mysql-5.7 Using Asynchronous I/O on Linux

一、mysql - innodb 使用异步IO的场景

　　总的来说innodb 只会对数据文件采用异步IO，为了保存日志是真正被写入到磁盘，innodb不会对日志文件启用异步IO

　　更新细一步的说，innodb只会对数据文件的read-ahead ,write 这两个操作启用异步IO

二、异步IO在mysql中解决了什么问题：

　　在没有IO这个功能之前，innodb对数据的读写请求先放入任务队列，后台read-thread ，write-thread从任务队列中

　　拿出任务并执行读写操作；后台读写线程的个数可以通过show engine innodb status 语句来查看

show engine innodb status;

--------
FILE I/O
--------
I/O thread 0 state: waiting for i/o request (insert buffer thread)
I/O thread 1 state: waiting for i/o request (log thread)
I/O thread 2 state: waiting for i/o request (read thread)
I/O thread 3 state: waiting for i/o request (read thread)
I/O thread 4 state: waiting for i/o request (read thread)
I/O thread 5 state: waiting for i/o request (read thread)
I/O thread 6 state: waiting for i/o request (write thread)
I/O thread 7 state: waiting for i/o request (write thread)
I/O thread 8 state: waiting for i/o request (write thread)
I/O thread 9 state: waiting for i/o request (write thread)
Pending normal aio reads: [0, 0, 0, 0] , aio writes: [0, 0, 0, 0] ,
 ibuf aio reads:, log i/o‘s:, sync i/o‘s:
Pending flushes (fsync) log: 0; buffer pool: 0
376 OS file reads, 54 OS file writes, 7 OS fsyncs
41.77 reads/s, 21886 avg bytes/read, 6.00 writes/s, 0.78 fsyncs/s

　　由上面的内容可以看出默认情况下innodb会有4个read-thread 和4个write-thread ，问题就出现在这里，如果业务繁重，那么

　　读写任务就会非常的多，而innodb只有八个读写线程，这样的话队列中的其它读写请求就没能得到及时的响应。

　　引入异步IO之后就变天啦，读写请求不再像之前那样先放入队列，等着后台的读写线程去执行任务；

----