How to create an Oracle instance?

Before you know how to create an Oracle instance, you must understand the instance definition. When talking about Oracle databases, we are likely to refer to the entire Oracle Database Management System DBMS). But as an Oracle database personnel, we must recognize the major difference between databases and instances-a difference that often confuses non-Oracle system administrators.

To provide services, flexibility, and performance that Oracle customers expect, many of the work of the database is done by the instance. The instance is a series of complex memory structures and operating system processes. Unless you use the parallel Oracle server option, each Oracle database has an instance associated with it, and a database is loaded by a unique instance. The instance structure allows RDBMS to provide services for requests from different types of transactions from multiple users at the same time, providing first-class performance, fault tolerance, data consistency and security.

It is worth noting that the term "process" defined here refers to a task that is running without user intervention. Your operating system may call it a "process" or use other terms, such as tasks, jobs, threads, and other similar terms.

After UNIX implements a multi-task operating system, instances are loosely structured. A discrete process that works together completes the specified task in the RDBMS that achieves the target of the instance. Each process has its own memory block used to store private variables, address stacks, and other runtime information. Use a public shared area between processes and complete their work in the public shared area. A public shared area is a memory area that can be read and written by different programs and processes at the same time. This memory block is called the system global zone SGA ). Because SGA resides in a shared memory segment, it is often called a shared global zone.

You can think of backend processes as database workers and directly process database components. You can also think that SGA is like a brain, where a local dispatcher can process their information and storage and retrieval as necessary. SGA participates in the processing of all information in the database and the server. A single user's Oracle configuration, such as personalpoliclelite), does not use multi-process execution of the database function. On the contrary, all database functions are completed by an Oracle process. For this reason, a single user is also called a single process Oracle.

After learning about the basic definition of an instance, let's take a look at how to create an instance. To open an Oracle database, follow these steps:
1) Create an Oracle instance not in the installation phase ).
2) install the database by the instance ).
3) Open the database open stage ).

An Oracle instance is created in the non-installation phase of the database startup. When the database is not installed, read the init. ora parameter file, start the background process, and initialize the system global zone SGA ). The init. ora file defines the instance configuration, including the size of the memory structure and the number and type of background processes. The instance name is set according to the environment variable OracleSID. It does not have to be the same as the name of the opened database, but it is usually the same ).

The next stage is called the installation stage. The parameter value of the control file in the init. ora file determines the database installation instance. In the installation phase, you can read the control file and make it accessible. You can query and modify the data stored in the control file.

The final stage is to open the database. At this stage, the database file whose name is stored in the control file is locked by the instance in order to make the database accessible to common users. Open is the normal operation status of the database. Before the database is opened, only the DBA can access the database and access the database only through the Server Manager.

To change the database operation status, you must connect to the database internally or have the SYSDBA privilege. When the database is in the closed state to the open state, you can clearly debug each operation state of the database in a single step. However, when the database is closed, it can only be switched from the current running state to the fully closed state. For example, you can run the startup nomount command in the Server Manager tool, which puts the database in a non-installation stage, next, you can run alter database mount or the alter database open command to debug the operation in one step. No matter what the operation status is, If you execute the SHUTDOWN command, the database will be completely shut down. For example, the database cannot be switched from the open status to the installation status.

An instance without a database installed is called idle-it uses memory, but does not work. One instance can only be connected to one database, and only one instance is allocated to one database unless a parallel server is used. An instance is the core of data management-it does all the work, and the database stores all the data.

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