Each disk in the cold storage gear can hold 4 terabytes of data, and each 2U system contains two levels of 15 disks. In other words, each unit can handle 120 terabytes. A rack could hold 16 of these storage systems, allowing for 2 petabytes of cold storage in a rack.

Source: http://www.datacenterknowledge.com/archives/2013/10/16/first-look-facebooks-oregon-cold-storage-facility/

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