nixos/modules/base/impermanence.nix

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{impermanence, ...}: {
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# 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
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# https://wiki.nixos.org/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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# 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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# The uid and gid maps for entities without a static id is saved in
# /var/lib/nixos. Persist to ensure they aren't changed between reboots.
{
directory = "/var/lib/nixos";
user = "root";
group = "root";
mode = "0755";
}
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# Save the last run time of persistent timers so systemd knows if they were missed
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{
directory = "/var/lib/systemd/timers";
user = "root";
group = "root";
mode = "0755";
}
{
directory = "/var/log";
user = "root";
group = "root";
mode = "0755";
}
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# /var/tmp is meant for temporary files that are preserved across
# reboots. Some programs might store files too big for in-memory /tmp
# there. Files older than 10 days are cleaned by systemd.
{
directory = "/var/tmp";
user = "root";
group = "root";
mode = "1777";
}
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];
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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];
};
};
}