Overlayfs

NixOS offers a convenient abstraction to create both read-only as well writable overlays.

fileSystems = {
  "/writable-overlay" = {
    overlay = {
      lowerdir = [ writableOverlayLowerdir ];
      upperdir = "/.rw-writable-overlay/upper";
      workdir = "/.rw-writable-overlay/work";
    };
    # Mount the writable overlay in the initrd.
    neededForBoot = true;
  };
  "/readonly-overlay".overlay.lowerdir = [
    writableOverlayLowerdir
    writableOverlayLowerdir2
  ];
};

If upperdir and workdir are not null, they will be created before the overlay is mounted.

To mount an overlay as read-only, you need to provide at least two lowerdirs.

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Overlayfs

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