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用MySQL慢日志分析解决MySQL CPU占用高的问题

发布时间:2017-12-13   编辑:jiaochengji.com
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我们可以通过MySQL慢日志分析来分析网站执行的sql语句占用的时间与性能从而我们对此条sql进行优化操作了,下面一起来看小编整理的用MySQL慢日志分析解决MySQL CPU占用高的问题的方法。

首先找到MySQL的配置文件my.cnf,根据不同版本的mysql开启慢查询的配置也不一样

mysql 5.0

[mysqld]
long_query_time = 1
log-slow-queries = /var/log/mysql/slow.log

mysql 5.1

[mysqld]
long_query_time = 1
slow_query_log=1
slow_query_log_file = /var/log/mysql/slow.log

long_query_time 是指执行超过多久的sql会被log下来,这里是1秒。
log-slow-queries和slow_query_log_file 设置把日志写在哪里

把上述参数打开,运行一段时间,就可以关掉了,省得影响生产环境

接下来就是分析了,我这里的文件名字叫 /var/log/mysql/slow.log。
先mysqldumpslow –help下,主要用的是

-s ORDER what to sort by (t, at, l, al, r, ar etc), ‘at’ is default
-t NUM just show the top n queries
-g PATTERN grep: only consider stmts that include this string

-s,是order的顺序,说明写的不够详细,主要有
c,t,l,r和ac,at,al,ar,分别是按照query次数,时间,lock的时间和返回的记录数来排序,前面加了a的时倒序
-t,是top n的意思,即为返回前面多少条的数据
-g,后边可以写一个正则匹配模式,大小写不敏感的

mysqldumpslow -s c -t 20  /var/log/mysql/slow.log
mysqldumpslow -s r -t 20  /var/log/mysql/slow.log

上述命令可以看出访问次数最多的20个sql语句和返回记录集最多的20个sql。

mysqldumpslow -t 10 -s t -g “left join”  /var/log/mysql/slow.log
这个是按照时间返回前10条里面含有左连接的sql语句。

例子,正确安全清空在线慢查询日志slow log的流程

1, see the slow log status;
mysql> show variables like '%slow%';
--------------------- ------------------------------------------
| Variable_name       | Value                                    |
--------------------- ------------------------------------------
| log_slow_queries    | ON                                       |
| slow_launch_time    | 2                                        |
| slow_query_log      | ON                                       |
| slow_query_log_file | /mysqllog/slow_log/slow_queries_3306.log |
--------------------- ------------------------------------------
4 rows in set (0.00 sec)


2, stop the slow log server.
mysql> set global slow_query_log=0;
Query OK, 0 rows affected (0.27 sec)


mysql> show variables like '%slow%';
--------------------- ------------------------------------------
| Variable_name       | Value                                    |
--------------------- ------------------------------------------
| log_slow_queries    | OFF                                      |
| slow_launch_time    | 2                                        |
| slow_query_log      | OFF                                      |
| slow_query_log_file | /mysqllog/slow_log/slow_queries_3306.log |
--------------------- ------------------------------------------
4 rows in set (0.00 sec)


mysql>
mysql> show variables like '%slow%'; -- check slow log status
--------------------- ------------------------------------------
| Variable_name       | Value                                    |
--------------------- ------------------------------------------
| log_slow_queries    | OFF                                      |
| slow_launch_time    | 2                                        |
| slow_query_log      | OFF                                      |
| slow_query_log_file | /mysqllog/slow_log/slow_queries_3306.log |
--------------------- ------------------------------------------
4 rows in set (0.00 sec)


3, reset the new path of slow log
mysql> set global slow_query_log_file='/mysqllog/slow_log/slow_queries_3306_new.log';
Query OK, 0 rows affected (0.03 sec)


4, start the slow log server
mysql>
mysql>
mysql> set global slow_query_log=1;
Query OK, 0 rows affected (0.01 sec)


mysql> show variables like '%slow%';
--------------------- ----------------------------------------------
| Variable_name       | Value                                        |
--------------------- ----------------------------------------------
| log_slow_queries    | ON                                           |
| slow_launch_time    | 2                                            |
| slow_query_log      | ON                                           |
| slow_query_log_file | /mysqllog/slow_log/slow_queries_3306_new.log |
--------------------- ----------------------------------------------
4 rows in set (0.00 sec)


5, check the slow sql in the new slow log file.
mysql> select sleep(10) as a, 1 as b;
--- ---
| a | b |
--- ---
| 0 | 1 |
--- ---
1 row in set (10.00 sec)


mysql>
[mysql@xxx-xxx ~]$ more /mysqllog/slow_log/slow_queries_3306_new.log
......
Time                 Id Command    Argument
# Time: 140213  6:44:24
# User@Host: root[root] @ localhost []
# Query_time: 10.000365  Lock_time: 0.000000 Rows_sent: 1  Rows_examined: 0
SET timestamp=1392273864;
select sleep(10) as a, 1 as b;


6, backup the old big slow log file to other directory.
mv /mysqllog/slow_log/slow_queries_3306.log /mysqlbackup/slow_log/slow_queries_3306.log.bak.20140213

用了这个工具就可以查询出来那些sql语句是性能的瓶颈,进行优化,比如加索引,该应用的实现方式等

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