InnoDB 存储引擎体系架构

1. Master Thread

这是最核心的一个线程,主要负责将缓冲池中的数据异步刷新到磁盘,保证数据的一致性,包括赃页的刷新、合并插入缓冲、UNDO 页的回收等.

2. IO Thread

在 InnoDB 存储引擎中大量使用了异步 IO 来处理写 IO 请求, IO Thread 的工作主要是负责这些 IO 请求的回调.

可以通过命令来观察 InnoDB 中的 IO Thread:

mysql> SHOW ENGINE INNODB STATUS\G
*************************** 1. row ***************************
Type: InnoDB
Name:
Status:
=====================================
2016-03-23 20:19:53 0x700000d51000 INNODB MONITOR OUTPUT
….
…
——–
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)
……
—————————-
END OF INNODB MONITOR OUTPUT

可以看到, InnoDB 共有10个 IO Thread, 分别是 4个 write、4个 read、1个 insert buffer和1个 log thread.

3. Perge Thread

事物被提交之后, undo log 可能不再需要,因此需要 Purge Thread 来回收已经使用比分配的 undo页. InnoDB 支持多个 Purge Thread, 这样做可以加快 undo 页的回收
InnoDB 引擎默认设置为4个 Purge Thread

mysql> SHOW VARIABLES LIKE “innodb_purge_threads”\G
*************************** 1. row ***************************
Variable_name: innodb_purge_threads
Value: 4
1 row in set (0.00 sec)