A failure group is one or more disks within a disk group that share a common resource, such as a
disk controller, whose failure would cause the entire set of disks to be unavailable to the group. In
most cases, an ASM instance does not know the hardware and software dependencies for a given
disk. Therefore, unless you specifically assign a disk to a failure group, each disk in a disk group
is assigned to its own failure group.
Once the failure groups have been defined, you can define the mirroring for the disk group;
the number of failure groups available within a disk group can restrict the type of mirroring
available for the disk group. There are three types of mirroring available: external redundancy,
normal redundancy, and high redundancy.
External Redundancy External redundancy requires only one disk location and assumes that the
disk is not critical to the ongoing operation of the database or that the disk is managed externally
with high-availability hardware such as a RAID controller.
Normal Redundancy Normal redundancy provides two-way mirroring and requires at least two
failure groups within a disk group. Failure of one of the disks in a failure group does not cause
any downtime for the disk group or any data loss other than a slight performance hit for queries
against objects in the disk group; when all disks in the failure group are online, read performance
is typically improved because the requested data is available on more than one disk.
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