oracle体系结构深入学习Buffer Cache

oracle体系结构深入学习Buffer Cache - Overview
oracle 2009-09-02 23:13:26 阅读40 评论0 字号：大中小
涛涛深入学习oracle体系结构
我发觉，英文的一些技术文章在描述oracle原理非常的恰当到位，表达的非常的美，呵呵。英文不好，献丑翻译并学习了，认真看了，相信你受益匪浅。
oracle体系结构之：Buffer Cache - Overview
All Oracle data is obtained by users from the buffer cache. The basic purpose of the cache is to minimize physical
disk I/O by holding (buffering) copies of requested data blocks in memory. Data that is buffered in the cache can
be served to users at memory access speed, much faster than going to disk for it. The large performance penalty of
disk I/O makes tuning the buffer cache an extremely important task for the DBA.
Oracle data is stored on disk in identically sized units called blocks. Block size is determined at database creation
 by the DB_BLOCK_SIZE initialization parameter. The buffer cache is also divided into buffers of this same size, and
 each buffer can hold exactly one database block. Thus, the block is the basic unit of data transfer between disk and
  memory in Oracle.
 
 所有的oracle数据获取，都是用户通过高速数据缓存取得的，最基本的目的就是为了减少物理磁盘的I/O操作通过把请求的数据保存在内存当中。
 被缓存在高速缓冲区的数据能被用户快速的从内存中获取到，那比从磁盘去取得数据要快很多。磁盘的I/O所消耗的巨大的性能代价，这使得调优
 数据库高速缓存成为dba一个非常明确的重要任务。
 
 oracle数据被存储在磁盘中等同大小的单元中，这些单元被叫做块；块大小在数据库被创建时通过初始参数db_block_size来定义。数据高速缓存
 也被划分成同样大小的缓存块。并且每个缓存块持有一个数据库块，因此块是基本的数据传送单元在数据文件和内存之间。
 
 oracle数据库高速缓存之Cache Hit Ratio
The buffer cache is a shared resource, accessible by all users. When a user process requests data, Oracle firsts looks
 for that data's block in the cache. If the data is buffered in the cache it is returned to the requ