If you specify mirroring for a file, then Oracle ASM automatically stores redundant copies of the file extents in separate failure groups. Failure groups apply only to normal and high redundancy disk groups. You can define the failure groups for each disk group when you create or alter the disk group.
There are three types of disk groups based on the Oracle ASM redundancy level. Table 4-1 lists the types with their supported and default mirroring levels. The default mirroring levels indicate the mirroring level with which each file is created unless a different mirroring level is designated.
Table 4-1 Mirroring options for Oracle ASM disk group types
| Disk Group Type | Supported Mirroring Levels | Default Mirroring Level |
|---|---|---|
|
External redundancy |
Unprotected (none) |
Unprotected |
|
Normal redundancy |
Two-way, three-way, unprotected (none) |
Two-way |
|
High redundancy |
Three-way |
Three-way |
The redundancy level controls how many disk failures are tolerated without dismounting the disk group or losing data. Each file is allocated based on its own redundancy, but the default comes from the disk group.
The redundancy levels are:
External redundancy
Oracle ASM does not provide mirroring redundancy and relies on the storage system to provide RAID functionality. Any write error causes a forced dismount of the disk group. All disks must be located to successfully mount the disk group.
Normal redundancy
Oracle ASM provides two-way mirroring by default, which means that all files are mirrored so that there are two copies of every extent. A loss of one Oracle ASM disk is tolerated. You can optionally choose three-way or unprotected mirroring.
A file specified with HIGH redundancy (three-way mirroring) in a NORMAL redundancy disk group provides additional protection from a bad disk sector, not protection from a disk failure.
High redundancy
Oracle ASM provides triple mirroring by default. A loss of two Oracle ASM disks in different failure groups is tolerated.
If there are not enough online failure groups to satisfy the file mirroring (redundancy attribute value) specified in the disk group file type template, Oracle ASM allocates as many mirrors copies as possible and subsequently allocates the remaining mirrors when sufficient online failure groups are available. For information about specifying Oracle ASM disk group templates, see "Managing Disk Group Templates".
Failure groups enable the mirroring of metadata and user data. System reliability can diminish if your environment has an insufficient number of failure groups.
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