1.3.2.2.2 Oracle ASM Failure Group

An Oracle ASM failure group is a subset of disks in an Oracle ASM disk group that can fail together because they share the same hardware.

Oracle ASM considers failure groups when making redundancy decisions.

For Oracle Exadata Storage Servers, all grid disks, which consist of the Oracle ASM disk group members and candidates, can effectively fail together if the storage cell fails. Because of this scenario, all Oracle ASM grid disks sourced from a given storage cell should be assigned to a single failure group representing the cell.

For example, if all grid disks from two storage cells, A and B, are added to a single Oracle ASM disk group with normal redundancy, then all grid disks on storage cell A are designated as one failure group, and all grid disks on storage cell B are designated as another failure group. This enables Oracle Exadata System Software and Oracle ASM to tolerate the failure of either storage cell.

Failure groups for Oracle Exadata Storage Server grid disks are set by default so that the disks on a single cell are in the same failure group, making correct failure group configuration simple for Oracle Exadata Storage Servers.

You can define the redundancy level for an Oracle ASM disk group when creating a disk group. An Oracle ASM disk group can be specified with normal or high redundancy. Normal redundancy double mirrors the extents, and high redundancy triple mirrors the extents. Oracle ASM normal redundancy tolerates the failure of a single cell or any set of disks in a single cell. Oracle ASM high redundancy tolerates the failure of two cells or any set of disks in two cells. Base your redundancy setting on your desired protection level. When choosing the redundancy level, ensure the post-failure I/O capacity is sufficient to meet the redundancy requirements and performance service levels. Oracle recommends using three cells for normal redundancy. This ensures the ability to restore full redundancy after cell failure. Consider the following:

• If a cell or disk fails, then Oracle ASM automatically redistributes the cell or disk contents across the remaining disks in the disk group as long as there is enough space to hold the data. For an existing disk group using Oracle ASM redundancy, the USABLE_FILE_MB and REQUIRED_FREE_MIRROR_MB columns in the V$ASM_DISGKROUP view give the amount of usable space and space for redundancy, respectively.

• If a cell or disk fails, then the remaining disks should be able to generate the IOPS necessary to sustain the performance service level agreement.

After a disk group is created, the redundancy level of the disk group cannot be changed. To change the redundancy of a disk group, you must create another disk group with the appropriate redundancy, and then move the files.

Each Exadata Cell is a failure group. A normal redundancy disk group must contain at least two failure groups. Oracle ASM automatically stores two copies of the file extents, with the mirrored extents placed in different failure groups. A high redundancy disk group must contain at least three failure groups. Oracle ASM automatically stores three copies of the file extents, with each file extent in separate failure groups.

System reliability can diminish if your environment has an insufficient number of failure groups. A small number of failure groups, or failure groups of uneven capacity, can lead to allocation problems that prevent full use of all available storage.