<chapter xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" 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 (<literal>/nix/store</literal>).
    You should periodically run Nix’s <emphasis>garbage
    collector</emphasis> to remove old, unreferenced packages. This is
    easy:
  </para>
  <programlisting>
$ nix-collect-garbage
</programlisting>
  <para>
    Alternatively, you can use a systemd unit that does the same in the
    background:
  </para>
  <programlisting>
# systemctl start nix-gc.service
</programlisting>
  <para>
    You can tell NixOS in <literal>configuration.nix</literal> to run
    this unit automatically at certain points in time, for instance,
    every night at 03:15:
  </para>
  <programlisting language="bash">
nix.gc.automatic = true;
nix.gc.dates = &quot;03:15&quot;;
</programlisting>
  <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:
  </para>
  <programlisting>
$ nix-collect-garbage -d
</programlisting>
  <para>
    You can also do this for specific profiles, e.g.
  </para>
  <programlisting>
$ nix-env -p /nix/var/nix/profiles/per-user/eelco/profile --delete-generations old
</programlisting>
  <para>
    Note that NixOS system configurations are stored in the profile
    <literal>/nix/var/nix/profiles/system</literal>.
  </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.
  </para>
  <programlisting>
$ nix-store --optimise
</programlisting>
  <para>
    Since this command needs to read the entire Nix store, it can take
    quite a while to finish.
  </para>
  <section xml:id="sect-nixos-gc-boot-entries">
    <title>NixOS Boot Entries</title>
    <para>
      If your <literal>/boot</literal> partition runs out of space,
      after clearing old profiles you must rebuild your system with
      <literal>nixos-rebuild boot</literal> or
      <literal>nixos-rebuild switch</literal> to update the
      <literal>/boot</literal> partition and clear space.
    </para>
  </section>
</chapter>
