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You probably have heard of the cool new kid on the file system block, btrfs (pronounced "butter-eff-ess") - after all, Linus Torvalds is using it as his root file system on one of his laptops. But you might not know much about it beyond a few high-level keywords - copy-on-write, checksums, writable snapshots - and a few sensational rumors and stories - the Phoronix benchmarks, btrfs is a ZFS ripoff, btrfs is a secret plan for Oracle domination of Linux, etc. When it comes to file systems, it's hard to tell truth from rumor from vile slander: the code is so complex, the personalities are so exaggerated, and the users are so angry when they lose their data. You can't even settle things with a battle of the benchmarks: file system workloads vary so wildly that you can make a plausible argument for why any benchmark is either totally irrelevant or crucially important. In this article, we'll take a behind-the-scenes look at the design and development of btrfs on many levels - technical, political, personal - and trace it from its origins at a workshop to its current position as Linus's root file system. Knowing the background and motivation for each step will help you understand why btrfs was started, how it works, and where it's going in the future. By the end, you should be able to hand-wave your way through a description of btrfs's on-disk format.