Not at all, originally the whole purpose of DragonFly was a test ground for its scheduler. It moved on to being much more though, refactoring the entire source tree and rewriting parts of the system. The whole purpose of DragonFly is DragonFly.

Dragonfly also has lightweight kernel threads and some other different architectural decisions. Not just HAMMER. But you are right, it will never reach Linux.

Can you go into more detail please? That sounds interesting. I thought it had to do with concurrency.

You're not wrong. Interview from 2004 [0]: "DragonFly split off from FreeBSD-5 over major architectural differences, not anything else. We really do feel that FreeBSD-5 is taking the wrong approach to SMP and building something that is so complex that it will ultimately not be maintainable. We think we have a better way."

But their original architectural decisions were done with this goal in mind [1]: "The ultimate goal of the DragonFly project at its inception was to provide native clustering support in the kernel. This type of functionality requires a sophisticated cache management framework for filesystem namespaces, file spaces and VM spaces." It's not difficult to see how HAMMER came to be three to four years later, given their stated design goals.

[0] http://www.onlamp.com/pub/a/bsd/2004/07/08/dragonfly_bsd_int... [1] https://www.dragonflybsd.org/history/