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Writing on the Open Compute blog, Facebook Engineer Eran Tal makes an astute observation. Essentially, he says, HDDs must remaining spinning even when their data is no longer accessed often because should someone need something, the 30 seconds it might take for disk to resume spinning might seem like an eternity. The natural response to this point might be to suggest buying slower disks, but, as Tal points out, disks don’t start off cold:

HDDs however aren’t “born” cold; they progress into that state. In early stages, when user data is constantly being uploaded, or when data is recovered to the system due to a failure on a different machine, high disk bandwidth is a valuable asset. However, when a system’s data turns cold, there is no value to the high bandwidth.

Solid-state drives might be one solution to the problem, but they’re still relatively expensive, especially at scale. Tal has another idea, which is to put a switch on HDDs that could be flipped to slow their speeds when their data becomes cold. According to his calculations, a 3TB SATA HDD reduced from 7,2oo RPM to 3,600 RPM would cut its power consumption from 7W to 3W.