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ASM is not a software RAID. ASM works on the basis of file extents and provides data redundancy by using the free disk space of disk failure groups. By default, each disk is a failure group. For a 2-way (normal) mirror you need at least 2 disk failure groups; for 3-way (high) you need at least 3. You can also create a 2-way mirror using 3 disks, in which case data will be written in a round robin fashion. To have normal ASM redundancy in your example you need to have at least two 1 TB disks, but you can also use 3 and will have more space. You can also use an external RAID and use no (external) ASM redundancy, though ASM will give you better performance and flexibility since you can add disks to increase space without having to rebuild your storage.