Once upon a time, there was a magical kingdom called btrfs, with many CoWs in it. In this kingdom was a castle, called Castle Carfax. One of the most important people living in Castle Carfax was called Amelia, and in the castle she kept eight circles of adamant. These hard disks, as they were called, held all the important knowledge of the castle.
Here's the email I sent to the btrfs mailing list a few moments ago:
Over the last few weeks, I've been playing with a foolish idea, mostly triggered by a cluster of people being confused by btrfs's free space reporting (df vs btrfs fi df vs btrfs fi show). I also wanted an excuse, and some code, to mess around in the depths of the FS data structures.