正确理解innodb参数innodb_thread_concurrency

    看了这篇文章，曾经一度对innodb_thread_concurrency这个概念产生了混淆，先来回顾一下当时自己的错误理解：

首先查看了系统的信息，

innodb_thread_concurrency               | 8

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FILE I/O

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I/O thread 0 (thread id: 18578) state: waiting for completed aio requests (insert buffer thread)

I/O thread 1 (thread id: 18579) state: waiting for completed aio requests (log thread)

I/O thread 2 (thread id: 18580) state: waiting for completed aio requests (read thread)

I/O thread 3 (thread id: 18581) state: waiting for completed aio requests (read thread)

I/O thread 4 (thread id: 18582) state: waiting for completed aio requests (read thread)

I/O thread 5 (thread id: 18583) state: waiting for completed aio requests (read thread)

I/O thread 6 (thread id: 18584) state: waiting for completed aio requests (write thread)

I/O thread 7 (thread id: 18585) state: waiting for completed aio requests (write thread)

I/O thread 8 (thread id: 18586) state: waiting for completed aio requests (write thread)

I/O thread 9 (thread id: 18587) state: waiting for completed aio requests (write thread)

用show engine innodb status出来的结果就有10 IO个线程，而innodb_thread_concurrency却为8。

这是为什么呢？原来这个innodb_thread_concurrency指的是mysql调用innodb的线程最大并发数，大家知道由于mysql特有的插件式存储引擎架构，先上mysql架构图：

    所以上层要使用innodb存储引擎的时候，肯定是mysql开启线程去调用innodb存储引擎进行相关工作，所以这里的innodb_thread_concurrency指的就是innodb接受mysql线程调用的最大线程数，而不是在innodb内部工作的线程数。之前用show engine innodb status显示的线程就是innodb内部的线程。

    OK，问题解决了，顺便还提一句，还有一个参数thread_concurrency 这个代表的什么呢?

手册上有解释：This variable is specific to Solaris systems，而且在mysql5.6.1中就被废弃了