Oracle ASM keeps redundancy quite simple by providing only 3 options. you can choose to have no redundancy or mirroring provided by the ASM instance. This is the ‘External’ option and assumes that you are handling mirroring somewhere else perhaps through hardware raid on your storage network.
You can chose ‘normal’ 2 way mirroring r ‘High’ for 3 way mirroring.
This option is set for each disk group and can be different for every one.
Basically, the redundancy level determines how many disks you can afford to have fail before the disk group is dismounted or data loss occurs. With external mirroring, you can’t afford to lose any disks whereas with 3 way mirroring, you could lose 2 disks and there would still be one copy available.
With normal RAID, the level of mirroring is set for the disk (or disk group). ASM gives you a more granular level of control. For example, it is possible to set different mirroring levels for different files on a single disk group.
When data is created or an extent is allocated, ASM creates a primary copy and a mirrored copy. these copies are distributed across failure groups so that losing a failure group completely would not in and of itself cause the loss of any data.
You cannot change the redundancy levels of a disk group once it is created. If failure groups are not specified, ASM places each disk into its own failure group (unless the disk group contains Exadata cells.