Getting some space free in NixOs

NixOS is a great operating system. However, by time to time, I just realise that my /nix gets bigger and bigger. I of course use the standard garbage collector in order to make some space free :

$ nix-collect-garbage

is used to delete unused derivations from the nix-store, and

$ nix-collect-garbage -d

is used to remove past generations (which makes it impossible at that time to perform a rollback after it). But despite this, nix still continue to grow. So what did I miss ?

Simple : NixOS is just super awsome. What happens is that when I develop and when add packages to nixpkgs, I use the nix-build command to test my work. nix-build instantiates and realises the derivation, and at the end places a symlink to the build derivation directory named result. The same kind of thing happens wen using nixops. All the built targets are kept somewhere in the store, referenced by symlinks.

As long as those symlinks exists, the referenced derivations will not be removed from the nix store (as does its dependencies). Therefore, to allow the garbage collector to do some more cleaning, it is important to remove this symlink.

For those who cannot remember where they where when they used nix-build, it is possible to use the following command to retrieve all the roots :

$ nix-store --gc --print-roots

It is also possible to find those same roots in /nix/var/nix/gcroots/. This folder contains symlinks to all the existing roots. The so called roots are what the garbage collector looks for when doing its job. Are preserves those roots, and all their (indirect and indirect) dependencies.

For those who are interested in this topic, you can check additional informations here.