MySQL最大连接数配置与调优实践

-- 查看当前最大连接数
SHOW VARIABLES LIKE 'max_connections';

-- 查看当前实际连接数
SHOW STATUS LIKE 'Threads_connected';

-- 查看历史最大连接数
SHOW STATUS LIKE 'Max_used_connections';

-- 动态修改最大连接数（立即生效，重启后失效）
SET GLOBAL max_connections = 500;

-- 验证修改结果
SHOW VARIABLES LIKE 'max_connections';

[mysqld]
max_connections = 1000

# 相关参数配置
back_log = 300
thread_cache_size = 32

// 在编译前修改源码中的默认值
// 文件位置：sql/mysqld.cc
{"max_connections", OPT_MAX_CONNECTIONS,
 "The number of simultaneous clients allowed.", 
 &max_connections, &max_connections, 0, GET_ULONG,
 REQUIRED_ARG, 150, 1, 16384, 0, 1, 0},

-- 估算单个连接的内存开销
SHOW STATUS LIKE 'Bytes_received';
SHOW STATUS LIKE 'Bytes_sent';

-- 计算总内存占用估算
SELECT @@max_connections * 
       (@@read_buffer_size + @@sort_buffer_size + 
        @@join_buffer_size + @@thread_stack) AS estimated_memory_usage;

-- 电商系统建议配置
SET GLOBAL max_connections = 800;
SET GLOBAL thread_cache_size = 64;
SET GLOBAL table_open_cache = 4000;

-- 内部管理系统配置
SET GLOBAL max_connections = 300;
SET GLOBAL wait_timeout = 600;  -- 减少空闲连接占用

-- 监控连接数趋势
SHOW STATUS LIKE 'Max_used_connections';
SHOW STATUS LIKE 'Threads_created';

-- 查看连接状态分布
SHOW PROCESSLIST;

# 逐步调整策略
[mysqld]
max_connections = 500
wait_timeout = 300
interactive_timeout = 300

// Java 连接池配置示例
@Configuration
public class DataSourceConfig {
    
    @Bean
    public DataSource dataSource() {
        HikariConfig config = new HikariConfig();
        config.setMaximumPoolSize(50);        // 最大连接数
        config.setMinimumIdle(10);            // 最小空闲连接
        config.setIdleTimeout(300000);        // 空闲超时时间
        config.setMaxLifetime(1800000);       // 连接最大生命周期
        config.setConnectionTimeout(30000);   // 连接超时时间
        return new HikariDataSource(config);
    }
}

# Linux 系统文件描述符调整
echo "* soft nofile 65535" >> /etc/security/limits.conf
echo "* hard nofile 65535" >> /etc/security/limits.conf

# 验证当前限制
ulimit -n

[mysqld]
# 连接相关内存配置
key_buffer_size = 256M
query_cache_size = 128M
tmp_table_size = 64M
max_heap_table_size = 64M

-- 检查连接状态
SHOW STATUS LIKE 'Threads_connected';
SHOW PROCESSLIST;

-- 查看错误日志中的连接相关错误
SELECT * FROM performance_schema.events_errors_summary_global_by_error;

-- 监控内存使用情况
SHOW STATUS LIKE 'Memory_used';
SHOW STATUS LIKE 'Memory_used_%';

#!/bin/bash
# MySQL 连接数监控脚本

while true; do
    connections=$(mysql -e "SHOW STATUS LIKE 'Threads_connected'" | grep Threads_connected | awk '{print $2}')
    max_connections=$(mysql -e "SHOW VARIABLES LIKE 'max_connections'" | grep max_connections | awk '{print $2}')
    usage_percentage=$(( (connections * 100) / max_connections ))
    
    echo "$(date): 当前连接数: $connections, 使用率: $usage_percentage%"
    
    if [ $usage_percentage -gt 80 ]; then
        echo "警告：连接数使用率超过80%！"
    fi
    
    sleep 60
done