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

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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="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>
This will create an entry in <filename>/etc/fstab</filename>, which will
generate a corresponding
<link xlink:href="https://www.freedesktop.org/software/systemd/man/systemd.mount.html">systemd.mount</link>
unit via
<link xlink:href="https://www.freedesktop.org/software/systemd/man/systemd-fstab-generator.html">systemd-fstab-generator</link>.
The filesystem will be mounted automatically unless
<literal>"noauto"</literal> is present in <link
linkend="opt-fileSystems._name_.options">options</link>.
<literal>"noauto"</literal> filesystems can be mounted explicitly using
<command>systemctl</command> e.g. <command>systemctl start
data.mount</command>.
Mount points are created automatically if they dont already exist. For
<option><link linkend="opt-fileSystems._name_.device">device</link></option>,
its best to use the topology-independent device aliases in
<filename>/dev/disk/by-label</filename> and
<filename>/dev/disk/by-uuid</filename>, as these dont 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 its 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>