nix-super/doc/manual/conf-file.xml

<sect1 id="sec-conf-file"><title>Nix configuration file</title>

<para>A number of persistent settings of Nix are stored in the file
<filename><replaceable>prefix</replaceable>/etc/nix/nix.conf</filename>.
This file is a list of <literal><replaceable>name</replaceable> =
<replaceable>value</replaceable></literal> pairs, one per line.
Comments start with a <literal>#</literal> character.  An example
configuration file is shown in <xref linkend="ex-nix-conf" />.</para>

<example id='ex-nix-conf'><title>Nix configuration file</title>

<programlisting>
gc-keep-outputs = true       # Nice for developers
gc-keep-derivations = true   # Idem
env-keep-derivations = false
</programlisting>
</example>

<para>The following variables are currently available: 

<variablelist>

  <varlistentry id="conf-gc-keep-outputs"><term><literal>gc-keep-outputs</literal></term>

    <listitem><para>If <literal>true</literal>, the garbage collector
    will keep the outputs of non-garbage derivations.  If
    <literal>false</literal> (default), outputs will be deleted unless
    they are GC roots themselves (or reachable from other roots).</para>
 
    <para>In general, outputs must be registered as roots separately.
    However, even if the output of a derivation is registered as a
    root, the collector will still delete store paths that are used
    only at build time (e.g., the C compiler, or source tarballs
    downloaded from the network).  To prevent it from doing so, set
    this option to <literal>true</literal>.</para></listitem>

  </varlistentry>

  <varlistentry id="conf-gc-keep-derivations"><term><literal>gc-keep-derivations</literal></term>

    <listitem><para>If <literal>true</literal> (default), the garbage
    collector will keep the derivations from which non-garbage store
    paths were built.  If <literal>false</literal>, they will be
    deleted unless explicitly registered as a root (or reachable from
    other roots).</para>

    <para>Keeping derivation around is useful for querying and
    traceability (e.g., it allows you to ask with what dependencies or
    options a store path was built), so by default this option is on.
    Turn it off to safe a bit of disk space (or a lot if
    <literal>gc-keep-outputs</literal> is also turned on).</para></listitem>

  </varlistentry>

  <varlistentry id="conf-gc-reserved-space"><term><literal>gc-reserved-space</literal></term>

    <listitem><para>This option specifies how much space should be
    reserved in normal use so that the garbage collector can run
    succesfully.  Since the garbage collector must perform Berkeley DB
    transactions, it needs some disk space for itself.  However, when
    the disk is full, this space is not available, so the collector
    would not be able to run precisely when it is most needed.</para>

    <para>For this reason, when Nix is run, it allocates a file
    <filename>/nix/var/nix/db/reserved</filename> of the size
    specified by this option.  When the garbage collector is run, this
    file is deleted before the Berkeley DB environment is opened.
    This should give it enough room to proceed.</para>

    <para>The default is <literal>1048576</literal> (1
    MiB).</para></listitem>

  </varlistentry>

  <varlistentry><term><literal>env-keep-derivations</literal></term>

    <listitem><para>If <literal>false</literal> (default), derivations
    are not stored in Nix user environments.  That is, the derivation
    any build-time-only dependencies may be garbage-collected.</para>

    <para>If <literal>true</literal>, when you add a Nix derivation to
    a user environment, the path of the derivation is stored in the
    user environment.  Thus, the derivation will not be
    garbage-collected until the user environment generation is deleted
    (<command>nix-env --delete-generations</command>).  To prevent
    build-time-only dependencies from being collected, you should also
    turn on <literal>gc-keep-outputs</literal>.</para>

    <para>The difference between this option and
    <literal>gc-keep-derivations</literal> is that this one is
    “sticky”: it applies to any user environment created while this
    option was enabled, while <literal>gc-keep-derivations</literal>
    only applies at the moment the garbage collector is
    run.</para></listitem>

  </varlistentry>

</variablelist>

</para>

</sect1>