System Global Area is the sum of Shared pool, DB buffers and log buffers
It is defined by shared_pool_size which is a part of SGA (System Global Area)
It contains DBCACHE and LIBCACHE and they are nothing but parsed SQL Statements, Authorizations, Table and Field definitions and packages.
Each query/ request should pass through this area...
DB BUFFER : Used to store the content that is accessed from the database. It is used to reduce the hits on the database there by reducing the load on the database. Its quality is measured by database buffer hit ratio. In general for an optimized database the ratio should be not less than 94%.
From ST04 - [Logical Reads - Physical Reads/ Logical Reads] * 100
i.e. Out of 100 request 6 requests goes to the database.
DB Buffer is used to store the data until its size is full (or) the content gets invalidated by a change or least recently used. It is defined by a parameter db_block_buffers.
SQL> select name, bytes/1024/1034 "MB" from V$sgastat where pool....
Its size is defined by multiplying by 8kb because each block size is 8 kb.
SQL> show SGA
LOG BUFFER: It is used to perform the data manipulations by database work process. Every request that needs modification/ update / delete has to be performed only in this area. It is defined by the parameter log_buffer generally its size varies between 1MB to 4MB. It should not be too big or too small.
SYSTEM/ SHARED GLOBAL AREA: It is a memory area which resides purely on memory and it is lost when the system is stopped.
SGA is lost the complete cache information is lost. It takes time to build SGA when system is started that is why the system is slow after a restart.
1. Shadow Process
2. Background Process/ Server Process
These are used to handle the user requests. They perform the task assigned by an R/3 WP. They called as shadow processes because they perform the task non-interactively. These are also called as User processes (In Oracle Terms)
The process are established at the rate of 1:1 during the startup of SAP system.
Background/ Server Process
These are non-interactive to R/3 Processes. These are initiated by system based on requirement.
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