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+<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>
+<prompt>$ </prompt>nix-collect-garbage
+</screen>
+  Alternatively, you can use a systemd unit that does the same in the
+  background:
+<screen>
+<prompt># </prompt>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>
+<prompt>$ </prompt>nix-collect-garbage -d
+</screen>
+  You can also do this for specific profiles, e.g.
+<screen>
+<prompt>$ </prompt>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>
+<prompt>$ </prompt>nix-store --optimise
+</screen>
+  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 <filename>/boot</filename> partition runs out of space, after
+   clearing old profiles you must rebuild your system with
+   <literal>nixos-rebuild</literal> to update the <filename>/boot</filename>
+   partition and clear space.
+  </para>
+ </section>
+</chapter>