''main memory'', ''internal memory'', or ''prime memory''), often referred to simply as ''memory'', is storage 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 a uniform manner. Historically, [[History of computing hardware|early computers]] used [[Delay-line memory|delay lines]], [[Williams tube]]s, or rotating [[Drum memory|magnetic drums]] as primary storage. By 1954, those unreliable methods were mostly replaced by [[Magnetic-core memory|magnetic-core memory]]. Core memory remained dominant until the 1970s, when advances in [[Engineering:Integrated circuit|integrated circuit]] technology allowed [[Engineering:Semiconductor memory|semiconductor memory]] to become economically competitive.<!-- Please do not expand above text! Change but no expand any further. Anyone desiring to know more types, let it be [[Twistor memory|twistor memory]] or [[Bubble memory|bubble memory]] etc, will click the History link. Do not confuse other readers. -->

This led to modern [[Random-access memory|random-access memory]], which is small-sized, light, and relatively expensive. RAM used for primary storage is [[Volatile memory|volatile]], meaning that it loses the information when not powered. Besides storing opened programs, it serves as [[Page cache|disk cache]] and [[Write buffer|write buffer]] to improve both reading and writing performance. Operating systems borrow RAM capacity for caching so long as it's not needed by running software.<ref>{{cite web| url = https://www.kernel.org/doc/html/latest/admin-guide/sysctl/vm.html| title = Documentation for /proc/sys/vm/ — The Linux Kernel documentation}}</ref> Spare memory can be utilized as [[RAM drive]] for temporary high-speed data storage. Besides main large-capacity RAM, there are two more sub-layers of primary storage:
* [[Processor register]]s are the fastest of all forms of data storage, being located inside the processor, with each register typically holding a [[Word (computer architecture)|word]] of data (often 32 or 64&nbsp;bits). CPU instructions instruct the [[Arithmetic logic unit|arithmetic logic unit]] to perform various calculations or other operations on this data.
* Processor cache is an intermediate stage between faster registers and slower main memory, being faster than main memory but with much less capacity. Multi-level [[Memory hierarchy|hierarchical cache]] setup is also commonly used, such that primary cache is the smallest and fastest, while secondary cache is larger and slower.<!-- Please DO NOT EXPAND above text, especially with L1/L2/etc variants - reader can always click the link. -->

Primary storage, including [[Read-only memory|ROM]], [[EEPROM]], NOR flash, and RAM,<ref>{{cite book |url=https://books.google.com/books?id=QGPHAl9GE-IC&dq=size+of+a+memory+address&pg=PA321 |title=The Essentials of Computer Organization and Architecture |date=2006 |publisher=Jones & Bartlett Learning |isbn=978-0-7637-3769-6}}</ref> is usually byte-addressable. Such memory is directly or indirectly connected to the central processing unit via a [[Engineering:Bus (computing)|memory bus]], comprising an [[Engineering:Address bus|address bus]] and a data bus. The CPU firstly sends a number called the [[Memory address|memory address]] through the address bus 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|memory management unit]] (MMU) is a small device between CPU and RAM recalculating the actual memory address. Memory management units allow for [[Memory management|memory management]]; they may, for example, provide an [[Abstraction (computer science)|abstraction]] of [[Virtual memory|virtual memory]] or other tasks.

===== BIOS =====
[[Non-volatile memory|Non-volatile primary storage]] contains a small startup program ([[BIOS]]) is used to [[Finance: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 [[Read-only memory|read-only memory]] (ROM). Most types of "ROM" are not literally read only but are difficult and slow to write to''.'' Some [[Embedded system|embedded system]]s run programs directly from ROM, because such programs are rarely changed. Standard computers largely do not store many programs in ROM, apart from [[Firmware|firmware]], and use large capacities of secondary storage.

==== Secondary ====<!-- 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. Computers use 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, [[Engineering:Hard disk drive|hard disk drive]]s (HDDs) or [[Solid-state drive|solid-state drive]]s (SSDs) are usually used as secondary storage. The [[Access time|access time]] per byte for HDDs or SSDs is typically measured in [[Millisecond|millisecond]]s, while the access time per byte for primary storage is measured in [[Nanosecond|nanosecond]]s. Rotating [[Engineering:Optical disc drive|optical storage]] devices, such as [[Engineering:CD|CD]] and [[Engineering:DVD|DVD]] drives, have even longer access times. Other examples of secondary storage technologies include [[Engineering:USB flash drive|USB flash drive]]s, [[Floppy disk|floppy disk]]s, [[Physics:Magnetic-tape data storage|magnetic tape]], paper tape, [[Engineering:Punched card|punched card]]s, and [[RAM drive|RAM disks]].

To reduce the seek time and rotational latency, secondary storage, including [[Engineering:Hard disk drive|HDD]], [[Engineering:Optical disc drive|ODD]] and SSD, are transferred to and from [[Disk formatting|disks]] in large contiguous blocks. Secondary storage is [[Address space|addressable]] by block; 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. 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=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|file system]] format, which provides the abstraction necessary to organize data into [[Computer file|files]] and [[Directory (computing)|directories]], while also providing [[Metadata|metadata]] describing the owner of a certain file, the access time, the access permissions, and other information. Most computer [[Operating system|operating system]]s use the concept of [[Virtual memory|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.

==== Tertiary ====<!-- Note that additional direct links point to this section by its name. -->
[[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''' typically involves a [[Engineering:Industrial robot|robotic arm]] which mounts and dismount removable mass storage media from a catalog [[Database|database]] into a storage device according to the system's demands. 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, [[Engineering:Optical jukebox|optical jukebox]]es, and massive arrays of idle disks (MAID). Tertiary storage is also known as [[Nearline storage|nearline storage]] because it is "near to online".<ref name="pearson2010">{{cite web |last=Pearson |first=Tony |year=2010 |url=https://www.ibm.com/developerworks/community/blogs/InsideSystemStorage/entry/the_correct_use_of_the_term_nearline2 |url-status=dead|title=Correct use of the term nearline|work=IBM developer-works, inside system storage|archive-url=https://web.archive.org/web/20151124110253/https://www.ibm.com/developerworks/community/blogs/InsideSystemStorage/entry/the_correct_use_of_the_term_nearline2?lang=en|archive-date=24 November 2015|access-date=16 August 2015}}</ref> [[Hierarchical storage management]] is an archiving strategy involving automatically migrating long-unused files from fast hard disk storage to libraries or jukeboxes.

==== Offline ====<!-- Additional direct links point to this section by name. -->
'''Offline storage''' is computer data storage on a medium or a device that is not under the control of a [[Central processing unit|processing unit]].<ref>{{Cite tech report
 |last         = National Communications System
 |title        = Federal Standard 1037C – Telecommunications: Glossary of Telecommunication Terms
 |publisher    = General Services Administration
 |date         = August 7, 1996 
 |id           = FS-1037C
 |url          = http://www.its.bldrdoc.gov/fs-1037/fs-1037c.htm
 |access-date  = 2007-10-08
 |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.</ref> The medium is recorded, usually in a secondary or tertiary storage device, and then physically removed or disconnected. Unlike tertiary storage, it cannot be accessed without human interaction. It is used to [[Data communication|transfer information]] since the detached medium can easily be physically transported. In modern personal computers, most secondary and tertiary storage media are also used for offline storage.

=== Network connectivity ===
A secondary or tertiary storage may connect to a computer utilizing [[Computer network|computer network]]s. This concept does not pertain to the primary storage.

* [[Direct-attached storage]] (DAS) is a traditional mass storage, that does not use any network.
* [[Network-attached storage]] (NAS) is mass storage attached to a computer which another computer can access at file level over a [[Engineering:Local area network|local area network]], a private [[Wide area network|wide area network]], or in the case of [[Software:File-hosting service|online file storage]], over the [[Internet]]. NAS is commonly associated with the [[Network File System|NFS]] and CIFS/SMB protocols.
* [[Storage area network]] (SAN) is a specialized network, that provides other computers with storage capacity. SAN is commonly associated with [[Engineering:Fibre Channel|Fibre Channel]] networks.

== Cloud ==
[[Cloud storage]] is based on highly [[Virtualization|virtualized]] infrastructure.<ref>{{Cite web |date=18 August 2023 |title=Disaster Recovery on AWS Cloud |url=https://zenbit.tech/blog/disaster-recovery-on-aws-cloud/}}</ref> A subset of [[Cloud computing|cloud computing]], it has particular cloud-native interfaces, near-instant elasticity and [[Scalability|scalability]], multi-tenancy, and metered resources. Cloud storage services can be used from an off-premises service or deployed on-premises.<ref>{{cite web |title=On-premises private cloud storage description, characteristics, and options |url=http://www.vion.com/capacity-services/vion-capacity-services.html |url-status=dead |archive-url=https://web.archive.org/web/20160322022215/http://www.vion.com/capacity-services/vion-capacity-services.html |archive-date=2016-03-22 |access-date=2012-12-10}}</ref> 

=== Deployment models ===
Cloud deployment models define the interactions between cloud providers and customers.<ref name=":1">{{Cite journal |date=September 2023 |title=ISO/IEC 22123-2:2023(E) - Information technology — Cloud computing — Part 2: Concepts |journal=International Organization for Standardization}}</ref> 

* [[Private cloud computing infrastructure|Private clouds]], for example, are used in cloud security to mitigate the increased [[Attack surface|attack surface area]] of [[Outsourcing|outsourcing]] data storage.<ref>{{cite web |title=The Attack Surface Problem |url=http://www.sans.edu/research/security-laboratory/article/did-attack-surface |url-status=dead |archive-url=https://web.archive.org/web/20130921221257/http://www.sans.edu/research/security-laboratory/article/did-attack-surface |archive-date=2013-09-21 |access-date=2013-07-08 |publisher=Sans.edu}}</ref> A private cloud is cloud infrastructure operated solely for a single organization, whether managed internally or by a third party, or hosted internally or externally.<ref name="nist">{{cite tech report |title=The NIST Definition of Cloud Computing |author1=Mell, Peter |author2=Timothy Grance |date=September 2011 |number=Special publication 800-145 |institution=National Institute of Standards and Technology: U.S. Department of Commerce |doi=10.6028/NIST.SP.800-145}}</ref>
* [[Hybrid cloud storage]] are another cloud security solution, involving storage infrastructure that uses a combination of on-premises storage resources with cloud storage. The on-premises storage is usually managed by the organization, while the public cloud storage provider is responsible for the management and security of the data stored in the cloud.<ref name="jones1">{{cite web |last=Jones |first=Margaret |date=July 2019 |title=Hybrid Cloud Storage |url=https://www.techtarget.com/searchstorage/definition/hybrid-cloud-storage |url-status=live |archive-url=https://web.archive.org/web/20220621203942/https://www.techtarget.com/searchstorage/definition/hybrid-cloud-storage |archive-date=2022-06-21 |access-date=2022-06-22 |work=SearchStorage}}</ref><ref name="searchstorage12">{{cite web |last=Barrett |first=Mike |date=July 2014 |title=Definition: cloud storage gateway |url=https://www.techtarget.com/searchstorage/definition/cloud-storage-gateway |url-status=live |archive-url=https://web.archive.org/web/20220621201806/https://www.techtarget.com/searchstorage/definition/cloud-storage-gateway |archive-date=2022-06-21 |access-date=2022-06-22 |work=SearchStorage}}</ref> Using a hybrid model allows data to be ingested in an encrypted format where the key is held within the on-premise infrastructure and can limit access to the use of on-premise [[Cloud storage gateway|cloud storage gateways]], which may have options to encrypt the data prior to transfer.<ref name="searchstorage1">{{cite web |last=Barrett |first=Mike |date=July 2014 |title=Definition: cloud storage gateway |url=https://www.techtarget.com/searchstorage/definition/cloud-storage-gateway |url-status=live |archive-url=https://web.archive.org/web/20220621201806/https://www.techtarget.com/searchstorage/definition/cloud-storage-gateway |archive-date=2022-06-21 |access-date=2022-06-22 |work=SearchStorage}}</ref>
* Cloud services are considered "public" when they are delivered over the public Internet.<ref>{{cite web |last=Rouse |first=Margaret |title=What is public cloud? |url=http://searchcloudcomputing.techtarget.com/definition/public-cloud |url-status=live |archive-url=https://web.archive.org/web/20141016022343/http://searchcloudcomputing.techtarget.com/definition/public-cloud |archive-date=16 October 2014 |access-date=12 October 2014 |publisher=Definition from Whatis.com}}</ref>

=== Types ===
There are three types of cloud storage: 

* [[Object storage]]<ref>{{cite book |last1=Kolodner |first1=Elliot K. |title=2011 IEEE Third International Conference on Cloud Computing Technology and Science |last2=Tal |first2=Sivan |last3=Kyriazis |first3=Dimosthenis |last4=Naor |first4=Dalit |last5=Allalouf |first5=Miriam |last6=Bonelli |first6=Lucia |last7=Brand |first7=Per |last8=Eckert |first8=Albert |last9=Elmroth |first9=Erik |year=2011 |isbn=978-1-4673-0090-2 |pages=357–366 |chapter=A Cloud Environment for Data-intensive Storage Services |citeseerx=10.1.1.302.151 |doi=10.1109/CloudCom.2011.55 |last10=Gogouvitis |first10=Spyridon V. |last11=Harnik |first11=Danny |last12=Hernandez |first12=Francisco |last13=Jaeger |first13=Michael C. |last14=Bayuh Lakew |first14=Ewnetu |last15=Manuel Lopez |first15=Jose |last16=Lorenz |first16=Mirko |last17=Messina |first17=Alberto |last18=Shulman-Peleg |first18=Alexandra |last19=Talyansky |first19=Roman |last20=Voulodimos |first20=Athanasios |last21=Wolfsthal |first21=Yaron |s2cid=96939}}</ref><ref name="oceanstore">S. Rhea, C. Wells, P. Eaton, D. Geels, B. Zhao, H. Weatherspoon, and J. Kubiatowicz, Maintenance-Free Global Data Storage. IEEE Internet Computing, Vol 5, No 5, September/October 2001, pp 40–49. [http://oceanstore.cs.berkeley.edu/publications/papers/pdf/ieeeic.pdf] {{Webarchive|url=https://web.archive.org/web/20120329061438/http://oceanstore.cs.berkeley.edu/publications/papers/pdf/ieeeic.pdf|date=2012-03-29}} [http://oceanstore.cs.berkeley.edu/info/overview.html] {{Webarchive|url=https://web.archive.org/web/20110623031932/http://oceanstore.cs.berkeley.edu/info/overview.html|date=2011-06-23}}</ref>
* File storage
* [[Block-level storage]] is a concept in cloud-hosted [[Persistent data structure|data persistence]] where [[Cloud computing|cloud services]] emulate the behaviour of a traditional [[Device file#Block devices|block device]], such as a physical [[Engineering:Hard disk drive|hard drive]],<ref>{{Cite book |last1=Wittig |first1=Andreas |title=Amazon web services in action |last2=Wittig |first2=Michael |publisher=Manning press |year=2015 |isbn=978-1-61729-288-0 |pages=204–206}}</ref> where storage is organised as [[Block (data storage)|blocks]]. Block-level storage differs from [[Object storage|object stores]] or 'bucket stores' or to [[Cloud database|cloud databases]]. These operate at a higher level of abstraction and are able to work with entities such as files, documents, images, videos or database records.<ref>{{Cite web |last=Taneja |first=Arun |title=How an object store differs from file and block storage |url=http://searchcloudstorage.techtarget.com/feature/How-an-object-store-differs-from-file-and-block-storage |url-status=dead |archive-url=https://web.archive.org/web/20160213202025/http://searchcloudstorage.techtarget.com/feature/How-an-object-store-differs-from-file-and-block-storage |archive-date=2016-02-13 |access-date=2016-02-22 |website=TechTarget.com}}</ref> At one time, block-level storage was provided by [[Storage area network|SAN]], and NAS provided file-level storage.<ref>{{Cite web |title=What is file level storage versus block level storage? |url=http://www.iscsi.com/resources/File-Level-Storage-vs-Block-Level-Storage.asp |url-status=dead |archive-url=https://web.archive.org/web/20121015033321/https://stonefly.com/resources/File-Level-Storage-vs-Block-Level-Storage.asp |archive-date=15 October 2012 |website=Stonefly}}</ref> With the shift from on-premises hosting to cloud services, this distinction has shifted.{{Sfnp|Wittig|Wittig|2015|page=205}}
** Instance stores are a form of cloud-hosted block-level storage, being provided as part of a cloud [[Instance (computer science)|instance]].{{Sfnp|Wittig|Wittig|2015|pages=212-214}} Unlike other forms of block storage, instance store data will be lost the cloud instance is stopped.{{Sfnp|Wittig|Wittig|2015|page=212}}

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

* [[Volatile memory|Volatility]]
** An [[Uninterruptible power supply|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, have integrated batteries that maintain volatile storage for several minutes.
* [[Immutable object|Mutability]]
** Storage can be classified into read/write, slow-write/fast-read (e.g. [[Engineering:CD-RW|CD-RW]], [[Solid-state drive|SSD]]), [[Engineering:Write once read many|write-once/read-many]] or WORM (e.g. [[Programmable read-only memory|programmable read-only memory]], [[Engineering:CD-R|CD-R]]), read-only storage (e.g. [[Read-only memory#Factory programmed|mask ROM ICs]], [[CD-ROM]]).
* Accessibility
** Types of access include [[Random access|random access]] and [[Sequential access|sequential access]]. In random access, any location in storage can be accessed at any moment in approximately the same amount of time. In 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.
* [[Addressing mode|Addressability]]
** Storage can be location accessible (i.e. selected with its numerical [[Memory address|memory address]]), [[File system|file addressable]], or [[Content-addressable memory|content-addressable]].
* Capacity and [[Density (computer storage)|density]]
* Performance
** Storage performance metrics include [[Engineering:Latency (engineering)|latency]], [[Network throughput|throughput]], granularity and reliability.
* Energy
** Low capacity [[Solid-state drive|solid-state drive]]s have no moving parts and consume less power than hard disks.<ref>{{Cite web |date=12 July 2006 |title=Super Talent's 2.5" IDE flash hard drive |url=http://techreport.com/articles.x/10334/13 |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 |website=The tech report |page=13}}</ref><ref>{{Cite web |date=20 September 2006 |title=Power consumption – Tom's hardware : Conventional hard drive obsoletism? Samsung's 32 GB flash drive previewed |url=https://www.tomshardware.com/reviews/conventional-hard-drive-obsoletism,1324-5.html |access-date=18 June 2011 |website=tomshardware.com}}</ref><ref name="xbitSSDvsHD">{{cite web |author=Aleksey Meyev |date=23 April 2008 |title=SSD, i-RAM and traditional hard disk drives |url=http://www.xbitlabs.com/articles/storage/display/ssd-iram.html |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 |publisher=X-bit labs}}</ref> Also, memory may use more power than hard disks.<ref name="xbitSSDvsHD" /> Large caches, which are used to avoid hitting the memory wall, may also consume a large amount of power.
* Security<ref>{{Cite web |author1=Karen Scarfone |author2=Murugiah Souppaya |author3=Matt Sexton |date=November 2007 |title=Guide to storage encryption technologies for end user devices |url=https://www.hhs.gov/sites/default/files/ocr/privacy/hipaa/administrative/securityrule/nist800111.pdf |publisher=National Institute of Standards and Technology}}</ref>

{| class=wikitable style="text-align:center;"
|+ Overview
|-
! Characteristic
! [[Engineering:Hard disk drive|Hard disk drive]]
! [[Engineering:Optical disc|Optical disc]]
! [[Flash memory]]
! [[Random-access memory]]
! [[Linear Tape-Open|Linear tape-open]]
|-
! Technology
| Magnetic disk
| [[Physics:Laser|Laser]] beam
| colspan=2 | Semiconductor
| Magnetic tape
|-
! Volatility
| No
| No
| No
| Volatile
| No
|-
! Random access
| Yes
| Yes
| Yes
| Yes
| No
|-
! [[Engineering:Latency (engineering)|Latency]] (access time)
| [[Hard disk drive performance characteristics|~15 ms]] (swift)
| ~150 ms (moderate)
| None (instant)
| None (instant)
| Lack of random access (very slow)
|-
! Controller
| Internal
| External
| [[Flash memory controller|Internal]]
| Internal
| External
|-
! Failure with imminent [[Data loss|data loss]]
| [[Head crash]]
| —
|colspan=2| [[Engineering:Failure of electronic components|Circuitry]]
| —
|-
! Error detection
| Diagnostic ([[Engineering:Self-Monitoring, Analysis and Reporting Technology|S.M.A.R.T.]])
| [[Engineering:Optical disc#Surface error scanning|Error rate measurement]]
| Indicated by downward spikes in transfer rates
| (Short-term storage)
| Unknown
|-
! Price per space
| Low
| Low
| High
| Very high
| Very low (but expensive drives)
|-
! Price per unit
| Moderate
| Low
| Moderate
| High
| Moderate (but expensive drives)
|-
! Main application
| Mid-term archival, routine backups, server, workstation storage expansion
| Long-term archival, [[Hard copy|hard copy]] distribution
| Portable electronics; operating system
| Real-time
| Long-term archival
|-
|}

== Media ==

=== Semiconductor ===
{{Main|Engineering:Semiconductor memory}}

[[Engineering:Semiconductor memory|Semiconductor memory]] uses [[Physics:Semiconductor|semiconductor]]-based [[Engineering:Integrated circuit|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 [[Engineering:MOSFET|MOS field-effect transistors]] (MOSFETs) and/or [[Engineering:MOSFET#MOS capacitor|MOS capacitors]]. Both ''volatile'' and ''non-volatile'' forms of semiconductor memory exist, the former using standard MOSFETs and the latter using [[Engineering:Floating-gate MOSFET|floating-gate MOSFET]]s.

In modern computers, primary storage almost exclusively consists of dynamic volatile semiconductor [[Random-access memory|random-access memory]] (RAM), particularly [[Dynamic random-access memory|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]] 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, [[Engineering:Laptop|notebook]] and [[Engineering:Desktop computer|desktop computer]] manufacturers started using flash-based [[Solid-state drive|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>

=== Magnetic ===
[[Magnetic storage]] uses different patterns of [[Physics:Magnetization|magnetization]] on a 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;
** [[Engineering:Hard disk drive|Hard disk drive]], used for secondary storage.
* [[Physics: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:

* [[Microcode]] storage in [[Engineering:Transformer read-only storage|transformer read-only storage]];
* Primary storage in a form of [[Drum memory|magnetic memory]], or [[Core memory|core memory]], [[Core rope memory|core rope memory]], [[Thin-film memory|thin-film memory]] and/or [[Twistor memory|twistor memory]];
* Magnetic-tape was often used for secondary storage;
* Tertiary (e.g. [[NCR CRAM]]) or off line storage in the form of magnetic cards.

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 ===
[[Engineering:Optical storage|Optical storage]], the typical [[Engineering:Optical disc|optical disc]], stores information in deformities on the surface of a circular disc and reads this information by illuminating the surface with a [[Engineering:Laser diode|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>

* [[Engineering:CD|CD]], [[CD-ROM]], [[Engineering:DVD|DVD]], BD-ROM: Read only storage, used for mass distribution of digital information (music, video, computer programs);
* [[Engineering:CD-R|CD-R]], DVD-R, DVD+R, BD-R: Write once storage, used for tertiary and off-line storage;
* [[Engineering:CD-RW|CD-RW]], DVD-RW, DVD+RW, [[Engineering:DVD-RAM|DVD-RAM]], BD-RE: Slow write, fast read storage, used for tertiary and off-line storage;
* [[Engineering:Ultra Density Optical|Ultra Density Optical]] or UDO is similar in capacity to [[Engineering:Blu-ray Disc recordable|BD-R or BD-RE]] and is slow write, fast read storage used for tertiary and off-line storage.

[[Physics:Magneto-optical drive|Magneto-optical disc storage]] is optical disc storage where the magnetic state on a [[Physics:Ferromagnetic|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.

[[Physics:3D optical data 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.<ref name="Náfrádi 2016">{{cite journal|last1=Náfrádi|first1=Bálint|title=Optically switched magnetism in photovoltaic perovskite CH3NH3(Mn:Pb)I3|journal=Nature Communications|date=24 November 2016|volume=7|article-number=13406|doi=10.1038/ncomms13406|pmc=5123013|pmid=27882917|bibcode=2016NatCo...713406N|arxiv=1611.08205}}</ref>

=== Paper ===
[[Paper data storage]], typically in the form of paper tape or [[Engineering:Punched card|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 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 ===
* Vacuum-tube memory: 
** A [[Williams tube]] used a [[Engineering:Cathode-ray tube|cathode-ray tube]], and a [[Selectron tube]] used a large [[Physics:Vacuum tube|vacuum tube]] to store information.
* Electro-acoustic memory: [[Delay-line memory]] used [[Physics:Longitudinal wave|sound waves]] in a substance such as [[Chemistry:Mercury (element)|mercury]] to store information.
* [[Engineering:Optical tape|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.
* [[Phase-change memory]] uses different mechanical phases of [[Physics:Phase-change material|phase-change material]] to store information in an X–Y addressable matrix and reads the information by observing the varying [[Physics:Electrical resistance|electrical resistance]] of the material.
* [[Holographic data storage]] stores information optically inside [[Earth:Crystal|crystal]]s or [[Physics:Photopolymer|photopolymer]]s, for example, in HVDs ([[Holographic Versatile Disc|Holographic Versatile Discs]]). 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.
* Magnetic photoconductors store magnetic information, which can be modified by low-light illumination.<ref name="Náfrádi 2016" />
* [[Molecular memory]] stores information in [[Chemistry:Polymer|polymers]] that can store electric charge.<ref>{{Cite web|url=https://www.sciencedaily.com/releases/2009/02/090219141438.htm|title=New method of self-assembling nanoscale elements could transform data storage industry|archive-url=https://web.archive.org/web/20090301235710/https://www.sciencedaily.com//releases//2009//02//090219141438.htm|archive-date=1 March 2009|website=sciencedaily.com|date=1 March 2009|access-date=18 June 2011}}</ref>
* [[Biology:DNA digital data storage|DNA]] stores digital information in DNA [[Biology:Nucleotide|nucleotide]]s.<ref>{{cite news|last1=Yong |first1=Ed|title=This speck of DNA contains a movie, a computer virus, and an Amazon gift card|url=https://www.theatlantic.com/science/archive/2017/03/this-speck-of-dna-contains-a-movie-a-computer-virus-and-an-amazon-gift-card/518373/|access-date=3 March 2017|work=The Atlantic|url-status=live|archive-url=https://web.archive.org/web/20170303010826/https://www.theatlantic.com/science/archive/2017/03/this-speck-of-dna-contains-a-movie-a-computer-virus-and-an-amazon-gift-card/518373/|archive-date=3 March 2017}}</ref><ref>{{cite web|title=Researchers store computer operating system and short movie on DNA|url=https://phys.org/news/2017-03-short-movie-dna.html|website=phys.org |access-date=3 March 2017|url-status=live|archive-url=https://web.archive.org/web/20170302221126/https://phys.org/news/2017-03-short-movie-dna.html|archive-date=2 March 2017}}</ref><ref>{{cite web|title=DNA could store all of the world's data in one room|url=https://www.science.org/content/article/dna-could-store-all-worlds-data-one-room|publisher=Science Magazine|access-date=3 March 2017|date=2 March 2017|url-status=live|archive-url=https://web.archive.org/web/20170302225525/http://www.sciencemag.org/news/2017/03/dna-could-store-all-worlds-data-one-room|archive-date=2 March 2017}}</ref><ref>{{cite journal|last1=Erlich|first1=Yaniv|last2=Zielinski|first2=Dina|title=DNA Fountain enables a robust and efficient storage architecture|journal=Science|date=2 March 2017|volume=355|issue=6328|pages=950–954|doi=10.1126/science.aaj2038|pmid=28254941|url=https://zenodo.org/record/889697|bibcode=2017Sci...355..950E|s2cid=13470340}}</ref>

== See also ==

*[[Aperture (computer memory)]]
* [[Mass storage]]
* [[Memory leak]]
* [[Memory protection]]
* [[Page address register]]
* [[Stable storage]]

=== Secondary, tertiary and off-line storage topics ===
* [[Data deduplication]]
* [[Data proliferation]]
* [[Data storage tag]] used for capturing research data
* [[Software:Disk utility|Disk utility]]
* [[File system]]
* [[Flash memory]]
* [[Geoplexing]]
* [[Information repository]]
* [[Noise-predictive maximum-likelihood detection]]
* [[Removable media]]
* [[Engineering:Hard disk drive#Spindle|Spindle]]
* [[Virtual tape library]]
* [[Wait state]]
* [[Write buffer]]
* [[Write protection]]
* [[Cold data]]

== Notes ==
{{notelist}}

== References ==
{{reflist}}

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

{{Basic computer components}}
{{Primary storage technologies}}{{Magnetic storage media}}{{Optical storage media}}{{Paper data storage media}}


{{DEFAULTSORT:Computer Data Storage}}
[[Category:Computer data storage| ]]
[[Category:Computer architecture]]

{{Sourceattribution|Computer data storage}}