Oracle Tips by Burleson

ASM Failure Groups

Failure groups are manually defined groups of disks sharing a common resource. Failure group definitions determine which ASM disks are used for storing mirror copies of data. Failure groups ensure that data and its redundant copy do not both reside on disks that are likely to fail together.

Each set of failure groups is site-specific. This is because failure group decisions are based on how your system tolerates the failure of specific components. For example: suppose you have ten disks and one SCSI controller. The failure of the one SCSI controller makes all ten disks unavailable. In this scenario, you should put each SCSI disk in its own failure group.

Each disk is assigned to its one failure group in ASM by default. However this assignment of failure groups can be over-ridden by the administrator when creating a disk group or adding a disk to a disk group. The layout of data is then optimized by ASM to reduce the unavailability of data due to failures.

A failure group is maintained in a disk group, and multiple failure groups can exist within any given disk group. However, changing a disk’s failure group requires dropping the disk from the disk group and then adding the disk back to the disk group under the new failure group name.

 

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