Computer data storage: Difference between revisions

Revision as of 13:12, 15 July 2025

Revision as of 13:12, 15 July 2025

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=== Primary storage ===

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{{Main|Computer memory}}

''Primary storage'' (also known as ''main memory'', ''internal memory'', or ''prime memory''), often referred to simply as ''memory'', is the only one directly accessible to the CPU. The CPU continuously reads instructions stored there and executes them as required. Any data actively operated on is also stored there in a uniform manner.

Historically, [[History of computing hardware|early computers]] used [[delay-line memory|delay lines]], [[Williams tubestube]]s, or rotating [[drum memory|magnetic drums]] as primary storage. By 1954, those unreliable methods were mostly replaced by [[magnetic-core memory]]. Core memory remained dominant until the 1970s, when advances in [[integrated circuit]] technology allowed [[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]] or [[bubble memory]] etc, will click the History link. Do not confuse other readers. -->

This led to modern [[random-access memory]] (RAM). It is small-sized, light, but quite expensive at the same time. The particular types of RAM used for primary storage are [[volatile memory|volatile]], meaning that they lose the information when not powered. Besides storing opened programs, it serves as [[Page cache|disk cache]] and [[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.

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

* [[Processor registersregister]]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.

* [[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 [[address bus]] and a [[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]]'').

Many types of "ROM" are not literally ''read only'', as updates to them are possible; however it is slow and memory must be erased in large portions before it can be re-written. Some [[embedded systemssystem]]s run programs directly from ROM (or similar), because such programs are rarely changed. Standard computers do not store non-rudimentary programs in ROM, and rather, use large capacities of secondary storage, which is non-volatile as well, and not as costly.

Recently, ''primary storage'' and ''secondary storage'' in some uses refer to what was historically called, respectively, ''secondary storage'' and ''tertiary storage''.<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 ===

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=== Tertiary storage ===

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''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 libraries and optical jukeboxes.

{{See also|Nearline storage|Cloud storage}}

[[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 libraries]] and [[optical jukeboxesjukebox]]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.

Tertiary storage is also known as ''[[nearline storage]]'' because it is "near to online". The formal distinction between online, nearline, and offline storage is:<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>

* Online storage is immediately available for I/O.

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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 libraries]], are nearline storage, while tape cartridges that must be manually loaded are offline storage.

=== Off-line storage ===