nixos/modules/base/impermanence.nix

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{ impermanence, ... }: {
# Impermanence in NixOS is where the root directory isn't permanent, but gets
# wiped every reboot (such as by mounting it as tmpfs). Such a setup is
# possible because NixOS only needs /boot and /nix in order to boot, all
# other system files are simply links to files in /nix.
# The impermanence module bind-mounts persistent files and directories,
# stored in /nix/persist, into the tmpfs root partition on startup. For
# example: /nix/persist/etc/machine-id is mounted to /etc/machine-id.
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# https://github.com/nix-community/impermanence
# https://nixos.wiki/wiki/Impermanence
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# https://elis.nu/blog/2020/05/nixos-tmpfs-as-root/
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imports = [
impermanence.nixosModules.impermanence
];
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# We *don't* want to use tmpfs for /tmp in case we have to put big files
# there. Instead, we mount it to the disk and instruct systemd to clean it on
# boot.
# TODO: There might be a way to configure /tmp to be in-memory storage until
# it gets too big.
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boot.tmp.cleanOnBoot = true;
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# Each module will configure the paths they need persisted. Here we define
# some general system paths that don't really fit anywhere else.
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environment.persistence."/nix/persist" = {
hideMounts = true;
directories = [
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# See comment above for /tmp
{ directory = "/tmp"; user = "root"; group = "root"; mode = "1777"; }
# With great power comes great responsibility, we get it
{ directory = "/var/db/sudo/lectured"; user = "root"; group = "root"; mode = "0700"; }
# Save the last run time of persistent timers so systemd knows if they was missed
{ directory = "/var/lib/systemd/timers"; user = "root"; group = "root"; mode = "0755"; }
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{ directory = "/var/log"; user = "root"; group = "root"; mode = "0755"; }
];
files = [
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"/etc/machine-id" # needed for /var/log
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];
users.caspervk = {
directories = [
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"/" # entire home directory (TODO)
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];
};
};
}