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+# systemd {#sec-systemd-state}
+
+## `machine-id(5)` {#sec-machine-id}
+
+`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.
+
+[UUIDv4]: https://en.wikipedia.org/wiki/Universally_unique_identifier#Version_4_(random)
+[discourage]: https://github.com/NixOS/nixpkgs/pull/268995
+
+
+## `/var/lib/systemd` {#sec-var-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}` {#sec-var-journal}
+
+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";`](#opt-services.journald.storage)),
+which disables journal persistence and causes it to be written to
+`/run/log/journal`.