Each cell is not a full copy of the other cells. ASM diskgroups on Exadata utilize each cell as a failgroup, meaning that no 2 copies of an allocation unit are placed on the same cell. A half rack of high capacity storage (3TB drives) has 252TB of raw space (12 disks * 7 cells) ~126TB of usable space with normal redundancy, and ~84TB of usable space in high redundancy. As far as disk sizes, on ASM you generally want all disks to be the same size to keep everything balanced evenly. I have a short writeup on Exadata diskgroup storage at http://blog.oracle-ninja.com/2011/12/exadata-diskgroup-planning
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