CIO 并发IO / DIO 直接IO

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CIO/DIO

直接 I/O(DIO)由于可以绕过在文件系统级别进行高速缓存,从而改进内存性能。此过程可减少CPU开销并使得更多的内存可用于数据库实例。并发I/O(CIO)具有DIO的优点,并且还可以消除串行化写访问权。与使用文件系统缓冲I/O相比,在具有大量事务处理工作负载和回滚时CIO/DIO机制可增大吞吐量。

DIO和CIO在HP-UX、Solaris、Linux和Windows操作系统最新版本上都受支持。

关键字NO FILE SYSTEM CACHING和FILE SYSTEM CACHING是CREATE和ALTER TABLESPACESQL语句的一部分,允许您指定将对每个表空间使用DIO还是CIO。当 NO FILE SYSTEM CACHING有效时,只要可能,数据库管理器都会尝试使用"并发 I/O"。在不支持CIO的情况下(例如,当使用了JFS 时),将取而代之使用DIO。

建议在表空间级别启用或禁用UNIX、Linux和Windows上的非缓冲I/O。这将允许您在特定表空间上启用或禁用非缓冲I/O,同时避免数据库的物理布局中的任何依赖性。它还允许数据库管理器确定每个文件最适合使用哪种I/O,缓冲的还是非缓冲的。

NO FILE SYSTEM CACHING子句用于启用非缓冲I/O,从而禁用特定表空间的文件高速缓存。一旦启用了非缓冲 I/O,数据库管理器就会根据平台自动确定将使用直接 I/O还是并发I/O。由于使用CIO可以提高性能,所以只要支持CIO,数据库管理器就会使用它。

FILE SYSTEM CACHING选项并不是总没有好处,例如当一个应用程序检索LOB或LONG数据时,这些大对象数据不能经过数据库缓冲池,每次应用程序需要其中一个页时,数据库管理器必须从磁盘对其进行直接读取。但是,如果LOB或LONG数据存储在SMS或DMS文件容器中,文件系统高速缓存可提供缓冲,因此也就改善了性能。

未在CREATE TABLESPACE语句或CREATE DATABASE命令中指定此属性时,数据库管理器将使用基于平台和文件系统类型的默认行为处理请求。
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