nixpkgs/nixos/doc/manual/administration/cleaning-store.xml

<chapter xmlns="http://docbook.org/ns/docbook"
        xmlns:xlink="http://www.w3.org/1999/xlink"
        xmlns:xi="http://www.w3.org/2001/XInclude"
        version="5.0"
        xml:id="sec-nix-gc">
 <title>Cleaning the Nix Store</title>
 <para>
  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 (<filename>/nix/store</filename>). You should periodically run
  Nix’s <emphasis>garbage collector</emphasis> to remove old, unreferenced
  packages. This is easy:
<screen>
$ nix-collect-garbage
</screen>
  Alternatively, you can use a systemd unit that does the same in the
  background:
<screen>
# systemctl start nix-gc.service
</screen>
  You can tell NixOS in <filename>configuration.nix</filename> to run this unit
  automatically at certain points in time, for instance, every night at 03:15:
<programlisting>
<xref linkend="opt-nix.gc.automatic"/> = true;
<xref linkend="opt-nix.gc.dates"/> = "03:15";
</programlisting>
 </para>
 <para>
  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:
<screen>
$ nix-collect-garbage -d
</screen>
  You can also do this for specific profiles, e.g.
<screen>
$ nix-env -p /nix/var/nix/profiles/per-user/eelco/profile --delete-generations old
</screen>
  Note that NixOS system configurations are stored in the profile
  <filename>/nix/var/nix/profiles/system</filename>.
 </para>
 <para>
  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.
<screen>
$ nix-store --optimise
</screen>
  Since this command needs to read the entire Nix store, it can take quite a
  while to finish.
 </para>
</chapter>