In Oracle ASM, the disk group is used to manage several storage properties such as: redundancy, access control, and allowed repair time before drop. Physical disks are assigned to disk groups. Disk groups provide the functionality of RAID with much more flexibility. Disk groups have unique names in the Oracle ASM instance. All disks in a disk group must be of equal size. A disk can belong to one and only one disk group.
You specify redundancy property for each disk group. The possible settings are EXTERNAL, NORMAL, HIGH, FLEX and EXTENDED. When you specify EXTERNAL redundancy, Oracle ASM does not provide mirroring. If you specify NORMAL redundancy Oracle ASM provides two-way mirroring by default for most files. When HIGH redundancy is specified, Oracle ASM provides three-way mirroring. When you specify FLEX or EXTENDED redundancy the disk group redundancy is overridden by the file group redundancy property. EXTENDED redundancy provides two-way mirroring by default within a site, and assumes that you have a vendor supplied disk mirroring solution between sites.
By default each disk is in a different failure group, optionally disks may be grouped into failure groups. Normal redundancy requires a minimum of two disks or two failure groups. High redundancy requires a minimum of three disks or failure groups. A disk group will have a default redundancy that can be overridden at the file or file group level, if there are sufficient failure groups. The example shows only two failure groups so high redundancy is not possible. Depending on the redundancy setting the blocks of a file placed in the disk group or file group will be scattered across the required number of physical disks.
File Groups are optional, but provide more flexible storage management when multiple database, PDBs, or clusters are using the same Oracle ASM storage set. To use Oracle ASM File Groups, set the Disk group redundancy to FLEX or EXTENDED. File Groups allow you to control redundancy, and access by database to specified files within a disk group.