ceph-disk prepare writes osd log 0 with root owner
The ceph-disk prepare call calls the following lines to check journal options with a hard coded osd id 0:
/usr/bin/ceph-osd --check-allows-journal -i 0 --cluster ceph
/usr/bin/ceph-osd --check-wants-journal -i 0 --cluster ceph
/usr/bin/ceph-osd --check-needs-journal -i 0 --cluster ceph
This causes /var/log/<cluster>-osd-0.log to be written with owner and group as root. This happens on every node that ceph-disk prepare is run on. What happens downstream is the node where osd 0 will really run has problems because the OSD daemon, which runs as user 'ceph' cannot write to its own log because it has already been created with root/root.
The ceph-osd command has --setuser and --setgroup parameters that when added to this call make the file have the ceph user and group. The fix is likely just adding those parameters to the call.