After upgrading to 5 on a systemd based environment it seems there are now 3 or 4 possible locations to define startup and configuration parameters. These being
/etc/sysconfig/elasticsearch (for heap, paths and various miscellany)
/etc/elasticsearch/jvm.options (for heap)
/etc/elasticsearch/elasticsearch.yml (ES config and paths)
/usr/lib/systemd/system/elasticsearch.service (all of the above?)
This is CRAZY confusing, and should probably be normalised?