Below the following commands will help assist in mirroring the rootvg on AIX 5.x to 7.1.
Have an empty disk available to mirror the existing rootvg. Add the empty disk to the existing vg by doing the following.
extendvg rootvg hdisk1
Now we are going to mirror the VG. This will take a while to run depending on your system.
Change the bootlist so that the system can fail to hdisk1 if hdisk 0 should go offline, or in reverse order.
bootlist -o -m normal
bootlist -m normal hdisk0 hdisk1
Install the software needed for boot on both disks.
bosboot -ad hdisk0
bosboot -ad hdisk1