Database Reference
In-Depth Information
SGA.
Whenever the word SGA comes, imagine as if we are
talking about a big sheet of paper for rough work or rough
calculations and whenever the word “process” comes, imagine as
if we are talking about a “live” person with a assigned job
or in
other words person who has being assigned a specific job.
The difference between “process” (e.g. Oracle Background
Process like Dispatcher, Server Process, DBWR etc.) and
“software application” (e.g. like MS Word, Excel etc.) is that
you can or users can interact with the “software application”
whereas a process is also a software but it has a pre-assigned task
set by the “process developer” or computer programmer.
Software process never allows users to interact and more
precisely certain environmental events result in the cause of its
execution. It re-executes if that event happens gain or terminates
itself after executing once just like in the movie “Mission
Impossible”. The mission is described to the team through a
“system” which after completion of mission description destroys
itself. It all depends on the programmer who designed or wrote
the software code for the process. Similarly, Oracle background
processes have been assigned a specific job. Each process gets
executed under certain circumstances. If those circumstances
happen again, the process gets executed again.
DBWn
DBWn or Database Writer process writes the contents of dirty
buffers of Database Buffer Cache to the data files. When you
start the database, Oracle starts one Database Writer process
(DBW0). For situations where we have multiple users accessing
the Oracle system, we can add nine more (DBW1 to DBW9)
Database Writer processes. DBWn writes the contents of Dirty
buffers of the Database Buffer Cache to the Data files under
following circumstances or events.
When the checkpoint occurs. Checkpoint occurs periodically or
when “log switch” occurs. A log switch occurs when Oracle
finishes writing one redo log file and moves to the next one.
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