原因1. MySQL 服務宕了

判斷是否屬于這個原因的方法很簡單，執行以下命令，查看mysql的運行時長

$ mysql -uroot -p -e "show global status like 'uptime';"

+---------------+-------+

| Variable_name | Value |

+---------------+-------+

| Uptime | 68928 |

+---------------+-------+

1 row in set (0.04 sec)

或者查看MySQL的報錯日志，看看有沒有重啟的信息

$ tail /var/log/mysql/error.log

130101 22:22:30 InnoDB: Initializing buffer pool, size = 256.0M

130101 22:22:30 InnoDB: Completed initialization of buffer pool

130101 22:22:30 InnoDB: highest supported file format is Barracuda.

130101 22:22:30 InnoDB: 1.1.8 started; log sequence number 63444325509

130101 22:22:30 [Note] Server hostname (bind-address): '127.0.0.1'; port: 3306

130101 22:22:30 [Note] - '127.0.0.1' resolves to '127.0.0.1';

130101 22:22:30 [Note] Server socket created on IP: '127.0.0.1'.

130101 22:22:30 [Note] Event Scheduler: Loaded 0 events

130101 22:22:30 [Note] /usr/sbin/mysqld: ready for connections.

Version: '5.5.28-cll' socket: '/var/lib/mysql/mysql.sock' port: 3306 MySQL Community Server (GPL)

如果uptime數值很大，表明mysql服務運行了很久了。說明最近服務沒有重啟過。

如果日志沒有相關信息，也表名mysql服務最近沒有重啟過，可以繼續檢查下面幾項內容。

2. 連接超時

如果程序使用的是長連接，則這種情況的可能性會比較大。

即，某個長連接很久沒有新的請求發起，達到了server端的timeout，被server強行關閉。

此后再通過這個connection發起查詢的時候，就會報錯server has gone away

$ mysql -uroot -p -e "show global variables like '%timeout';"

+----------------------------+----------+

| Variable_name | Value |

+----------------------------+----------+

| connect_timeout | 30 |

| delayed_insert_timeout | 300 |

| innodb_lock_wait_timeout | 50 |

| innodb_rollback_on_timeout | OFF |

| interactive_timeout | 28800 |

| lock_wait_timeout | 31536000 |

| net_read_timeout | 30 |

| net_write_timeout | 60 |

| slave_net_timeout | 3600 |

| wait_timeout | 28800 |

+----------------------------+----------+

mysql> SET SESSION wait_timeout=5;

## Wait 10 seconds

mysql> SELECT NOW();

ERROR 2006 (HY000): MySQL server has gone away

No connection. Trying to reconnect...

Connection id: 132361

Current database: *** NONE ***

+---------------------+

| NOW() |

+---------------------+

| 2013-01-02 11:31:15 |

+---------------------+

1 row in set (0.00 sec)

3. 進程在server端被主動kill

這種情況和情況2相似，只是發起者是DBA或者其他job。發現有長時間的慢查詢執行kill xxx導致。

$ mysql -uroot -p -e "show global status like 'com_kill'"

+---------------+-------+

| Variable_name | Value |

+---------------+-------+

| Com_kill | 0 |

+---------------+-------+

4. Your SQL statement was too large.

當查詢的結果集超過 max_allowed_packet 也會出現這樣的報錯。定位方法是打出相關報錯的語句。

用select * into outfile 的方式導出到文件，查看文件大小是否超過 max_allowed_packet ，如果超過則需要調整參數，或者優化語句。

mysql> show global variables like 'max_allowed_packet';

+--------------------+---------+

| Variable_name | Value |

+--------------------+---------+

| max_allowed_packet | 1048576 |

+--------------------+---------+

1 row in set (0.00 sec)

修改參數：

mysql> set global max_allowed_packet=1024*1024*16;

mysql> show global variables like 'max_allowed_packet';

+--------------------+----------+

| Variable_name | Value |

+--------------------+----------+

| max_allowed_packet | 16777216 |

+--------------------+----------+

1 row in set (0.00 sec)

總結

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