Introduction to Oracle architecture instances

If you are not familiar with the basic concepts and instances in the Oracle architecture, the following articles describe the related content of the instance, in fact, the basic concept instance in the Oracle architecture is a common means for Oracle database operations.

Basic concepts of Oracle architecture, instances

In general, instances are a means of operating Oracle databases.

A database instance is also called a server. It is used to access the storage structure of the database file set and the collection of background processes.

A database can be accessed by multiple instances (called the true application cluster option ).

Determines the size and composition of the instance. ora initialization file, or hidden in the server parameter file in the database. this file is referenced through spfile, which is stored in spfile. ora file.

When the instance starts, it reads the initialization file. The database system administrator can modify the file. Modifications to the initialization file are only valid at the next startup.

Instances are divided into two parts:

1: memory structure (memory structure)

Memory structure is divided into two parts: the SGA (System Global Area) Area is the memory Area used to store database information, which is shared by the database process. It contains data and control information of the Oracle server,

It is allocated in the actual memory of the computer where the Oracle server resides. If the actual memory is insufficient, it is written into the virtual memory.

Including:

 
 

  
  
.share pool  
  
  
.datafase buffer cache  
  
  
.redo log buffer  
  
  
.other structures  
 
 

The Program Global Area contains the data and control information of a single server process or a single background process. The SGA shared with several processes is the opposite. PGA is only used by one process, PGA is allocated during Process Creation and recycled upon Process Termination

2: background process)

Including:. PMON is responsible for clearing resources when an Oracle process fails.

. SMON checks Database Consistency and starts database recovery when the database is opened if necessary

. DBWR is responsible for writing changed data from the database buffer cache to the data file

. LGWR writes changes in the redo log buffer to the online redo log file.

. CKPT updates the database status information in control files and data files whenever changes in the buffer cache are permanently recorded in the database.