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Oracle 11g等待事件：db file async I/O submit

Jun 07, 2016 pm 05:34 PM

Oracle 11g等待事件：db file async I/O submit

SQL> select * from v$version;

BANNER
--------------------------------------------------------------------------------
Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit Production
PL/SQL Release 11.2.0.1.0 - Production
CORE 11.2.0.1.0 Production
TNS for Linux: Version 11.2.0.1.0 - Production
NLSRTL Version 11.2.0.1.0 - Production

有时候出现后台等待事件：db file async I/O submit

该等待跟oracle的异步io有关。

与oracle异步io相关的参数有2个：

SQL> show parameter filesystem

NAME TYPE VALUE
------------------------------------ -------------------------------- ------------------------------
filesystemio_options string none
SQL> show parameter disk_asynch

NAME TYPE VALUE
------------------------------------ -------------------------------- ------------------------------
disk_asynch_io boolean TRUE

disk_asynch_io 这个参数默认是true，，但是filesystemio_options默认是none。这种情况会出现db file async I/O submit等待事件。

如果不想这个等待事件出现，可以采取两种方法：

1：alter system set disk_asynch_io=false scope=spfile; 关闭异步io

2：alter system set filesystemio_options=asynch scope=spfile;

如果想开启oracle的异步io，除了设置上面的两个参数为true和asynch以外，还得操作系统支持异步io才行。

检查linux是否允许异步io：

另一个是：trace –p

做dbwr进程的跟踪，如果操作是pwrite，那么是同步io，如果是io_getevents，那么是异步io。

下面是参数的解释：

FILESYSTEMIO_OPTIONS可以设置的值如下：

ASYNCH: enable asynchronous I/O on file system files, which has no timing requirement for transmission.

DIRECTIO: enable direct I/O on file system files, which bypasses the buffer cache.

SETALL: enable both asynchronous and direct I/O on file system files.

NONE: disable both asynchronous and direct I/O on file system files.

看下面的表格就一目了然了：

disk_asynch_io filesystemio_options strace DBWR AIO DBWR waits

FALSE NONE pwrite64 NO db file parallel write

FALSE ASYNCH pwrite64 NO db file parallel write

TRUE ASYNCH io_submit/io_getevents YES db file parallel write

TRUE NONE pwrite64 NO db file async I/O submit

另外，如果disk_asynch_io=false 的话，可以设置参数dbwr_io_slaves为一个大于0的值来模仿异步io

查看数据库中文件的异步io情况：

SQL> select file_no,filetype_name,asynch_io from v$iostat_file;

FILE_NO FILETYPE_NAME ASYNCH_IO
---------- ---------------------------- ---------
0 Other ASYNC_OFF
0 Control File ASYNC_OFF
0 Log File ASYNC_OFF
0 Archive Log ASYNC_OFF
0 Data File Backup ASYNC_OFF
0 Data File Incremental Backup ASYNC_OFF
0 Archive Log Backup ASYNC_OFF
0 Data File Copy ASYNC_OFF
0 Flashback Log ASYNC_OFF
0 Data Pump Dump File ASYNC_OFF
1 Data File ASYNC_OFF
2 Data File ASYNC_OFF
3 Data File ASYNC_OFF
4 Data File ASYNC_OFF
5 Data File ASYNC_OFF
6 Data File ASYNC_OFF