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Question: each 'server' is made up of cheap commodity hardware - but is the storage side of things handled elsewhere? YouTube alone takes 2 days worth of new video every minute - and stores multiple bitrate versions and the source video. That alone, for example, would cause problem for 900,000 commodity-sized hard disks...

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Google's storage is built on servers. http://labs.google.com/papers/gfs.html

You can easily put 6 disks in a server, and they may have some specialized designs that hold more.

Google has moved past GFS, but the basic concept is right: every machine in the cluster is a storage server.

There are some machines that are equipped with more disks than others where local density is needed.

As a ballpark estimate 2 days worth of video in multiple formatats is around 24 blue ray disks @ 25 gigabytes per disk or 600 gigabytes per minute. (Note: most of their video is not HD yet so this is a fairly high estimate.) That means a 2TB disk lasts around 3.3 minutes and they need around 150,000 disks per year. Which including redundancy is in the right ballpark for 900,000 disks. But, they could easily have 6+ disks per server.

But you can easily handel that much bandwidth on 5% of those disks so aslong as your not trying to fill up one disk at a time it's really not an issue.

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