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What happens during a system switch?
Running nixos-rebuild switch is one of the more common tasks under NixOS.
This chapter explains some of the internals of this command to make it simpler
for new module developers to configure their units correctly and to make it
easier to understand what is happening and why for curious administrators.
nixos-rebuild, like many deployment solutions, calls switch-to-configuration
which resides in a NixOS system at $out/bin/switch-to-configuration. The
script is called with the action that is to be performed like switch, test,
boot. There is also the dry-activate action which does not really perform
the actions but rather prints what it would do if you called it with test.
This feature can be used to check what service states would be changed if the
configuration was switched to.
If the action is switch or boot, the bootloader is updated first so the
configuration will be the next one to boot. Unless NIXOS_NO_SYNC is set to
1, /nix/store is synced to disk.
If the action is switch or test, the currently running system is inspected
and the actions to switch to the new system are calculated. This process takes
two data sources into account: /etc/fstab and the current systemd status.
Mounts and swaps are read from /etc/fstab and the corresponding actions are
generated. If the options of a mount are modified, for example, the proper .mount
unit is reloaded (or restarted if anything else changed and it's neither the root
mount or the nix store). The current systemd state is inspected, the difference
between the current system and the desired configuration is calculated and
actions are generated to get to this state. There are a lot of nuances that can
be controlled by the units which are explained here.
After calculating what should be done, the actions are carried out. The order of actions is always the same:
- Stop units (
systemctl stop) - Run activation script (
$out/activate) - See if the activation script requested more units to restart
- Restart systemd if needed (
systemd daemon-reexec) - Forget about the failed state of units (
systemctl reset-failed) - Reload systemd (
systemctl daemon-reload) - Reload systemd user instances (
systemctl --user daemon-reload) - Set up tmpfiles (
systemd-tmpfiles --create) - Reload units (
systemctl reload) - Restart units (
systemctl restart) - Start units (
systemctl start) - Inspect what changed during these actions and print units that failed and that were newly started
Most of these actions are either self-explaining but some of them have to do with our units or the activation script. For this reason, these topics are explained in the next sections.
unit-handling.section.md
activation-script.section.md