disk, with listener.ready_min_free_bytes set (YAML node.min_free_bytes): the file
system holding data_dir has at least that many bytes free. Below it the
reason is below_floor, and the node is unready before the store hits
ENOSPC, which is what lets the load balancer drain it while the operator
still has room to act.
Free space comes from :disksup (:os_mon), which core does not start and
the server image does. A floor with no :disksup running, an answer that is
not a disk entry, or one that takes longer than @timeout_ms (a hung mount)
is reason unmeasured: a configured floor that cannot be checked fails, it
is never silently skipped.