Could you think of any situation where coreos.inst.ignition_url and ignition.config.url are used
at the same time?
Maybe it would be enough just to have one of them if they are never used together?
To me it seems they are mutually exclusive.
If you’re using PXE to do a persistent install (a common use case), then indeed you provide both.
The ignition.config.url is the Ignition for the live PXE boot - it defines what you want to do there, which would include e.g. running coreos-installer to write to the disk you specify. Another thing you might want to do in this Ignition is call back to a “provisioning” system to report success. See e.g. https://github.com/coreos/coreos-installer/pull/25
The coreos.inst.ignition_url applies for coreos-installer - it is for the subsequent boot with the installed system, so that’s where you want most of your actual configuration to take place.
In the documentation there is a PXE installation example but it only uses coreos.inst.ignition_url and no ignition.config.url.
APPEND ip=dhcp rd.neednet=1 initrd=fedora-coreos-30.20191014.1-live-initramfs.x86_64.img console=tty0 console=ttyS0 coreos.inst.install_dev=/dev/sda coreos.inst.ignition_url=http://192.168.1.101:8000/config.ign
So it is possible to add ignition.config.url to that line. Did I understand you correctly?
Yup, that’s correct.