Comment # 13 on bug 1193172 from Dr. Werner Fink

Yep there is a reason why klog.service is a oneshot as well it refuses to be
started manually (means via e.g. systemctl) as otherwise it would trash/smash
the /var/log/boot.msg as well as /var/log/boot.omsg

And klog.service should be able to access the character devices below /dev as
otherwise /usr/bin/setlogcons as well as /bin/dmesg can not work ... also
boot.msg  and boot.omsg should be in the real upper filesystem and not within a
temporary userspace