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1.2 databases and instances

Database is a specific concept, it refers to the database provided by the manufacturer of the database program running and user database files created using this program and parameter file. These files are dead things. Through these programs we can't access data to achieve our purpose. Therefore leads to a instance of concepts.

Instance refers to processes started by a database program and run some memory allocated for the database structure of the system. Oracle memory structures called the SGA (System Global Area), which is composed of the following parts:

Database cache

Shared pool

Redo log buffer

Buffer pool

Fixed SGA

Appropriate to set the size of the structure in the SGA for database performance is absolutely essential. You can adjust the

Init.ora parameter file to adjust the system's SGA structures

Oracle Management database system through a set of background processes to run. This set of background processes are as follows:

The DBW0 database writer processes

Log writer process LGWR

SMON system monitor process

PMON process monitor process

REC0 recovery process

SNP0 snapshot process

QMN0 queue monitor processes

LCK0 locking process

CKPT checkpoint process

ARC0 archive process


For Oracle, and some processes are optional, such as the SNP0 process.

Above SGA and process to form the Oracle instance. Remote clients via Net8 (Oracle networking software)

And this instance to connect to remote database functionality.

1.2 Database and examples

The database is a concrete concept, it refers to the database
manufacturers provide the procedures and run the database files
and parameter files with this program created by the user.

These files are the fixed things. We cannot achieve the purpose of
data access though these programs, so it leads to the concrete
concept to illustrate the examples.

The example is a process started by some of the database program,
and some memory structure was allocated by system running the
database.

Oracle's memory structure was called SGA (System Global Area),
which was composed by following parts:
Database cache area
Shared pool
Redo log buffer
Large cache pool
Fixed SGA

In the SGA, the appropriately setting of the structure size was
absolutely essential to the performance of the database, that which
can be adjusted.

The init.ora parameter file was used to adjust the structure of SGA,
Oracle through a set of processes to manage and run the database
system.
This set of background processes as follows:
DBW0 database writer process
LGWR Log Writer process
SMON System Monitor Process
PMON Process Monitor Process
REC0 The recovery process
SNP0 snapshot process
QMN0 queue monitor process
LCK0 lock process
CKPT checkpoint process
ARC0 archiving process

For Oracle, some processes are optional, as SNP0 process.
SGA and process above was composed an example of Oracle.
Distal client can access the function of database through the
Net8 (Oracle network software) and this example was connected.

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