很多开发人员都会遇见”MySQL: ERROR 1040: Too many connections”的异常情况,造成这种情况的一种原因是访问量过高,MySQL服务器抗不住,这个时候就要考虑增加从服务器分散读压力;另一种原因就是MySQL配置文件中max_connections值过小。
首先,我们来查看mysql的最大连接数:
mysql> show variables like '%max_connections%';+-----------------+-------+| Variable_name | Value |+-----------------+-------+| max_connections | 151 |+-----------------+-------+1 row in set (0.00 sec)
其次,查看服务器响应的最大连接数:
mysql> show global status like 'Max_used_connections';+----------------------+-------+| Variable_name | Value |+----------------------+-------+| Max_used_connections | 2 |+----------------------+-------+1 row in set (0.00 sec)
可以看到服务器响应的最大连接数为2,远远低于mysql服务器允许的最大连接数值。
对于mysql服务器最大连接数值的设置范围比较理想的是:服务器响应的最大连接数值占服务器上限连接数值的比例值在10%以上,如果在10%以下,说明mysql服务器最大连接上限值设置过高。
Max_used_connections / max_connections * 100% = 2/151 *100% ≈ 1%
我们可以看到占比远低于10%(因为这是本地测试服务器,结果值没有太大的参考意义,大家可以根据实际情况设置连接数的上限值)。
再来看一下自己 linode VPS 现在(时间:2013-11-13 23:40:11)的结果值:
mysql> show variables like '%max_connections%';+-----------------+-------+| Variable_name | Value |+-----------------+-------+| max_connections | 151 |+-----------------+-------+1 row in set (0.19 sec) mysql> show global status like 'Max_used_connections';+----------------------+-------+| Variable_name | Value |+----------------------+-------+| Max_used_connections | 44 |+----------------------+-------+1 row in set (0.17 sec)
这里的最大连接数占上限连接数的30%左右。
上面我们知道怎么查看mysql服务器的最大连接数值,并且知道了如何判断该值是否合理,下面我们就来介绍一下如何设置这个最大连接数值。
方法1:
mysql> set GLOBAL max_connections=256; Query OK, 0 rows affected (0.00 sec)mysql> show variables like '%max_connections%';+-----------------+-------+| Variable_name | Value |+-----------------+-------+| max_connections | 256 |+-----------------+-------+1 row in set (0.00 sec)
方法2:
修改mysql配置文件my.cnf,在[mysqld]段中添加或修改max_connections值:
max_connections=128
重启mysql服务即可。

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