Oracle architecture-databases, tablespaces, and instances

I. Databases

A database is a collection of data, while Oracle is a software system that manages these data sets. It is an object-relational database management system.

 

Ii. tablespace

A tablespace is the logical ing of ORACLE data in a physical database. A database is logically divided into several tablespaces, each of which contains a logically associated structure, each database has at least one tablespace (called the system tablespace ).

Each tablespace consists of one or more files on the same disk. These files are called data files. A data file can belong to only one tablespace.

The relationships between databases, tablespaces, and data files are shown in:

The figure above shows the following points:

1. Each database is logically divided into one or more tablespaces;

2. Each tablespace consists of one or more data files;

3. The storage size of each tablespace is equal to the total size of all its data files;

4. The storage size of each database is equal to the total size of all its tablespaces;

5. A data file can belong to only one tablespace;

6. A tablespace can belong to only one database;

 

Iii. Instances

Each running database corresponds to an oracle instance. When the Oracle database on the server is started, Oracle first allocates an area in the memory-system global area (SGA ), start one or more Oracle processes.

Together with these processes, SGA is called an oracle instance. The memory and Process Manager in the instance are used to efficiently access database data and provide services to one or more users.

A database can be opened by multiple instances, but one instance can only open one database at any time. Multiple instances can run on the same machine at the same time and access each other's independent physical
Database. (If a database is opened by multiple instances at the same time, multiple users can access the tables in the database, in this case, will Data Access and operations also use the lock mechanism ?). After the instance is started, Oracle associates the instance with its corresponding physical database. This process is called "mounting ".
When the database is ready to be opened, you can operate the database after it is opened (you can operate the database after it is not loaded, but you can also open this step ).

 

 

The instance is a memory plus background process, and the tablespace is used to store data files. It is a logical concept.