问题概述
最近我们遇到一个MySQL的问题,分析后很有代表意义,特地写出来供大家参考。
出现问题是,数据库先是被置为只读,然后过了一段时间,MySQL直接Crash掉了
发生Crash时MySQL的error日志中打印了以下内容:
----------
SEMAPHORES
----------
OS WAIT ARRAY INFO: reservation count 1246555
--Thread 140363572082432 has waited at row0upd.cc line 2354 for 253.00 seconds the semaphore:
X-lock (wait_ex) on RW-latch at 0x7fa949340740 created in file buf0buf.cc line 1069
a writer (thread id 140363572082432) has reserved it in mode wait exclusive
number of readers 1, waiters flag 1, lock_word: ffffffffffffffff
Last time read locked in file btr0sea.cc line 931
Last time write locked in file/export/home/pb2/build/sb_0-17068951-1447697721.44/mysql-5.6.28/storage/innobase/row/row0upd.cc line 2354
根据日志中我们可以看到,线程140363572082432要对记录上一个X锁,但是等待0x7fa949340740线程的RW-latch的释放
我们再向下看查询到如下信息(涉及到用户信息 谓词就用xxx代替):
173 lock struct(s), heap size 30248, 7925 row lock(s), undo log entries 7924
MySQL thread id 5709783, OS thread handle 0x7fa8f0da7700, query id 92213034 10.23.163.54 citicqyh updating
update TB_DEPARTMENT_INFO set TOTAL_USER=1 where ID='ac84f17e-82d3-4519-a1da-0d5a5a835d44'
---TRANSACTION 53065242, ACTIVE 313 sec fetching rows, thread declared inside InnoDB 2081
mysql tables in use 2, locked 0
MySQL thread id 5428690, OS thread handle 0x7fa8f0136700, query id 92213061 10.23.163.55 citicqyh Sending data
SELECT COUNT(DISTINCT r.user_id) FROM TB_DEPARTMENT_INFO d left join tb_qy_user_department_ref r on r.department_id = d.id WHERE d.org_id = 'xxx' AND (d.dept_full_name LIKE 'xxx%' or d.dept_full_name = 'xxx
Trx read view will not see trx with id >= xxx, sees < xxx
......还有很多select语句省略
根据上面信息我们去数据库中查看了这些select语句,发现执行计划都是全表扫描。
首先数据库变成了只读,最后数据库crash了,crash输出的信息如下:
----------------------------
END OF INNODB MONITOR OUTPUT
============================
InnoDB: ###### Diagnostic info printed to the standard error stream
InnoDB: Error: semaphore wait has lasted > 600 seconds
InnoDB: We intentionally crash the server, because it appears to be hung.
7fa8f9580700 InnoDB: Assertion failure in thread 140363714529024 in file srv0srv.cc line 1754
InnoDB: We intentionally generate a memory trap.
InnoDB: Submit a detailed bug report to http://bugs.mysql.com.
InnoDB: If you get repeated assertion failures or crashes, even
InnoDB: immediately after the mysqld startup, there may be
InnoDB: corruption in the InnoDB tablespace. Please refer to
InnoDB: http://dev.mysql.com/doc/refman/5.6/en/forcing-innodb-recovery.html
InnoDB: about forcing recovery.
InnoDB: Error: semaphore wait has lasted > 600 seconds 提示600秒没有响应 数据库选择了Crash 强制重启
从报错信息来看:
update语句需要在记录上面加X锁,但是必须等待RW-Latch的释放
由于有大量的select语句是全表扫描,一直占用latch没有释放,update迟迟竞争不到RW-latch
Innodb 的Diagnostic线程检查到RW-Latch等待超过了600秒还没有返回,认为系统出现了严重问题,于是触发了MySQL服务的Crash。
进一步分析
这里首先需要补充一下Latch的概念:Latch在MySQL中是用于保护高速缓冲区中共享数据的,举个例子:
当我们执行select时,数据是缓存在buffer pool中的,多个线程并发访问或者修改这个数据必然需要一个并发控制机制,这个就是Latch
大家知道,数据库要访问的数据都必须先存在缓存中,而缓存一般比磁盘空间要小,数据缓存使用hash表来记录数据页是否在内存中。在Oracle中的并发控制比较精细:首先会对hash桶加latch,并根据hash桶查找对应的数据并加上pin,然后释放Latch。而MySQL相对没有控制得这么精细,对应的RW-Latch在errlog中说的很清楚,该RW-Latch是在buf0buf.cc的1069行创建的
RW-latch at 0x7fa949340740 created in file buf0buf.cc line 1069
对应的代码摘录如下:
rw_lock_create(PFS_NOT_INSTRUMENTED,&block->lock,SYNC_LEVEL_VARYING);
跟踪源码,知道这个Latch是MySQL在数据库启动,初始化 innodb_buffer_pool时,将Latch创建好的。对应的函数调用过程:
buf_pool_init_instance()->buf_chunk_init()->buf_block_init()
正是由于这个RW-Latch被长时间占用了,其他的线程一直竞争不到,才导致了这个问题
修复建议
这类问题的发生多数都是因为SQL写的不好,在表上面进行了大量的全表扫描占用了大量的Latch,解决方案就是避免SQL长时间占用latch:
修改select查询避免全表扫描,避免latch长期被占用
适当的加索引,让select执行更快,也避免一个select锁的数据更少
适当加大buffer pool instance,每个buffer pool都有自己独立的Latch,避免latch竞争。