[pletnes]

Btrfs does stuff like instantaneous snapshots for a multi terabyte size filesystem. I have no idea how they do it, but apparently it’s 100% possible.

[waynesonfire]

Well since it's claiming to not mutate state, it must be simple! Go read the code and tell us what you learn.

In the event you find the source code TLDR, indeed it seems implementing an immutable api is complex.