This section discusses frequently asked questions about failure group under the following topics:
Choosing the number of failure groups to create depends on the types of failures that must be tolerated without data loss. For small numbers of disks, such as fewer than 20, it is usually best to use the default failure group creation that puts every disk in its own failure group.
Using the default failure group creation for small numbers of disks is also applicable for large numbers of disks where your main concern is disk failure. For example, a disk group might be configured from several small modular disk arrays. If the system must continue operating when an entire modular array fails, then a failure group should consist of all of the disks in one module. If one module fails, then all of the data on that module is relocated to other modules to restore redundancy. Disks should be placed in the same failure group if they depend on a common piece of hardware whose failure must be tolerated with no loss of availability.
A simultaneous failure can occur if there is a failure of a piece of hardware used by multiple failure groups. This type of failure usually forces a dismount of the disk group if all disks are unavailable.
Oracle ASM mirroring runs on the database server and Oracle recommends to off load this processing to the storage hardware RAID controller by using external redundancy. You can use normal redundancy in the following scenarios:
Storage system does not have RAID controller
Mirroring across storage arrays
Extended cluster configurations
In general, Oracle ASM mirroring is the Oracle alternative to third party logical volume managers. Oracle ASM mirroring eliminates the deployment of additional layers of software complexity in your Oracle Database environment.