Oracle Database Architecture

   Introduction to Oracle Database

The purpose of a database is to store and retrieve information. In general, a  server reliably manages a large amount of data in a multiuser environment so that many users can concurrently access the same data. A database server also prevents unauthorized access and provides efficient solutions for failure recovery.

Oracle Database is the first database designed for enterprise grid computing, the most flexible and cost effective way to manage information and applications. With this architecture, each new system can be rapidly provisioned from the pool of components. There is no need for peak workloads, because capacity can be easily added or reallocated from the resource pools as needed.

The database has logical structures and physical structures. Because the physical and logical structures are separate, the physical storage of data can be managed without affecting the access to logical storage structures.

 

Oracle Database Architecture

There are two main components of Oracle database –– instance and database itself. An instance consists of some memory structures and the background processes, whereas a database refers to the disk resources.

 There are two basic memory structures on the Oracle instance.

        - SGA (System Global Area)

        - PGA (Program Global Area)  

  SGA (System Global Area):

The Oracle SGA is the most important memory structure in Oracle. The SGA stores several different components of memory usage that are designed to execute processes to obtain data for user queries as quickly as possible while also maximizing the number of concurrent users that can access the Oracle instance. Main Components in SGA.

        - Shared Pool

        - Buffer Cache

        - Redo Log Buffer

        - Large Pool

        - Java Pool

Shared Pool:

The shared SQL library cache is designed to store parse information for SQL statements executing against the database. Parse information includes the set of database operations that the SQL execution mechanism will perform in order to obtain data requested by the user processes. If another user process run the same query that Oracle has already parsed for another user, the database will recognize the opportunity for reuse and let the user process utilize the parse information already available in the shared pool.

The other mandatory component of the shared pool is the data dictionary cache(row cache). This memory structure is designed to store the data from the Oracle data dictionary in order to improve response time on data dictionary queries.

Buffer cache:

The database buffer cache is the portion of the SGA that holds copies of data blocks read from datafiles. All users concurrently connected to the instance share access to the database buffer cache.

Redo log buffer :

One fact that is important to remember about redo logs and user processes is that every process that makes a change to the database must write an entry to the redo log in order to allow Oracle to recover the change. When the database is set up to archive redo logs, these database changes are kept in order to rebuild database objects in the event of a disk failure. Since every data change process has to write a redo log entry, it makes sense that processes be able to write that change as quickly as possible in order to boost speed and avoid problems.

Large pool :

It is an optional area of memory in the SGA. It is used to relieves the burden place on the shared pool. It is also used for I/O processes. The large pool size can be set by LARGE_POOL_SIZE parameter in init.ora initialization parameter file.

Java pool :           

It is used to services parsing of the Java commands. Its size can be set by JAVA_POOL_SIZE parameter in init.ora initialization parameter file.

Streams Pool :

The streams pool is used exclusively by Oracle Streams. The Streams pool stores buffered queue messages, and it provides memory for Oracle Streams capture processes and apply processes.

 

PGA (Program Global Area) :

Oracle Database allocates a program global area (PGA) for each server process. The PGA is used to process SQL statements and to hold logon and other session information. For the purposes of memory management, the collection of all PGAs is known as the instance PGA. Using an initialization parameter, you set the size of the instance PGA, and the database distributes memory to individual PGAs as needed.

 

Major Oracle Database’s background processes

The following are the major background processes of an Oracle instance:

PMON : (Process Monitor)

The process monitor that regulates all other processes. PMON cleans up abnormally connected database connections and automatically registers a database instance with the listener process. PMON is a process that must be alive in an Oracle database.

SMON: (System Monitor)

The system monitor process that performs system-level clean-up operation. It has two primary responsibilities including automatically instance recovery in the event of a failed instance, e.g., power failure and cleaning up of temporary files.

DBWn:(Database Writers)

Oracle performs every operation in memory instead of the disk because processing in memory is faster and more efficient than on disk. The DBWn process reads data from disk and writes it back to the disk. An Oracle instance has many database writers DBW0, DBW1, DBW2, and so on.

CKPT:(CheckPoint)

Data that is on disk is called block and the data which in memory is called buffer. When a block is written to the buffer and changed, the buffer becomes dirty, and it needs to be written down to the disk. The CKPT process updates the control and data file headers with checkpoint information and signals writing of dirty buffers to disk.

LGWR: (Log Writer)

The Process which is the key to the recoverability architecture. Every change occurs in the database is written out to a file called redo log for recovery purposes. And these changes are written and logged by LGWR process. The LGWR process first writes the changes to memory and then disk as redo logs which then can be used for recovery.

ARCn :(Archiver Process)

 The archiver process that copies the content of redo logs to archive redo log files. The archiver process can have multiple processes such as ARC0, ARC1, and ARC3 and so on.