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>:
Mount points are created automatically if they don’t already exist. For <option><linklinkend="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).
You can usually omit the file system type (<option><linklinkend="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.
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><linklinkend="opt-fileSystems._name__.options">options</link> = [ "nofail" ];</literal>.
