Comment # 1 on bug 1095315 from Martin Vidner

Created attachment 771980 [details]
extracting the screenshot of the device graph.png

Thanks for the report!

Josef, can you please look at this device graph.

I think that the warning is correct: there is no boot partition and the root
partition is on a logical volume composed from sda and sdb. It is booting
because /boot happens to be on sda but might also be on sdb and then it would
not boot.