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1# systemd {#sec-systemd-state}
2
3## `machine-id(5)` {#sec-machine-id}
4
5`systemd` uses per-machine identifier — {manpage}`machine-id(5)` — which must be
6unique and persistent; otherwise, the system journal may fail to list earlier
7boots, etc.
8
9`systemd` generates a random `machine-id(5)` during boot if it does not already exist,
10and persists it in `/etc/machine-id`. As such, it suffices to make that file persistent.
11
12Alternatively, it is possible to generate a random `machine-id(5)`; while the
13specification allows for *any* hex-encoded 128b value, systemd itself uses
14[UUIDv4], *i.e.* random UUIDs, and it is thus preferable to do so as well, in
15case some software assumes `machine-id(5)` to be a UUIDv4. Those can be
16generated with `uuidgen -r | tr -d -` (`tr` being used to remove the dashes).
17
18Such a `machine-id(5)` can be set by writing it to `/etc/machine-id` or through
19the kernel's command-line, though NixOS' systemd maintainers [discourage] the
20latter approach.
21
22[UUIDv4]: https://en.wikipedia.org/wiki/Universally_unique_identifier#Version_4_(random)
23[discourage]: https://github.com/NixOS/nixpkgs/pull/268995
24
25
26## `/var/lib/systemd` {#sec-var-systemd}
27
28Moreover, `systemd` expects its state directory — `/var/lib/systemd` — to persist, for:
29- {manpage}`systemd-random-seed(8)`, which loads a 256b “seed” into the kernel's RNG
30 at boot time, and saves a fresh one during shutdown;
31- {manpage}`systemd.timer(5)` with `Persistent=yes`, which are then run after boot if
32 the timer would have triggered during the time the system was shut down;
33- {manpage}`systemd-coredump(8)` to store core dumps there by default;
34 (see {manpage}`coredump.conf(5)`)
35- {manpage}`systemd-timesyncd(8)`;
36- {manpage}`systemd-backlight(8)` and {manpage}`systemd-rfkill(8)` persist hardware-related
37 state;
38- possibly other things, this list is not meant to be exhaustive.
39
40In any case, making `/var/lib/systemd` persistent is recommended.
41
42
43## `/var/log/journal/{machine-id}` {#sec-var-journal}
44
45Lastly, {manpage}`systemd-journald(8)` writes the system's journal in binary
46form to `/var/log/journal/{machine-id}`; if (locally) persisting the entire log
47is desired, it is recommended to make all of `/var/log/journal` persistent.
48
49If not, one can set `Storage=volatile` in {manpage}`journald.conf(5)`
50([`services.journald.storage = "volatile";`](#opt-services.journald.storage)),
51which disables journal persistence and causes it to be written to
52`/run/log/journal`.