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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>
+$ 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>
+nix.gc.automatic = true;
+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>
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