Mirror a Volume Group

Create a new Lun with the same capacity [ in my case hdisk8 ]
cfgmgr
chdev -l hdisk8 -a pv=yes
chdev -l hdisk8 -a pv=clear
chdev -l hdisk8 -a reserve_policy=no_reserve -a queue_depth=8 -a rw_timeout=60
extendvg rootvg hdisk8
mirrorvg rootvg
bootlist -o -m normal
bootlist -m normal hdisk1 hdisk8
bosboot -ad hdisk1
bosboot -ad hdisk8