Mirroring "rootvg" protects the operating system from a disk failure. Mirroring "rootvg" requires a couple extra steps compared to other volume groups. The mirrored rootvg disk must be bootable *and* in the bootlist. Otherwise, if the primary disk fails, you'll continue to run, but you won't be able to reboot.
In brief, the procedure to mirror rootvg on hdisk0 to hdisk1 is