That is of course perfectly fine, but to me that represents a
    not-so-ideal fit between your interests and your job.

If the day job were Clojure, I'd be hacking in Python in my spare time. Or some other language (Rust, Haskell, etc.). Or not even hacking at all, but reading a book, or doing something else. The entire point of having hobby projects for me is that they're not what I'm working on during the day. They're an opportunity to broaden my horizons.

    Me and a coworker started to dig into glusterfs to solve our
    issues. We spent months reading the code on our free time, reading
    papers on distributed filesystems, fixing issues in our fork
    (hoping that RH would pick up maintenance after the transition).

That just sounds like a lot of unpaid overtime to me. It sounds like you enjoyed it, but I'd find that experience absolutely miserable. Working intensely on a project is great, but I need time where I'm thinking about something completely different, otherwise I burn out fast.