Mirroring protects data integrity by storing copies of data on multiple disks. When you create a disk group, you specify an Oracle ASM disk group type based on one of the following three redundancy levels:
Normal for 2-way mirroring
High for 3-way mirroring
External to not use Oracle ASM mirroring, such as when you configure hardware RAID for redundancy
The redundancy level controls how many disk failures are tolerated without dismounting the disk group or losing data. The disk group type determines the mirroring levels with which Oracle creates files in a disk group. For information about disk group types and templates, see "Managing Disk Group Templates".
Oracle ASM mirroring is more flexible than traditional RAID mirroring. For a disk group specified as NORMAL redundancy, you can specify the redundancy level for each file. For example, two files can share the same disk group with one file being mirrored while the other is not.
When Oracle ASM allocates an extent for a mirrored file, Oracle ASM allocates a primary copy and a mirror copy. Oracle ASM chooses the disk on which to store the mirror copy in a different failure group than the primary copy. Failure groups are used to place mirrored copies of data so that each copy is on a disk in a different failure group. The simultaneous failure of all disks in a failure group does not result in data loss.
You define the failure groups for a disk group when you create an Oracle ASM disk group. After a disk group is created, you cannot alter the redundancy level of the disk group. If you omit the failure group specification, then Oracle ASM automatically places each disk into its own failure group, except for disk groups containing disks on Oracle Exadata cells. Normal redundancy disk groups require at least two failure groups. High redundancy disk groups require at least three failure groups. Disk groups with external redundancy do not use failure groups.
For more information about mirroring and failure groups, see "Mirroring_ Redundancy_ and Failure Group Options".