nixpkgs/nixos/doc/manual/administration/system-state.chapter.md

Necessary system state

Normally — on systems with a persistent rootfs — system services can persist state to the filesystem without administrator intervention.

However, it is possible and not-uncommon to create impermanent systems, whose rootfs is either a tmpfs or reset during boot. While NixOS itself supports this kind of configuration, special care needs to be taken.

nixos-state.section.md
systemd-state.section.md
zfs-state.section.md