Editing Computer data storage (section)

=== Tertiary storage ===
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{{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 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.

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.
* Nearline storage is not immediately available, but can be made online quickly without human intervention.
* Offline storage is not immediately available, and requires some human intervention to become online.

For example, always-on spinning hard disk drives are online storage, while spinning drives that spin down automatically, such as in massive arrays of idle disks ([[Non-RAID drive architectures#MAID|MAID]]), are nearline storage. Removable media such as [[tape cartridge]]s that can be automatically loaded, as in [[tape libraries]], are nearline storage, while tape cartridges that must be manually loaded are offline storage.