Failure groups are used to store mirror copies of data. When Oracle ASM allocates an extent for a normal redundancy file, Oracle ASM allocates a primary copy and a secondary copy. Oracle ASM chooses the disk on which to store the secondary copy so that it is in a different failure group than the primary copy. Each copy is on a disk in a different failure group so that the simultaneous failure of all disks in a failure group does not result in data loss.
A failure group is a subset of the disks in a disk group, which could fail at the same time because they share hardware. The failure of common hardware must be tolerated. Four drives that are in a single removable tray of a large JBOD (Just a Bunch of Disks) array should be in the same failure group because the tray could be removed making all four drives fail at the same time. Drives in the same cabinet could be in multiple failure groups if the cabinet has redundant power and cooling so that it is not necessary to protect against failure of the entire cabinet. However, Oracle ASM mirroring is not intended to protect against a fire in the computer room that destroys the entire cabinet.
There are always failure groups even if they are not explicitly created. If you do not specify a failure group for a disk, then Oracle automatically creates a new failure group containing just that disk, except for disk groups containing disks on Oracle Exadata cells.
A normal redundancy disk group must contain at least two failure groups. A high redundancy disk group must contain at least three failure groups. However, Oracle recommends using more failure groups. A small number of failure groups, or failure groups of uneven capacity, can create allocation problems that prevent full use of all of the available storage.
Oracle recommends a minimum of three failure groups for normal redundancy disk groups and five failure groups for high redundancy disk groups to maintain the necessary number of copies of the Partner Status Table (PST) and to ensure robustness with respect to storage hardware failures.
In the event of a system failure, three failure groups in a normal redundancy disk group allow a comparison among three PSTs to accurately determine the most up to date and correct version of the PST, which could not be done with a comparison between only two PSTs. Similarly with a high redundancy disk group, if two failure groups are offline, then Oracle ASM would be able to make a comparison among the three remaining PSTs.
If configuring an extra failure group presents a problem with storage capacity management, then a quorum failure group can be used as the extra failure group to store a copy of the PST. A quorum failure group does not require the same capacity as the other failure groups.
Failure groups can be specified as regular or quorum failure groups. For information about quorum failure groups, see "Storing Oracle Cluster Registry and Voting Files in Oracle ASM Disk Groups".