nanuqsaurus/impermanence.md

“Impermanent” (in NixOS terms) usually means your / is treated as disposable: on boot you either mount it as tmpfs or roll it back to a clean Btrfs snapshot, and you only keep state in explicitly persistent mounts (commonly /nix, /home, plus a /persist).

What changes overall

• Your system becomes “reprovisioned” from Nix each boot/rebuild: the OS + config is reliable and drift-resistant, but anything that writes state under /etc or /var will be lost unless you persist it.
• Declared things (NixOS options, environment.systemPackages, enabled services) will come back automatically; undeclared/manual tweaks won’t.

In your repo, the big practical impacts would be

• Nix store and system packages: fine as long as /nix is persistent (your modules/system/disko-btrfs.nix already has a /nix subvolume). environment.systemPackages (like git, flatpak, podman-compose, etc.) is unaffected.
• Flatpak (modules/system/flatpak.nix): the declarative intent stays, but the actual installed Flatpaks/remotes live in state dirs (typically system: /var/lib/flatpak, user: ~/.local/share/flatpak). If those aren’t persisted, you’ll re-download/reinstall frequently and offline boots can be annoying.
• Secure Boot keys (modules/system/secure-boot.nix): you already point Lanzaboote at /var/lib/secureboot. If that path isn’t persisted, you risk regenerating keys and getting into a “firmware enrolled keys vs disk keys” mismatch situation.
• SSH (modules/system/services.nix enables openssh): host keys are typically under /etc/ssh. If they aren’t persisted, the host key changes after resets, which breaks known-hosts trust and can lock you out in headless scenarios.
• Libvirt/VMs (modules/system/virtualization.nix enables libvirtd + virt-manager): libvirt state and VM images commonly live under /var/lib/libvirt. If /var is wiped, your VMs disappear unless you store/persist them elsewhere.
• Podman/containers: rootless images/volumes are in your home (fine if /home persists); rootful storage is usually under /var/lib/containers (lost if not persisted).
• Printing (modules/system/printer.nix): CUPS queues/config live under /etc/cups and /var/lib/cups. With an impermanent root, any printers you add via GUI will vanish unless you persist those paths or configure printers declaratively.
• Home Manager bootstrap marker (modules/users/admin.nix): it writes %h/.config/home-manager/.hm_bootstrap_done. If /home isn’t persistent, that “one-time” bootstrap will run every login.

Rule of thumb

• Make /nix persistent (required).
• Decide whether /home is persistent (strongly recommended with your current “HM standalone per-user” setup).
• Persist selected state from /etc and /var based on the services you run (at minimum for your config: /var/lib/secureboot, /etc/ssh, plus whichever of /var/lib/flatpak, /var/lib/libvirt, /etc/cups//var/lib/cups you care about).