Nix has a purely functional model, meaning that packages are never upgraded in place. Instead new versions of packages end up in a different location in the Nix store (/nix/store). You should periodically run Nix’s garbage collector to remove old, unreferenced packages. This is easy: $ nix-collect-garbage Alternatively, you can use a systemd unit that does the same in the background: # systemctl start nix-gc.service You can tell NixOS in configuration.nix to run this unit automatically at certain points in time, for instance, every night at 03:15: nix.gc.automatic = true; nix.gc.dates = "03:15"; The commands above do not remove garbage collector roots, such as old system configurations. Thus they do not remove the ability to roll back to previous configurations. The following command deletes old roots, removing the ability to roll back to them: $ nix-collect-garbage -d You can also do this for specific profiles, e.g. $ nix-env -p /nix/var/nix/profiles/per-user/eelco/profile --delete-generations old Note that NixOS system configurations are stored in the profile /nix/var/nix/profiles/system. Another way to reclaim disk space (often as much as 40% of the size of the Nix store) is to run Nix’s store optimiser, which seeks out identical files in the store and replaces them with hard links to a single copy. $ nix-store --optimise Since this command needs to read the entire Nix store, it can take quite a while to finish.

If your /boot partition runs out of space, after clearing old profiles you must rebuild your system with nixos-rebuild boot or nixos-rebuild switch to update the /boot partition and clear space.