Innodb后台线程

1.maste thread

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。

2.IO Thread
负责IO请求的回调处理。
1.0版本之前有4个IO Thread，负责write、read、insert buffer和log IO Thread
1.0.x开始，read thread和write thread分别增加到4个，不再使用innodb_file_io_threads参数，而是使用innodb_read_io_threads和innodb_write_io_threads

mysql> show variables like 'innodb_version'\G
*************************** 1. row ***************************
Variable_name: innodb_version
        Value: 5.6.25
1 row in set (0.00 sec)

mysql> show variables like 'innodb_%io_threads'\G
*************************** 1. row ***************************
Variable_name: innodb_read_io_threads
        Value: 4
*************************** 2. row ***************************
Variable_name: innodb_write_io_threads
        Value: 4
2 rows in set (0.00 sec)

mysql> show engine innodb status\G
*************************** 1. row ***************************
  Type: InnoDB
  Name:
Status:
=====================================
2015-12-20 21:53:50 7fcaccfe8700 INNODB MONITOR OUTPUT
=====================================
Per second averages calculated from the last 11 seconds
-----------------
BACKGROUND THREAD
-----------------
srv_master_thread loops: 93 srv_active, 0 srv_shutdown, 26243 srv_idle
srv_master_thread log flush and writes: 26336
----------
SEMAPHORES
----------
OS WAIT ARRAY INFO: reservation count 393
OS WAIT ARRAY INFO: signal count 347
Mutex spin waits 297, rounds 8910, OS waits 281
RW-shared spins 53, rounds 1590, OS waits 50
RW-excl spins 13, rounds 1860, OS waits 57
Spin rounds per wait: 30.00 mutex, 30.00 RW-shared, 143.08 RW-excl
------------
TRANSACTIONS
------------
Trx id counter 2545
Purge done for trx's n:o < 2540 undo n:o < 0 state: running but idle
History list length 22
LIST OF TRANSACTIONS FOR EACH SESSION:
---TRANSACTION 0, not started
MySQL thread id 5, OS thread handle 0x7fcaccfe8700, query id 491 localhost root init
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, 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
915 OS file reads, 1496 OS file writes, 1089 OS fsyncs
0.00 reads/s, 0 avg bytes/read, 0.00 writes/s, 0.00 fsyncs/s
-------------------------------------
INSERT BUFFER AND ADAPTIVE HASH INDEX
-------------------------------------
Ibuf: size 1, free list len 0, seg size 2, 0 merges
merged operations:
 insert 0, delete mark 0, delete 0
discarded operations:
 insert 0, delete mark 0, delete 0
Hash table size 276671, node heap has 2 buffer(s)
0.00 hash searches/s, 0.00 non-hash searches/s
---
LOG
---
Log sequence number 369664448
Log flushed up to   369664448
Pages flushed up to 369664448
Last checkpoint at  369664448
0 pending log writes, 0 pending chkp writes
356 log i/o's done, 0.00 log i/o's/second
----------------------
BUFFER POOL AND MEMORY
----------------------
Total memory allocated 137363456; in additional pool allocated 0
Dictionary memory allocated 96365
Buffer pool size   8191
Free buffers       53
Database pages     8136
Old database pages 2983
Modified db pages  0
Pending reads 0
Pending writes: LRU 0, flush list 0, single page 0
Pages made young 9747, not young 4864716
0.00 youngs/s, 0.00 non-youngs/s
Pages read 4212, created 12449, written 13322
0.00 reads/s, 0.00 creates/s, 0.00 writes/s
No buffer pool page gets since the last printout
Pages read ahead 0.00/s, evicted without access 0.00/s, Random read ahead 0.00/s
LRU len: 8136, unzip_LRU len: 0
I/O sum[0]:cur[0], unzip sum[0]:cur[0]
--------------
ROW OPERATIONS
--------------
0 queries inside InnoDB, 0 queries in queue
0 read views open inside InnoDB
Main thread process no. 2345, id 140508950664960, state: sleeping
Number of rows inserted 3919015, updated 0, deleted 0, read 8532141
0.00 inserts/s, 0.00 updates/s, 0.00 deletes/s, 0.00 reads/s
----------------------------
END OF INNODB MONITOR OUTPUT
============================

1 row in set (0.00 sec)

mysql>

3.purge thread
事务提交后，purge thread线程回收已经分配并被使用的undo页。
innodb 1.2开始，支持多个purge thread。
purge thread离散地读取undo页。

mysql> show variables like 'innodb_purge_threads';
+----------------------+-------+
| Variable_name        | Value |
+----------------------+-------+
| innodb_purge_threads | 1     |
+----------------------+-------+
1 row in set (0.00 sec)

mysql>

4.page cleaner thread
1.2.x版本中引入的。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。减轻master thread的工作。