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