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{{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}}
{{Use dmy dates|date=June 2020}}
{{broader|Data storage}}
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[[File:DDR2 ram mounted.jpg|thumb|1 [[Gibibyte|GiB]] of [[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]]
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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 [[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]], [[paper tape]], [[punched card]]s, and [[RAM drive|RAM disks]].
Once the [[disk read/write head]] on HDDs reaches the proper placement and the data, subsequent data on the track are very fast to access. To reduce the seek time and rotational latency, data are transferred to and from disks in large contiguous blocks. Sequential or block access on disks is orders of magnitude faster than random access, and many sophisticated paradigms have been developed to design efficient algorithms based 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=[[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.
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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
=== Off-line storage ===
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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=
=== Security ===
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* 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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== 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}}
* [
{{Basic computer components}}
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