Thank you. Unfortunately I have no spare disk slots or space on existing disks to set up a separate boot partition. I'll backup the data now with zfs send. Does the recommendation to have a non-zfs /boot partition mean, that it is not recommended to boot from ZFS any more? The I woul take to disks out of the zpool to set up as non-zfs system disks.
there are broken systems were booting is not possible or not stable - most notable involving hardware raid controllers.