Download What are the components of instance?

Instance
An Oracle server is a database management system that provides an open,
comprehensive, integrated approach to information management
It consists of an Oracle instance and an Oracle database
 What is Oracle Instance?
An Oracle instance is a means to access an Oracle database
It always opens one and only one database
An Oracle instance consists of the shared memory structures and
background processes that
run the Oracle database.
 Instance Management
A) Instance memory structure
It consists of SGA, PGA and Oracle Background Process
SGA
The SGA is a shared memory area that contains data and control information for
the instance. SGA has several components:
Data Buffer Cache: It stores copies of data blocks that have been retrieved from
the data files which give great performance when you obtain and update data
Redo log Buffer: It records all the changes made to the database which is used for
recovery purpose
Large Pool: This is an optional area that is used for buffering large I/O requests
for various server processes.
Java Pool: This is required for all java specific session services such as parsing
requirements of java commands.
PGA
A program global area (PGA) is a memory area used by a single Oracle server
process. The PGA is used to process SQL statements and to hold logon and other
session information. The amount of PGA memory used and its content depends on
the instance configuration, that is, whether the instance is running in dedicated
server or shared server mode.
B) Oracle Background Process
Oracle creates a set of background processes for an instance that interact with each
other and with the operating system to manage memory structure.
Mandatory background processes:
DBWn, PMON, CKPT, LGWR, SMON
Optional background processes:
ARCn, LMDn, QMNn, CJQ0, LMON, RECO, Dnnn, LMS, Snnn, LCKn, Pnnn
C) Accessing the Database
The database instance can be started only by an authorized database administrator
who has a special type of connection privilege to the Oracle instance. After the
database instance has been started, it is usually open for access by normal users for
whom database user accounts have been created.
How to start Oracle instance?
The administrator who starts up the Oracle instance and database must connect to
the instance with a special kind of connection privilege.
The process of starting the instance and database is as follows:
When you start up a database, you create an instance of that database and you
determine the state of the database. Normally, you start up an instance by mounting
and opening the database. Doing so makes the database available for any valid user
to connect to and perform typical data access operations.
Connect to Oracle Database as SYSDBA:
CONNECT username/password AS SYSDBA
Use the STARTUP statement using SQL*Plus.
We can also start an instance using Oracle Enterprise Manager (OEM).
Below is the steps to start an instance using OEM.
There are three states of an Oracle Instance
1) NO-MOUNT
2) MOUNT
3) OPEN
How to shutdown Oracle instance?
It is reverse of startup process.
To shutdown an instance
Use the SHUTDOWN statement using SQL*Plus.
We can also start an instance using Oracle Enterprise Manager (OEM).
Below are the steps to shutdown an Oracle Instance using OEM
The Oracle instance dismounts the database and updates relevant entries in
the control file to record a clean shutdown. The control file is closed. The
database is now closed and only the instance remains.
The Oracle instance stops the background processes of the instance and
deallocates the shared memory used by the SGA.
Shutdown mode:
• A = ABORT
• I = IMMEDIATE
• T = TRANSACTIONAL
• N = NORMAL
STARTUP & SHUTDOWN USING SQL*PLUS
C:\> sqlplus sys/oracle as sysdba
• SQL> shutdown immediate
Database closed.
Database dismounted.
ORACLE instance shut down.
• SQL> startup
ORACLE instance started.
Total System Global Area 285212672 bytes
Fixed Size
1218472 bytes
Variable Size
250177624 bytes
Database Buffers
33554432 bytes
Redo Buffers
262144 bytes
Database mounted.
Database opened.
D) Initialization Parameters
Instance management involves configuring parameters that affect the basic
operation of the database instance. These parameters are called initialization
parameters. The Oracle database server reads these parameters at database startup
and monitors them while the database is running. They are stored in memory,
where many of them can be changed dynamically.
These parameters are called initialization parameters. The Oracle instance reads
initialization parameters from a file at startup.
There are two types of parameter files, and whether these dynamic changes are
persistent across database shutdown and startup depends upon the type of
parameter file you are using.
SERVERPARAMETER FILES (SPFILE): The server parameter file is the
preferred form of initialization parameter file, and is a binary file that can be
written to and read by the database. It must not be edited manually. It resides on
the server side.
PARAMETR FILES (PFILE): In Pfile text file can be edited using a text editor.
When changes are made to the PFILE, the instance must be shut down and
restarted before it takes effect.
What are the components of instance?
The Systems Global Area (SGA) and shared pool are memory structures. The
process monitor is a background process (DBWn, LGWR, ARCn, PMON, and
other process.).
What are the Types of Process in Instance?
An Oracle Instance consists of User Process, SGA and PGA.
Below figure consists of types of Process in Instance.