针对mysql的连接参数和状态值，本文做些介绍和对比
一、MYSQL连接参数变量
1、常用连接数限制参数
show variables like '%connect%';
| max_connect_errors | 999999999 | ##允许单用户连接错误最大值，超过后在不刷新状态的情况下，禁止该用户新连接 | max_connections | 6000 | ##实例最大连接数限制 | max_user_connections | 0 | ##但用户连接最大限制，默认0表示无限制，遵守总连接数小于等于max_connections | connect_timeout | 10 | ##用户连接超时限制，超过10秒，如果依旧无法连接到mysql，则终止连接
2、超时参数
mysql -e "show variables like '%timeout%'"
+-----------------------------+----------+ | connect_timeout | 10 | ##连接超时，10秒 | delayed_insert_timeout | 300 | ##延迟插入超时时间，300秒 | have_statement_timeout | YES | ## | innodb_flush_log_at_timeout | 1 | ##刷新redo log buffer超时时间，1秒 | innodb_lock_wait_timeout | 120 | ##事务等待获取资源等待的最长时间，超过这个时间还未分配到资源则会返回应用失败，120秒 | innodb_rollback_on_timeout | ON | | interactive_timeout | 28800 | ##mysql客户端交互连接超时时间，默认8小时，用于控制sleep超时 | lock_wait_timeout | 31536000 | ##主要针对DDL产生的metadata locks超时时间 | net_read_timeout | 60 | ##网络读取数据超时时间，60秒 | net_write_timeout | 60 | ##为网络写入数据超时间60秒 | rpl_stop_slave_timeout | 31536000 | ##停止从库服务超时时间 | slave_net_timeout | 60 | ##slave网络超时时间 | thread_pool_idle_timeout | 60 | | wait_timeout | 28800 | ##jdbc/odbc连接超时时间，默认8小时，用于控制sleep超时 +-----------------------------+----------+
二、MySQL连接状态变量
1、常用链接状态变量
show global status like '%connect%';
+-----------------------------------------------+----------+ | Connection_errors_max_connections | 1906160 | ##用户最大错误连接数 | Connections | 87341259 | ##历史用户连接累积值 | Max_used_connections | 10242 | ##历史最大连接用户数,processlist看到的连接数 | Threads_connected | 298 | ##当前用户连接数，当前processlist数
show global status like '%Threads_running%';
| Threads_running | 2 | ##当前用户连接处于running状态的数目，非sleep
show global status like '%abort%';
+------------------+----------+ | Aborted_clients | 85050948 | ##非法终止客户端连接的状态统计，例如通过kill终止连接 | Aborted_connects | 1905528 | ##非法连接操作状态统计，例如用户密码错误
三、常见问题分析
1、最大连接数超出限制
动态调整最大连接参数变量，然后通过连接尝试
mysql> set global max_connections=2; [root@localhost ~]# mysql ERROR 1040 (HY000): Too many connections
##此时连接超出的尝试，不会经过权限验证，因此不会记录Aborted_connects变量值
如遇到此问题即表示用户连接超出了最大连接限制，需要增大连接数
mysql> set global max_connections=350; ##动态调整后，需要将/etc/my.cnf配置参数中的max_connections=350也要做相应调整，这个参数可以根据状态量值Max_used_connections 评估
2、用户连接错误数超出限制
动态调整最大连接错误数变量限制，然后通过尝试失败登录，主机对应的普通用户会被阻塞，超级用户不限
max_connect_errors官方解释如下：
If more than this many successive connection requests from a host are interrupted without a successful connection,
the server blocks that host from further connections. You can unblock blocked hosts by flushing the host cache.
To do so, issue a FLUSH HOSTS statement or execute a mysqladmin flush-hosts command.
If a connection is established successfully within fewer than max_connect_errors attempts after a previous connection was interrupted,
the error count for the host is cleared to zero. However, once a host is blocked, flushing the host cache is the only way to unblock it.
The default is 100.
手动调整参数测试（telnet 10.1.0.1 3306 五次断链，用户会被锁）
mysql> set global max_connect_errors=5;
通过测试发现用户直接错误密码连接，不会导致用户被锁，主要通过performance_schema下的host_cache表查看相关阻塞信息
mysql> select IP,HOST,HOST_VALIDATED,SUM_CONNECT_ERRORS ,COUNT_HANDSHAKE_ERRORS,COUNT_HOST_BLOCKED_ERRORS from host_cache; +----------------+-------+----------------+--------------------+------------------------+---------------------------+ | IP | HOST | HOST_VALIDATED | SUM_CONNECT_ERRORS | COUNT_HANDSHAKE_ERRORS | COUNT_HOST_BLOCKED_ERRORS | +----------------+-------+----------------+--------------------+------------------------+---------------------------+ | 10.1.0.1 | NULL | NO | 0 | 5 | 0 | | 10.1.0.100 | node1 | YES | 0 | 0 | 0 | +----------------+-------+----------------+--------------------+------------------------+---------------------------+
哪些连接会导致用户被锁？导致必须通过flush hosts释放host_cache，具体如下：
| COUNT_NAMEINFO_TRANSIENT_ERRORS | bigint(20) | NO | | NULL | | | COUNT_NAMEINFO_PERMANENT_ERRORS | bigint(20) | NO | | NULL | | | COUNT_FORMAT_ERRORS | bigint(20) | NO | | NULL | | | COUNT_ADDRINFO_TRANSIENT_ERRORS | bigint(20) | NO | | NULL | | | COUNT_ADDRINFO_PERMANENT_ERRORS | bigint(20) | NO | | NULL | | | COUNT_FCRDNS_ERRORS | bigint(20) | NO | | NULL | | | COUNT_HOST_ACL_ERRORS | bigint(20) | NO | | NULL | | | COUNT_NO_AUTH_PLUGIN_ERRORS | bigint(20) | NO | | NULL | | | COUNT_AUTH_PLUGIN_ERRORS | bigint(20) | NO | | NULL | | | COUNT_HANDSHAKE_ERRORS | bigint(20) | NO | | NULL | | | COUNT_PROXY_USER_ERRORS | bigint(20) | NO | | NULL | | | COUNT_PROXY_USER_ACL_ERRORS | bigint(20) | NO | | NULL | | | COUNT_AUTHENTICATION_ERRORS | bigint(20) | NO | | NULL | | | COUNT_SSL_ERRORS | bigint(20) | NO | | NULL | | | COUNT_MAX_USER_CONNECTIONS_ERRORS | bigint(20) | NO | | NULL | | | COUNT_MAX_USER_CONNECTIONS_PER_HOUR_ERRORS | bigint(20) | NO | | NULL | | | COUNT_DEFAULT_DATABASE_ERRORS | bigint(20) | NO | | NULL | | | COUNT_INIT_CONNECT_ERRORS | bigint(20) | NO | | NULL | | | COUNT_LOCAL_ERRORS | bigint(20) | NO | | NULL | | | COUNT_UNKNOWN_ERRORS | bigint(20) | NO | | NULL | |
解决方法就是：flush hosts或mysqladmin flush-hosts
mysql> select ip,host,HOST_VALIDATED,SUM_CONNECT_ERRORS ,COUNT_HANDSHAKE_ERRORS,COUNT_HOST_BLOCKED_ERRORS from host_cache; +----------------+-------+----------------+--------------------+------------------------+---------------------------+ | ip | host | HOST_VALIDATED | SUM_CONNECT_ERRORS | COUNT_HANDSHAKE_ERRORS | COUNT_HOST_BLOCKED_ERRORS | +----------------+-------+----------------+--------------------+------------------------+---------------------------+ | 10.1.0.1 | NULL | NO | 0 | 5 | 0 | | 10.1.0.100 | node1 | YES | 0 | 0 | 0 | +----------------+-------+----------------+--------------------+------------------------+---------------------------+ 2 rows in set (0.00 sec) mysql> flush hosts; Query OK, 0 rows affected (0.00 sec) mysql> select ip,host,HOST_VALIDATED,SUM_CONNECT_ERRORS ,COUNT_HANDSHAKE_ERRORS,COUNT_HOST_BLOCKED_ERRORS from host_cache; Empty set (0.00 sec)
3、aborted连接值非常高
abort连接状态值主要有两个，分别是 Aborted_clients和Aborted_connects，其中两个变量值含义如下：
Aborted_clients：The number of connections that were aborted because the client died without closing the connection properly.
Aborted_connects：The number of failed attempts to connect to the MySQL server
下面的状态变量主要是用户最大连接超过后，有多少尝试连接的错误统计：
Connection_errors_max_connections：The number of connections refused because the server max_connections limit was reached
Aborted_clients和Aborted_connects参数对比，两个状态值在什么情况下会变化？
Aborted_clients主要是因为客户端非正常关闭，次值就会不断增加，具体场景如下：
(1)手动杀死连接,kill的方式 (2)mysql客户端超时interactive_timeout，结束sleep (3)程序超时wait_timeout,异常结束sleep
(4)The client program did not callmysql_close()before exiting.
(5)The client program ended abruptly in the middle of a data transfer.
Aborted_connects主要是连接Mysql失败的次数，具体场景如下：
(1)用户名密码错误 (2)登陆权限不足
(3)连接超时，connect_timeout
(4)max_allowed_packetvariable value is too small or queries require more memory than you have allocated for mysqld
针对aborted连接非常高，首先判断是aborted_clients还是aborted_connects，然后具体情况具体分析，如果是aborted_connects可以通过tcpdump和error log日志判断具体连接失败、导致abort的主机、用户等信息。
tcpdump -s 1500 -w tcp.out port 3306
tcpdump参数选项如下： (1)tcp: ip icmp arp rarp 和 tcp、udp、icmp这些选项等都要放到第一个参数的位置，用来过滤数据报的类型 (2)-i eth1 : 只抓经过接口eth1的包 (3)-t : 不显示时间戳 (4)-s 0 : 抓取数据包时默认抓取长度为68字节。加上-S 0 后可以抓到完整的数据包 (5)-c 100 : 只抓取100个数据包 (6)dst port ! 22 : 不抓取目标端口是22的数据包 (7)src net 192.168.1.0/24 : 数据包的源网络地址为192.168.1.0/24 (8)-w ./target.cap : 保存成cap文件，方便用ethereal(即wireshark)分析
4、连接数不断增加
mysql连接数有时候会不断增加，这种情况一般由以下几种原因
(1)长连接，processlist不断递增 (2)连接池初始化设置过大，程序不关闭连接，只增加连接
解决方案：调整sleep合理超时时间；检查程序代码，确定是否关闭数据库连接操作；检查连接池配置，调整合适的初始化连接值和最大连接值