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I. oracle architecture

The architecture of Oracle is the composition and working process of the database, as well as the organization and management mechanism of the data in the database. In order to understand the architecture of oracle database, we must understand the main components and important concepts of oracle. The Oracle architecture consists of a series of components, including instances, user processes, service processes, data files, and other files, such as parameter files, password files, and archive log files. As shown in the following figure:

Instance and database are not only the core components of oracle database architecture, but also the two most important concepts. The main work of dba is to maintain instances and databases.

1) instance

An instance is a collection of background processes and memory, and you must start the instance to access data in the database.

When n Oracle starts, a system SGA is assigned and a series of oracle background processes are started

N an instance can only open and use one database

N-Oracle is divided into single-process instances and multi-process instances, which are generally multi-process instances, and multi-processes are multi-users. Different users use different processes to execute different parts of oracle.

2) Database

A database is a collection of data that is treated as a logical unit.

The Oracle database consists of operating system files that provide actual physical storage for database information.

Oracle database includes logical structure and physical structure.

Physical structure: a set of operating system files that store database information, each database has a logical structure and a physical structure. The physical structure refers to a set of operating system files that make up the database, which is mainly composed of three types of files: data files, control files and redo log files.

Logical structure: refers to the logical storage structure of database data, including: table space, data segment, table, view and so on.

2. Oracle storage structure

The storage structure of Oracle is divided into physical structure and logical structure, which are independent and related to each other.

three。

1. Physical structure:

The physical structure is the operating system physical files used after the creation of the oracle database, which can be divided into the following two types:

1) main document

N data file: a data file is a file that physically stores a database. The characteristics of a data file are that each data file is associated with only one database, and a table space can contain one or more data files. A data file can only belong to one table space. The data file contains the user or application data of the database, as well as metadata and data dictionaries.

N redo log file: it records all changes to the data and provides a data recovery mechanism if the database server crashes but no data files are lost. then the instance can use the information in these files to restore the database.

N control file: when the database is read, the data file should be found according to the information of the control file. The control file stores the name and location of the data file and redo log file. A database has at least two control files. Oracle11g includes 3 control files by default, and each control file has the same content, which can avoid starting the database because of the damage of one control file. The control file records the following key information: the location and size of the data file, the location and size of the redo log file, the database name and creation time, the log sequence number. Control files are critical to the database. Without these files, you cannot open the data file to access the data in the database.

2) other files

N parameter file: used to define the configuration when the instance is started

N password file: allows sysdba, sysoper, and sysasm to remotely connect to the instance and perform administrative tasks

N Archive log files: using these files and database backups, you can recover lost data files. That is, the archive log can recover the restored data files.

2. Oracle logical structure

The logical components of Oracle include table spaces, segments, extents, blocks, schemas, and so on.

1) tablespace

N tablespaces are the largest logical units in the database, SYSTEM and SYSAUX tablespaces are necessary tablespaces that must be created when the database is created, and there will be TEMP,USERS and so on.

N each tablespace consists of one or more data files, and a data file can only be associated with one tablespace.

The size of the n tablespace is equal to the sum of all the data file sizes that make up the tablespace.

N table space and data file is corresponding, if there is no data file, table space can not exist independently, if only table space without data file is not good, just like if the computer does not have a hard disk, you can no longer logically see C disk D disk, here the hard disk is equivalent to data file, and C disk is equivalent to tablespace. We can make multiple data files into a table space, and then store data in the table space, the table space and the data file are corresponding, a table space can be composed of one or more data files, but a data file can only be in one table space.

The role of tablespaces:

N assign different tablespaces to different users and different schema objects to facilitate the management of user operations and schema objects.

N different data files can be created on different disks, which helps to manage disk space, improve iUnix performance, backup and restore data, etc.

N SYSTEM tablespace: data used to store tables and data dictionaries within the oracle system, such as indication, column name, user name, etc.

N SYSAUX table space: as an auxiliary table space of SYSTEM, it is used to store data used by various database tools (such as enterprise manager, StasPack, logMiner, etc.). It is used to store object data of various patterns, such as intelligent agent user DBSNMP, data mining user ODM and so on.

N USERS tablespace: as a tablespace used by users, various objects can be created on this tablespace, such as creating tables, indexes, and so on.

N TEMP tablespace: a special tablespace for storing temporary data. For example, when sorting is needed, the system temporarily stores sorted data in this tablespace. After sorting is completed, the space occupied by sorted data can be released, which is called temporary tablespace.

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