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To understand the communication infrastructure, review the data flow and communications requirements.
The CVM, CFS, ODM, and Oracle RAC elements reflect the overall data flow, or data stack, from an instance running on a server to the shared storage. The various Oracle processes composing an instance -- such as DB Writers, Log Writer, Checkpoint, Archiver, and Server -- read and write data to the storage through the I/O stack in the diagram. Oracle communicates through the ODM interface to CFS, which in turn accesses the storage through the CVM.
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End-users on a client system are unaware that they are accessing a database hosted by multiple instances. The key to performing I/O to a database accessed by multiple instances is communication between the processes. Each layer or component in the data stack must reliably communicate with its peer on other nodes to function properly. RAC instances must communication to coordinate protection of data blocks in the database. ODM processes must communicate to coordinate data file protection and access across the cluster. CFS coordinates metadata updates for file systems, while CVM coordinates the status of logical volumes and maps.
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