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

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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。

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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。