>- process Pnnn works as a proxy on behalf of your pogram > - reads and writes ONLY to the System Global Area SGA) > - makes requests of the DB Writer, etc. to fill/flush the SGA
Pnnn processes are PQO (parallel query option) slaves and as such, are background processes. Unless MTS is in play, client sessions are serviced by an instance foreground process (shadow processes).
PQO slaves are not limited to "ONLY" SGA buffered IO. To the contrary, they spend the highest majority of their time performing direct path read and writes which are multi-block in size and buffered in the PGA. About the only task PQO slaves perform using SGA buffers is an index range scan.
> >- the DBWR reads and writes the files in the database area. > - neither your program, nor proxy Pnnn, writes to the disk.
DBWR does not perform reads. I have no idea what the term "proxy Pnnn" means since that is most certainly absent from the Concepts Guide. Again, Pnnn processes are PQO slaves. A user session in the server is served by either a shadow process (called two-task in the old days) or an MTS server/dispatcher pair. Either way, BOTH shadow processes and MTS server processes perform BOTH reads and writes to datafiles. They beside the typical db file sequential and scattered reads (which are buffered in the SGA), they also perform direct path read and writes. forground (shadow) process direct path writes are quite common for such tasks as spilling sort overflow.
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