Oracle数据库主要有数据文件database和数据库实例instance组成。用户通过用户进程链接到server process。在数据库启动的时候，需
从《Oracle服务器体系结构概述》找了一张介绍Oracle体系结构图，用于总体的描述Oracle的体系结构
Oracle服务器体系结构概述
1 Oracle数据库主要有数据文件database和数据库实例instance组成。用户通过用户进程链接到server process。在数据库启动的时候，需要依赖于参数文件parameter file，，如果我们的数据库是归档模式的话，Oracle数据库会把 redo log files 写入到 archived log files，避免redo log files 被覆盖
2 instance 介绍
数据库启动的时候，会读取参数文件spfile，并且根据参数文件分配一个System globle area(SGA),每个Oracle数据会话连接都共享的内存，对于SGA又可以分为shared pool的共享池，存放一些数据字典缓存和通用的函数库。对于SGA中，比较重要的有 database buffer cache，redo log buffer， java pool， large pool等
对于PGA和SGA的分配：PGA：每当有一个tcp连接Oracle的话就会产生一个session会话，分配一个PGA。对于SGA：是所有会话session共享的内存，当Oracle数据库启动就会初始化分配一个共享内存区域，启动以后这个SGA的大小不会改变
2 database数据库文件： data files，control files，Redo Log files。对于数据文件data files 主要是用于存放业务数据的文件，对于control files 存放的是控制文件，Redo log files存放的是对数据库操作的记录，如果数据库发生异常关闭，则启动的时候会读取Redo log files对数据库进行修复
3 Oracle数据库的database和instance之间是通过一系列的后台进行进行通信。不同的操作有不同的后台进程进行处理
Oracle体系结构系列相关文章：
Oracle体系结构之SCN、实例恢复
Oracle体系结构之检查点
Oracle体系结构之SQL语句的执行过程
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