Small changes in configuration yield full rebuilds


Second time I’m reaching a similar problem where a small change to configuration results in a full long rebuild, seemingly not benefiting from nix at all. I believe the nix configuration activation script should be modularly and hierachically split and built from smaller derivations, to allow for better caching and faster builds. I suspect it’s currently built in single huge derivation, is that correct?

I modify my configuration. I don’t change nixpkgs. I change filesystems option only. I don’t garbage collect. I expect the configuration to be quickly built again after the change.
Instead it takes equal time as previously to build. Correctly all the needed package are fetched from cache - these are already built (and were previously as well). But most of the time is taken in building the derivation.

[3/74/437 built, 16/5738/6337 copied (10373.4/16487.4 MiB), 2099.8/2156.3 MiB DL]

Can this be improved upon?

previous thread: Unexpected full rebuild with small change
I’m not sharing the details of configuration here. For some example check that above topic.