Would nixos benifit from being truly immutable? Or have the option to

4

What would you make immutable? Ok, let’s go though the root:

special filesystems:

  • proc
  • dev
  • sys

shouldn’t be read-only:

  • tmp
  • var
  • run
  • home

trivial to make read-only:

  • bin
  • lib
  • lib64
  • usr

already read-only:

  • nix

could be read-only:

  • etc
  • root

/root is the home of the root user, it could be made read-only, but I don’t think there’s much to gain from this and it would make rescuing a system slightly annoying.

/etc is really the only directory that would benefit from being read only. We already have /etc/static which is a symlink to /nix/store and contains all generated files. Every other file is either a link to /etc/static or was written imperatively, so you’d think you could make the whole /etc/ a symlink, however there are some caveats:

  1. passwords, users, groups are generated at activation time. These are intentionally not stored in /nix/store but should still be present under /etc.

  2. resolv.conf needs to be writable at runtime by programs using resolvconf.

  3. more generated files used by programs like CUPS or LVM.

So, the only way to make /etc read-only would be to use an overlay filesystem that combines the /etc generated by NixOS and some writable path under /var or /tmp. I think it shouldn’t be too difficult to achieve, but it doesn’t look like much of an improvement over the status quo.

Instead of trying to make the whole root read-only, I think the better approach is to make it impermanent. This means every change you don’t care about will be lost when restarting the system.

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