Preferred Read Failure Groups
When you configure Oracle ASM failure groups, it might be more efficient for a node to read from an extent that is closest to the node, even if that extent is a secondary extent. In other words, you can configure Oracle ASM to read from a secondary extent if that extent is closer to the node instead of Oracle ASM reading from the primary copy which might be farther from the node. Using the preferred read failure groups feature is most useful in extended clusters.
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To use this feature, Oracle recommends that you configure at least one mirrored extent copy from a disk that is local to a node in an extended cluster. However, a failure group that is preferred for one instance might be remote to another instance in the same Oracle RAC database. The parameter setting for preferred read failure groups is instance specific.
Both the Oracle ASM clients and Oracle ASM require Oracle Database 11g release 1 (11.1) or higher to use preferred read failure groups.
Note:
If you do not specify failure groups for a disk group, each disk in the disk group belongs to its own failure group. Oracle does not recommend that you configure multiple preferred read failure groups in a disk group for an Oracle ASM instance. For any given instance, if you specify multiple failure groups in the same disk group as preferred read, a warning message is written to the alert log.
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