2.4 KiB
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)withPersistent=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.