oracle directory permssions and listener 2004-10-12 - By Kevin Closson
Some corrections are needed here.
>- 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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