InnoDB后台线程

　　通过命令show engine innodb status来观察InnoDB中的IO hread：

mysql> show engine innodb status\G

*************************** 1. row ***************************
  Type: InnoDB
  Name:
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, 0] , aio writes: 0 [0, 0, 0, 0] ,
 ibuf aio reads: 0, log i/o‘s: 0, sync i/o‘s: 0
Pending flushes (fsync) log: 0; buffer pool: 0
425008825 OS file reads, 356489210 OS file writes, 265709650 OS fsyncs
0.05 reads/s, 16384 avg bytes/read, 100.60 writes/s, 62.48 fsyncs/s
-------------------------------------
INSERT BUFFER AND ADAPTIVE HASH INDEX
-------------------------------------

　　其中I/O thread 0 为 insert buffer thread。I/O thread 1 为 log thread 。后面就是根据参数innodb_read_io_threads 及 innodb_write_io_threads 来设置的读写线程。并且读线程的ID总是小于写线程ID。

　　3、Purge Thread

　　InnoDB 1.2版本 purge操作可以独立到单独线程进行，来减轻Master Thread的工作，提高CPU的使用率及提升存储引擎性能。还可以支持多个 Purge Thread.可以加快 undo页回收。同时 Purge Thread 需要离散地读取 undo 页，这样能更进一步利用磁盘的随机读取性能。

mysql> show variables like ‘innodb_purge%‘\G
*************************** 1. row ***************************
Variable_name: innodb_purge_batch_size
        Value: 300
*************************** 2. row ***************************
Variable_name: innodb_purge_threads
        Value: 1
2 rows in set (0.01 sec)

　　其中参数  innodb_purge_threads 为开启的Purge Thread数量，innodb_purge_batch_size 为 设置每次purge清理的undo页数，如果值太大则会导致CPU和磁盘IO过于集中（参考：https://www.cnblogs.com/suolu/p/6634592.html）

　　4、Page Cleaner Thread

　　Page Cleaner Thread 是在InnoDB 1.2.x 版本中引入的，作用是将之前版本中脏页的刷新操作都放入到单独的线程中来完成。以减轻原Master Thread 的工作及对用户查询线程的阻塞，进一步提高 InnoDB 存储引擎的性能。

InnoDB后台线程

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