A few days ago we open sourced TernFS, a distributed filesystem which has been used in production at XTX for a couple of years now. The project took up a big chunk of my waking hours for a year and a half, and I’m extremely happy that we are now sharing it with the world.
You can find a blog post describing it on XTX’s tech blog, and the source code on GitHub. The repository contains the full git history, which is somewhat unusual for these sort of projects: you can see how the codebase grew rapidly from prototype into a production system.