The collector bind-mounts /var/log/journal into its container and points
its journald receiver at that directory. journald only writes there when
it stores persistently: with storage = "volatile" the journal lives in
/run/log/journal, and with "none" there is none at all. Nothing in the
tree sets or asserts services.journald.storage, so a host with either
value deploys a collector that cannot start — nixos-container refuses to
start when a bind source is missing, which is the same trap the
swarm-bao-certs and hostClientSecretDir comments document.
A bindMounts entry never creates its hostPath, and unlike every other
runtime-written bind source in the tree there is no unit to order the
container after: each of those declares before + requiredBy on its
producing unit, and this directory has no producer. journald was told to
store persistently or it does not exist. That leaves an assertion as the
only place to say so.
"auto" is deliberately accepted: it uses the directory when it exists,
and evaluation cannot see whether it does. Certainty is what separates
an assertion from a warning, and only "volatile" and "none" are certain.
Verified by reading config.assertions for all three values rather than
system.build.toplevel, which raises the first failing assertion in the
whole config and so reports a minimal stub's unrelated ones instead:
persistent -> mine [] others 3
volatile -> mine [the message, quoting "volatile"] others 3
none -> mine [the message, quoting "none"] others 3
The others count is identical across the three, so the storage value is
the only thing that differs between them, and a populated list is what
makes the empty one mean "this assertion passed".
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