lukegb/depot
1
0
Fork 0
Code Issues 1 Pull requests Projects Packages Activity Actions
depot/third_party/nixpkgs/nixos/doc/manual/administration/systemd-state.section.md

2.4 KiB
Raw Blame History

systemd

machine-id(5)

systemd uses per-machine identifier — {manpage}machine-id(5) — which must be unique and persistent; otherwise, the system journal may fail to list earlier boots, etc.

systemd generates a random machine-id(5) during boot if it does not already exist, and persists it in /etc/machine-id. As such, it suffices to make that file persistent.

Alternatively, it is possible to generate a random machine-id(5); while the specification allows for any hex-encoded 128b value, systemd itself uses UUIDv4, i.e. random UUIDs, and it is thus preferable to do so as well, in case some software assumes machine-id(5) to be a UUIDv4. Those can be generated with uuidgen -r | tr -d - (tr being used to remove the dashes).

Such a machine-id(5) can be set by writing it to /etc/machine-id or through the kernel's command-line, though NixOS' systemd maintainers discourage the latter approach.

/var/lib/systemd

Moreover, systemd expects its state directory — /var/lib/systemd — to persist, for:

  • {manpage}systemd-random-seed(8), which loads a 256b “seed” into the kernel's RNG at boot time, and saves a fresh one during shutdown;
  • {manpage}systemd.timer(5) with Persistent=yes, which are then run after boot if the timer would have triggered during the time the system was shut down;
  • {manpage}systemd-coredump(8) to store core dumps there by default; (see {manpage}coredump.conf(5))
  • {manpage}systemd-timesyncd(8);
  • {manpage}systemd-backlight(8) and {manpage}systemd-rfkill(8) persist hardware-related state;
  • possibly other things, this list is not meant to be exhaustive.

In any case, making /var/lib/systemd persistent is recommended.

/var/log/journal/{machine-id}

Lastly, {manpage}systemd-journald(8) writes the system's journal in binary form to /var/log/journal/{machine-id}; if (locally) persisting the entire log is desired, it is recommended to make all of /var/log/journal persistent.

If not, one can set Storage=volatile in {manpage}journald.conf(5) (services.journald.storage = "volatile";), which disables journal persistence and causes it to be written to /run/log/journal.