Reality check: measuring Bytes in Tons

A tray of hard drives in a cold storage rack. Storing one day of Facebook photos "weights" a bit more than 1,100 Kg, over one ton. Credit: Facebook

We are so used to the idea that bits are floating somewhere in the cyberspace that seldom we come to think that bits "weight", or actually that it takes some physical device to store bits and these devices are solid objects (a bit also weights in terms of energy-mass, you can get many amazing facts by Googling, as an example if you fill your Kindle with books -4GB- it increases its weight by a billionth of a billionth of gram).

I was amazed by some data published on a blog by Facebook. A post I highly recommend to read.

The Internet, that appears to be a virtual space, has its roots in atoms, and plenty of them. So is the Cloud. It takes plenty of servers and connectivity to make a Cloud.

If you take the time to read the Facebook post you will feel “the heat” that is generated by the economics of running such huge infrastructure of atoms.

Storing the 2 billion photos that every day “Facebookers” upload and share with their friends requires over 1PB of storage. This turns out to required close to 500 hard discs (weighting over 1 ton) assembled into a single rack (interestingly the weight of the rack, and the need to move it around to fit in the data center has required the adoption of new rack movers…).

Powering these racks brings the energy bill out of control. Hence, the focus of Facebook engineers to dramatically contain the increase of energy requirements by adoption of cold storage architecture.

The basic idea is that as days go by a photo is less and less likely to be accessed. Still, it needs to remain accessible. Rather than managing all photos in the same way at Facebook they move the older photos in colder and colder storage spaces. Racks where only a subset of the hard discs are powered. If need arise the hard disc containing a photo will be power up. By this, and other tricks (there are plenty of them, read the article), Facebook can decrease the power requirements to one third.


I have to say, I am impressed by the sophistication and huge hardware deployment needed to support “bits”. Most of the time we forget about this huge hardware backstage that makes bit operation not just possible but also economically sustainable.

Author - Roberto Saracco

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