nixCraft posted a guide about setting up a software RAID 1 array under Redhat/CentOS

Recently, I've added another 73GB SAS disk to my Linux server after the installation. I've 30GB empty partition on old hard disk. How do I convert old /dev/sda3 and new /dev/sdb1 (both 30GB) into RAID 1 to improve NFS server speed and reliability?

  CentOS / Redhat: Create Software RAID 1 Array