Once your RDBMS instance has found the ASM instance it is relying on, what happens when the RDBMS needs to actually open a file that is held on ASM storage?
It seems fairly straightforward. The RDBMS maintains a series of connections to the ASM instance via the Onnn background processes. Requests to open a file go via these background processes to the ASM instance. The ASM instance then sends the extent map to the RDBMS instance.
The RDBMS can then use the extent map to perform I/O direct to the file. It is important to bare in mind that the I/O requests from the RDBMS do not go via the ASM instance, but straight to the devices by using the extent map.
The ASM instance only looks after the metadata that describes diskgroups and extent allocations. It would not be efficient for the RDBMS to have to send it’s I/O via the ASM instance and this does not happen.