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# Overlayfs {#sec-overlayfs}
NixOS offers a convenient abstraction to create both read-only as well writable
overlays.
```nix
{
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 `lowerdir`s.
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