RAID 0=min(S1,S2,......)*磁碟數 =20G*4=80G
RAID 1=min(S1,S2,......),20G
RAID 0+1 =min(S1,S2,......)/2 =40G
RAID 5 ,一顆磁碟容量用於同位位元區塊,所以可用空間剩下3顆磁碟,3*20=60G