Understanding ASM Capacity and Reservation of Free Space in Exadata (Doc ID 1551288.1)
Here's a good document which explains the concepts of ASM disk group failure coverage, how to reserve free space to cover disk or cell failure in an Exadata environment, and how to determine the resulting capacity of a disk group (DG).
Doc ID 1551288.1 : Understanding ASM Capacity and Reservation of Free Space in Exadata
It is a must read doc. for those that manage storage in the Exadata environment.
Cesar