Relationship between Oracle instances and databases

Relationship between Oracle instances and databases

Databases are physical and physical storage of data. They consist of files stored on disks, including control files, data files, and redo log files;

The instance is logical and consists of the server shared memory and background processes, providing access to information in the database.

Oracle databases and instances:

An instance can only load and open one database during operation, but a database in RAC can be loaded and opened by many instances at the same time.

If the database is an island, the instance can be seen as a temporary floating bridge through the island. The two are linked through the pfile parameter file.

While DB2 can run multiple instances at the same time, and one instance can contain multiple databases. Different databases in the same instance are completely independent and have their own system cataloguing tables. A database only belongs to one instance.

Oracle_sid and instance_name in oracle:

Oracle_sid appears in the Oracle System as an environmental variable. When an oracle instance is started, the fork process on the operating system is created based on this oracle_sid. The names of background processes and parameter files are determined by oracle_sid.

The oracle database has an internal initialization parameter instance_name, which is used to indicate the name of the database instance. The default value is usually oracle_sid. However, the initialization parameters instance_name and oracle_sid can be different. Different instances can have the same instance_name.

An instance_name is also recorded in the database's internal view V $ INSTANCE, which is always consistent with oracle_sid. The default instance_name In the parameter file is dynamically obtained from the System View.

In addition to the Instance name, instance_name is also used to register with the listener when the listener is dynamically registered.

DB_NAME indicates the name of the database to be attached to the instance, which is related to a specific physical file. It is recorded in data files, log files, and control files.

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