Oracle Architecture

                               
Overview of Primary Components
The Oracle architecture includes  many primary components, which are discussed below.
 
•Oracle instance: An Oracle instance is the combination of the background processes and the memory structures. The instance must be started to access the database. When instance started, a System Global Area (SGA) is allocated and Oracle background processes are started.  
•Oracle database: An Oracle database consists of operating system files, also called database files, that provide the actual physical storage for database information. The database files are used to ensure that the data is kept consistent and can be recovered in the event of a failure of the instance.
•User and server processes: The user and server processes are the primary processes which involved when a SQL statement is executed, however, other processes can also help the server complete the processing of the SQL statement.

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PCTFree & PCTUsed

PCTFree:
It is used to mention how much space should be Free in the Data Block for future update:

PCTUsed:
Its is used to mention that how much Data Block should be filled before any new insertion. This parameter worked when the data block reached PCT Free threshold and new insertion stop filling and then this block will start insertion when data block get empty until reached PCTUsed limit.

Lets suppose you have specified PCTFree as 10 % and PCTUsed as 20 %.

• Oracle will keep inserting new rows until the space is reached 90 %. It will reserve the remaining 10%(PCTFree) for future updates.
• To start adding new rows again to the data block, Oracle will check the space in the Data Block and the PCTUsed parameter.
• When the Data Block space falls below 20 %(PCTUsed, Oracle will start adding new rows to the Data Block.