Redundancy Level of Disk Group in Oracle

For Oracle ASM to mirror files, specify the redundancy level as NORMAL REDUNDANCY (2-way mirroring by default for most file types) or HIGH REDUNDANCY (3-way mirroring for all files). Specify EXTERNAL REDUNDANCY if you do not want mirroring by Oracle ASM. For example, you might choose EXTERNAL REDUNDANCY to use storage array protection features.

Various of Redundancy Level

• High Redundancy (3 Mirrors)

Oracle ASM does not provide mirroring redundancy and relies on the storage system to provide RAID functionality. Any write error causes a forced dismount of the disk group. All disks must be located to successfully mount the disk group.

• Normal Redundancy (2 Mirrors)

Oracle ASM provides two-way mirroring by default, which means that all files are mirrored so that there are two copies of every extent. A loss of one Oracle ASM disk is tolerated. You can optionally choose three-way or unprotected mirroring.

A file specified with HIGH redundancy (three-way mirroring) in a NORMAL redundancy disk group provides additional protection from a bad disk sector, not protection from a disk failure.

• External Redundancy (No Mirror)

Oracle ASM provides triple mirroring by default. A loss of two Oracle ASM disks in different failure groups is tolerated.

After a disk group is created, you cannot alter the redundancy level of the disk group. To change the redundancy level, you must create another disk group with the appropriate redundancy and then move the files to the new disk group.

Oracle recommends that you create failure groups of equal size to maintain space balance and even distribution of mirror data.

Source : Oracle Documents