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)
