Single-Instance vs. Oracle RAC Environment
Similar to in a single-instance environment, each instance in an Oracle RAC environment has its own System Global Area (SGA) and background processes. However, all the datafiles and control files are equally accessible to all the nodes, so these files must be placed on a shared disk subsystem. Each instance also has its own dedicated set of online redo log files. The online redo log files can be written to only by the instance to which they belong. However, these files must be readable by other instances during instance crash recovery. This requires that online redo log files reside on a shared disk subsystem and not on a node’s local storage, because the files would be lost if the node crashed.
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