But even looking how it never gained mainstream adoption for over 10 years when people always wanted something like zfs, it just feels there's something substantially wrong about its development model.
Personally, I tried btrfs as a root filesystem for a couple years. It definitely had weird gotchas like running out of inode space pretty frequently when making use of snapshots. A few years back I also ran into a problem with a bunch of zero length files (race between writing metadata and content?)
I switched this year and I found zfs to much more user friendly and reliable, although setting up zfs on a root on ubuntu is still a manual process.
But I guess if (big if?) FB uses it in production (without a lot of internal patches) it may be kinda stable now. Still, I don't see it as being useful for a lot of people nowadays, particularly if you sit on an "expendable" VM in a cloud.
https://en.wikipedia.org/wiki/Fusion-io
I met Josef at a FB recruiting event at my school.
Edit: it looks like a lot of the projects have already been open-sourced but were developed at Facebook (e.g. PSI and cgroups2), and Facebook is now announcing them all together along with some new tools.