The ASM_PREFERRED_READ_FAILURE_GROUPS initialization parameter value is a comma-delimited list of strings that specifies the failure groups that should be preferentially read by the given instance. The ASM_PREFERRED_READ_FAILURE_GROUPS parameter setting is instance specific. The default value is NULL. This parameter is generally used for clustered Oracle ASM instances and its value can be different on different nodes.
For example:
diskgroup_name1.failure_group_name1, ...
For more information about ASM_PREFERRED_READ_FAILURE_GROUPS, refer to "Preferred Read Failure Groups".
Oracle Real Application Clusters Administration and Deployment Guide for more information about configuring preferred disks in extended clusters
Oracle Database Reference for more information about the ASM_PREFERRED_READ_FAILURE_GROUPS initialization parameter