Redundancy
One can configure ASM diskgroups to have no redundancy (external), two-way mirroring (normal), or three-way mirroring (high). In the case of normal and high mirrors, good practice suggests having fail groups that talk to different controllers for performance and fail-safe reasons.
- In the case of external redundancy, ASM does not do any software mirroring, but only stripes its files across all the disks that belong to the disk group that does external redundancy.
- In the case of normal redundancy, ASM does two-way mirroring, meaning that ASM maintains two copies of the data through software mirroring. When querying for mirror information, DBAs will see two mirrors in this case.
- In the case of high redundancy, ASM does three-way mirroring, maintaining three copies of the data through software mirroring. When querying for mirror information, DBAs will see three mirrors in this case.
Read more about this topic: Automatic Storage Management