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the `nixos-rebuild` command has multiple subcommands, and not each of them would fix the problem of a large `/boot` partition, so let’s be more precise here.
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Cleaning the Nix Store
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.
NixOS Boot Entries
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.