82% of Facebook's traffic is focused on only 8% of its photos, so the company plans to put older images into cold storage, like one data hall under construction now in Prineville.
Each cold storage data hall will hold 1 exabyte of data -- equivalent to 1 million of the 1 terabyte hard drives now common in PCs. Facebook will fill each cold storage data hall with 500 racks of servers.
Most of those servers will be idle most of the time. Facebook will keep just a small number of hard drives running, and spin up additional ones when it needs to retrieve the photos they store from cold storage.
Read more at OregonLive.com.