I have very limited Btrfs experience, now or then; I've heard a lot of people complaining about it, but A) I hang out in ZFS circles, so the people I hear there

I have very limited Btrfs experience, now or then; I've heard a lot of people complaining about it, but A) I hang out in ZFS circles, so the people I hear there talking about Btrfs are a self-selecting group, and B) the plural of anecdote is not data (unless very large numbers are involved). One notable caveat is that in 2009, ZFS was basically available on FreeBSD and Solaris - zfs-fuse existed (and I even used it sometimes), but I would be astonished if you could boot from it, for example, sometimes it just broke, and FUSE is going to have at least some overhead by dint of being FUSE, which may be a nonstarter for performance applications. If Oracle had wanted to throw all their weight into ZFS on Linux (not the FOSS project that was spawned later, their own project), it'd have taken a bunch of engineering time for the initial port, plus ongoing maintenance on multiple OSes (unless they just decided to throw Solaris out, which, with support contracts, would have taken some years to do anyway), and they _still_ would have run into some of the fundamental limitations which make some of Btrfs's features infeasible on ZFS (though IIRC closed ZFS worked around one of them somehow, and I'm kind of curious how, but you know, closed source).