raid and mirroring

assume /dev/sda and /dev/sdb are local hard drives.
assume /dev/sda3 and /dev/sdb3 are of equal sized

mdadm --create --verbose /dev/md0 --level=mirror --raid-devices=2 /dev/sda3 /dev/sdb3

mkfs -t ext3 /dev/md0

mount /dev/md0 /mnt