Step-by-step operations in command line to setup Soft RAID.
Before setup:
sysctl kern.geom.debugflags=16
To create mirroring on disk /dev/ad0 :
gmirror label -v -b round-robin gm0 /dev/ad0
Changing configuration to boot from RAID:
echo geom_mirror_load="YES" > /boot/loader.conf
Changing /etc/fstab. Instead /dev/ad0s1a should be /dev/mirror/gm0s1a ( change "ad" to "mirror/gm"). If you have "ad4" change to "gm0".
Reboot.
Adding second disk:
gmirror insert gm0 /dev/ad2
Waiting for rebuilding new RAID:
gmirror list | more
# or:
gmirror status
If one disk is failed:
gmirror forget gm0
gmirror insert gm0 /dev/ad0
if first disk failed.
If you need rollback:
gmirror forget gm0
And change files to previous state: /boot/loader.conf and /etc/fstab.
Reboot.