Oracle Automatic Storage Management (Oracle ASM) provides software-based mirroring capabilities. Oracle ASM provides support for normal redundancy (mirroring) and high redundancy (triple mirroring). Oracle ASM also supports the use of external redundancy, in which case Oracle ASM does not perform additional mirroring. Oracle ASM normal redundancy can be compared to RAID 1 hardware mirroring.
With Oracle ASM mirroring, the mirror is produced by the database servers. Consequently, write operations require more I/O throughput when using Oracle ASM mirroring compared to using hardware-based mirroring. Depending on your configuration and the speed of the hardware RAID controllers, Oracle ASM mirroring or hardware RAID may introduce a bottleneck for data loads.
In Oracle ASM, the definition of failure groups enables redundancy, as Oracle ASM mirrors data across the boundaries of the failure group. For example, in a VLDB environment, you can define one failure group per disk array, in which case Oracle ASM ensures that mirrored data is stored on a different disk array. That way, you could not only survive a failure of a single disk in a disk array, but you could even survive the failure of an entire disk array or failure of all channels to that disk array. Hardware RAID configurations typically do not support this kind of fault tolerance.
Oracle ASM using normal redundancy requires double the amount of disk space needed to store the data. High redundancy requires triple the amount of disk space.