ASM redundancy and Failure Groups

A Failure group is a collection of disks that can become unavailable due to failure of one of its associated components (controllers, HBAs, Fiber channel switches, Disks, Entire arrays etc).

A diskgroup is divided in to failure groups. Each disks in the diskgroup is part of one failure group. The disks in different failure groups does not share the same failure component (HBAs, controllers etc).

ASM uses Mirroring for redundancy. Three types of redundancy are supported by ASM. They are,

Normal Redundancy: 2-way mirrored. At least two failure groups are needed.
High Redundancy: 3-way mirrored. At least three failure groups are needed.
External Redundancy: No ASM mirroring. 3rd party redundancy mechanism like RAID is used.

After creating a diskgroup you cannot change the redundancy level. If you want to change it then create a separate diskgroup and move the files to that diskgroup (using RMAN restore or DBMS_FILE_TRANSFER).

ASM mirrors extents instead of disks. The first copy of the extent is called the primary extent and its mirrored extent is called secondary extent. For high redundancy there are two secondary extents. The primary and secondary extents together are known as extent set. An extent set always contains same data. For read/write operations, each extent in an extent set is written in parallel and only primary extent is read.

Note: Disk group meta data is always triple mirrored with normal or high redundancy.

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