The lower levels of the hierarchy – {{dash}}from mass storage downwards – {{dash}}are also known as [[tiered storage]]. The formal distinction between online, nearline, and offline storage is:<ref name="pearson2010">{{cite web|last=Pearson|first=Tony|year=2010|title=Correct use of the term Nearline.|url=https://www.ibm.com/developerworks/community/blogs/InsideSystemStorage/entry/the_correct_use_of_the_term_nearline2|url-status=dead|archive-url=https://web.archive.org/web/20181127020712/https://www.ibm.com/developerworks/community/blogs/InsideSystemStorage/entry/the_correct_use_of_the_term_nearline2?lang=en|archive-date=2018-11-27|access-date=2015-08-16|work=IBM Developerworks, Inside System Storage}}</ref>
* Online storage is immediately available for I/O.
* Nearline storage is not immediately available, but can be made online quickly without human intervention.
