nixpkgs/nixos/doc/manual/configuration/file-systems.xml

<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="ch-file-systems">
 <title>File Systems</title>
 <para>
  You can define file systems using the <option>fileSystems</option>
  configuration option. For instance, the following definition causes NixOS to
  mount the Ext4 file system on device
  <filename>/dev/disk/by-label/data</filename> onto the mount point
  <filename>/data</filename>:
<programlisting>
<xref linkend="opt-fileSystems"/>."/data" =
  { device = "/dev/disk/by-label/data";
    fsType = "ext4";
  };
</programlisting>
  Mount points are created automatically if they don’t already exist. For
  <option><link linkend="opt-fileSystems._name__.device">device</link></option>,
  it’s best to use the topology-independent device aliases in
  <filename>/dev/disk/by-label</filename> and
  <filename>/dev/disk/by-uuid</filename>, as these don’t change if the
  topology changes (e.g. if a disk is moved to another IDE controller).
 </para>
 <para>
  You can usually omit the file system type
  (<option><link linkend="opt-fileSystems._name__.fsType">fsType</link></option>),
  since <command>mount</command> can usually detect the type and load the
  necessary kernel module automatically. However, if the file system is needed
  at early boot (in the initial ramdisk) and is not <literal>ext2</literal>,
  <literal>ext3</literal> or <literal>ext4</literal>, then it’s best to
  specify <option>fsType</option> to ensure that the kernel module is
  available.
 </para>
 <note>
  <para>
   System startup will fail if any of the filesystems fails to mount, dropping
   you to the emergency shell. You can make a mount asynchronous and
   non-critical by adding
   <literal><link linkend="opt-fileSystems._name__.options">options</link> = [
   "nofail" ];</literal>.
  </para>
 </note>
 <xi:include href="luks-file-systems.xml" />
</chapter>