<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>