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1<chapter xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xi="http://www.w3.org/2001/XInclude" xml:id="ch-file-systems">
2 <title>File Systems</title>
3 <para>
4 You can define file systems using the <literal>fileSystems</literal>
5 configuration option. For instance, the following definition causes
6 NixOS to mount the Ext4 file system on device
7 <literal>/dev/disk/by-label/data</literal> onto the mount point
8 <literal>/data</literal>:
9 </para>
10 <programlisting language="bash">
11fileSystems."/data" =
12 { device = "/dev/disk/by-label/data";
13 fsType = "ext4";
14 };
15</programlisting>
16 <para>
17 This will create an entry in <literal>/etc/fstab</literal>, which
18 will generate a corresponding
19 <link xlink:href="https://www.freedesktop.org/software/systemd/man/systemd.mount.html">systemd.mount</link>
20 unit via
21 <link xlink:href="https://www.freedesktop.org/software/systemd/man/systemd-fstab-generator.html">systemd-fstab-generator</link>.
22 The filesystem will be mounted automatically unless
23 <literal>"noauto"</literal> is present in
24 <link linkend="opt-fileSystems._name_.options">options</link>.
25 <literal>"noauto"</literal> filesystems can be mounted
26 explicitly using <literal>systemctl</literal> e.g.
27 <literal>systemctl start data.mount</literal>. Mount points are
28 created automatically if they don’t already exist. For
29 <literal>device</literal>, it’s best to use the topology-independent
30 device aliases in <literal>/dev/disk/by-label</literal> and
31 <literal>/dev/disk/by-uuid</literal>, as these don’t change if the
32 topology changes (e.g. if a disk is moved to another IDE
33 controller).
34 </para>
35 <para>
36 You can usually omit the file system type
37 (<literal>fsType</literal>), since <literal>mount</literal> can
38 usually detect the type and load the necessary kernel module
39 automatically. However, if the file system is needed at early boot
40 (in the initial ramdisk) and is not <literal>ext2</literal>,
41 <literal>ext3</literal> or <literal>ext4</literal>, then it’s best
42 to specify <literal>fsType</literal> to ensure that the kernel
43 module is available.
44 </para>
45 <note>
46 <para>
47 System startup will fail if any of the filesystems fails to mount,
48 dropping you to the emergency shell. You can make a mount
49 asynchronous and non-critical by adding
50 <literal>options = [ "nofail" ];</literal>.
51 </para>
52 </note>
53 <xi:include href="luks-file-systems.section.xml" />
54 <xi:include href="sshfs-file-systems.section.xml" />
55</chapter>