Il y a peut être un début de réponse dans
man systemd.unit
En particulier cette section:
Before=, After=
Configures ordering dependencies between units. If a unit
foo.service contains a setting Before=bar.service and both units
are being started, bar.service's start-up is delayed until
foo.service is started up. Note that this setting is independent of
and orthogonal to the requirement dependencies as configured by
Requires=. It is a common pattern to include a unit name in both
the After= and Requires= option in which case the unit listed will
be started before the unit that is configured with these options.
This option may be specified more than once, in which case ordering
dependencies for all listed names are created. After= is the
inverse of Before=, i.e. while After= ensures that the configured
unit is started after the listed unit finished starting up, Before=
ensures the opposite, i.e. that the configured unit is fully
started up before the listed unit is started. Note that when two
units with an ordering dependency between them are shut down, the
inverse of the start-up order is applied. i.e. if a unit is
configured with After= on another unit, the former is stopped
before the latter if both are shut down. If one unit with an
ordering dependency on another unit is shut down while the latter
is started up, the shut down is ordered before the start-up
regardless whether the ordering dependency is actually of type
After= or Before=. If two units have no ordering dependencies
between them they are shut down resp. started up simultaneously,
and no ordering takes place.
D'après ce que je comprend, on peut indiquer à un service de démarrer avant (ou après) un autre.
Par contre je n'ai pas compris comment cela se passe...