# File Systems

You can define file systems using the `fileSystems` configuration option. For instance, the following definition causes NixOS to mount the Ext4 file system on device `/dev/disk/by-label/data` onto the mount point `/data`:

    fileSystems."/data" =
      { device = "/dev/disk/by-label/data";
        fsType = "ext4";
      };

Mount points are created automatically if they don’t already exist. For `device`, it’s best to use the topology-independent device aliases in `/dev/disk/by-label` and `/dev/disk/by-uuid`, 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 (`fsType`), since `mount` 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 `ext2`, `ext3` or `ext4`, then it’s best to specify `fsType` to ensure that the kernel module is available.