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+<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>