your oppinion is correct ---- Just reboot into grub, hit c to get the command line and type "kopenbsd /bsd.rd" there, no need to edit the config twice ----
but i use rental server . so i have no time to hit c . so debian is booted . so i try another way . this install is convinet over network install . if there is not kopenbsd , i first send install57.iso and start to install openbsd . 1 hour or more is costed and whole disk may be ocupied with openbsd only . i want to use debian (typical linux server) also . set root=(hd0,4) is wrong. set root=(hd0,4) is right . and more i easily try snapshots upgrade . ------ regards

