The latest idea from Facebook on how to revolutionize the data center is elegant in its simplicity: Put a switch on hard-disk drives that slows their speed when their data is no longer hot. I’m not aware of any vendor actually doing this, which makes me wonder if Facebook will just take up the cause itself.
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.
As the post’s commenters point out, the idea of dynamic-speed HDDs isn’t entirely new (see this IEEE paper from 2003, for example), and the tactic wouldn’t work in every situation. As one commenter notes, on a site such as Amazon.com, where items such as textbooks receive little attention except with the start of each new semester, efforts at changing an HDD’s speed would have to be an ongoing process.
But the thing about Facebook — and Google, Yahoo and other large web sites — is that they innovate based on their unique needs, not what’s appropriate for mainstream users. Let’s not forget, this post appeared on the Open Compute blog, part of a project based around Facebook’s custom-built servers and data centers. If Facebook actually sees real value in this idea, it wouldn’t surprise me to see the specifications for its flipswitch HDDs show up in the Open Compute library at some point.
If that happens, those mainstream users can assess the designs and decide for themselves if they’ll follow in Facebook’s footsteps like some already have with servers.