MySQL server has gone away問題解決方法

一、應用程序（好比PHP）長時間的執行批量的MYSQL語句。最多見的就是採集或者新舊數據轉化。

　　解決方案：

　　在my.cnf文件中添加或者修改如下兩個變量：

wait_timeout=2880000
interactive_timeout = 2880000　　
關於兩個變量的具體說明能夠google或者看官方手冊。若是不能修改my.cnf，則能夠在鏈接數據庫的時候設置CLIENT_INTERACTIVE，好比：

sql = "set interactive_timeout=24*3600";
mysql_real_query(...)

　　二、執行一個SQL，但SQL語句過大或者語句中含有BLOB或者longblob字段。好比，圖片數據的處理

　　解決方案：

　　在my.cnf文件中添加或者修改如下變量：

max_allowed_packet = 10M
(也能夠設置本身須要的大小)

max_allowed_packet
參數的做用是，用來控制其通訊緩衝區的最大長度

MySQL: 詭異的MySQL server has gone away及其解決
來自：http://fz9493.blog.sohu.com/38472203.html

jimmy | 15 三月, 2007 20:32

在Mysql執行show status，一般更關注緩存效果、進程數等，每每忽略了兩個值：

Variable_name Value
Aborted_clients 3792
Aborted_connects 376

一般只佔query的0.0x%，因此並不爲人所重視。並且在傳統Web應用上，query錯誤對用戶而言影響並不大，只是從新刷新一下頁面就OK了。最近的基礎改造中，把不少應用做爲service運行，沒法提示用戶從新刷新，這種狀況下，可能就會影響到服務的品質。

經過程序腳本的日誌跟蹤，主要報錯信息爲「MySQL server has gone away」。官方的解釋是：

The most common reason for the MySQL server has gone away error is that the server timed out and closed the connection.

Some other common reasons for the MySQL server has gone away error are:

You (or the db administrator) has killed the running thread with a KILL statement or a mysqladmin kill command.

You tried to run a query after closing the connection to the server. This indicates a logic error in the application that should be corrected.

A client application running on a different host does not have the necessary privileges to connect to the MySQL server from that host.

You got a timeout from the TCP/IP connection on the client side. This may happen if you have been using the commands: mysql_options(..., MYSQL_OPT_READ_TIMEOUT,...) or mysql_options(..., MYSQL_OPT_WRITE_TIMEOUT,...). In this case increasing the timeout may help solve the problem.

You have encountered a timeout on the server side and the automatic reconnection in the client is disabled (the reconnect flag in the MYSQL structure is equal to 0).

You are using a Windows client and the server had dropped the connection (probably because wait_timeout expired) before the command was issued.

The problem on Windows is that in some cases MySQL doesn't get an error from the OS when writing to the TCP/IP connection to the server, but instead gets the error when trying to read the answer from the connection.

In this case, even if the reconnect flag in the MYSQL structure is equal to 1, MySQL does not automatically reconnect and re-issue the query as it doesn't know if the server did get the original query or not.

The solution to this is to either do a mysql_ping on the connection if there has been a long time since the last query (this is what MyODBC does) or set wait_timeout on the mysqld server so high that it in practice never times out.

You can also get these errors if you send a query to the server that is incorrect or too large. If mysqld receives a packet that is too large or out of order, it assumes that something has gone wrong with the client and closes the connection. If you need big queries (for example, if you are working with big BLOB columns), you can increase the query limit by setting the server's max_allowed_packet variable, which has a default value of 1MB. You may also need to increase the maximum packet size on the client end. More information on setting the packet size is given in Section A.1.2.9, 「Packet too large」.

An INSERT or REPLACE statement that inserts a great many rows can also cause these sorts of errors. Either one of these statements sends a single request to the server irrespective of the number of rows to be inserted; thus, you can often avoid the error by reducing the number of rows sent per INSERT or REPLACE.

You also get a lost connection if you are sending a packet 16MB or larger if your client is older than 4.0.8 and your server is 4.0.8 and above, or the other way around.

It is also possible to see this error if hostname lookups fail (for example, if the DNS server on which your server or network relies goes down). This is because MySQL is dependent on the host system for name resolution, but has no way of knowing whether it is working — from MySQL's point of view the problem is indistinguishable from any other network timeout.

You may also see the MySQL server has gone away error if MySQL is started with the --skip-networking option.

Another networking issue that can cause this error occurs if the MySQL port (default 3306) is blocked by your firewall, thus preventing any connections at all to the MySQL server.

You can also encounter this error with applications that fork child processes, all of which try to use the same connection to the MySQL server. This can be avoided by using a separate connection for each child process.

You have encountered a bug where the server died while executing the query.

據此分析，可能緣由有3：

1，Mysql服務端與客戶端版本不匹配。

2，Mysql服務端配置有缺陷或者優化不足

3，須要改進程序腳本

經過更換多個服務端與客戶端版本，發現只能部分減小報錯，並不能徹底解決。排除1。

對服務端進行了完全的優化，也未能達到理想效果。在timeout的取值設置上，從經驗值的10，到PHP默認的60，進行了屢次嘗試。而Mysql官方默認值(8小時)明顯是不可能的。從而對2也進行了排除。(更多優化的經驗分享，將在之後整理提供)

針對3對程序代碼進行分析，發現程序中大量應用了相似以下的代碼(爲便於理解，用原始api描述)：

$conn=mysql_connect( ... ... );

... ... ... ...

if(!$conn){ //reconnect

$conn=mysql_connect( ... ... );

}

mysql_query($sql, $conn);

這段代碼的含義，與Mysql官方建議的方法思路相符[ If you have a script, you just have to issue the query again for the client to do an automatic reconnection. ]。在實際分析中發現，if(!$conn)並非可靠的，程序經過了if(!$conn)的檢驗後，仍然會返回上述錯誤。

對程序進行了改寫：

if(!conn){ // connect ...}

elseif(!mysql_ping($conn)){ // reconnect ... }

mysql_query($sql, $conn);

經實際觀測，MySQL server has gone away的報錯基本解決。

BTW： 附帶一個關於 reconnect 的疑問，

在php4x+client3x+mysql4x的舊環境下，reconnet的代碼：

$conn=mysql_connect(...) 能夠正常工做。

可是，在php5x+client4x+mysql4x的新環境下，$conn=mysql_connect(...)返回的$conn有部分狀況下不可用。須要書寫爲：

mysql_close($conn);

$conn=mysql_connect(...);

返回的$conn才能夠正常使用。緣由未明。未作深刻研究，也未見相關討論。或許mysql官方的BUG彙報中會有吧。

～～呵呵～～

本文來自CSDN博客，轉載請標明出處：http://blog.csdn.net/brightsnow/archive/2009/03/17/3997705.aspx

description: remember that your MySQL "max_allowed_packet" configuration setting (default 1MB) mysql 默認最大可以處理的是1MB 若是你在sql使用了大的text或者BLOB數據，就會出現這個問題。 php手冊上的註釋 When trying to INSERT or UPDATE and trying to put a large amount of text or data (blob) into a mysql table you might run into problems. In mysql.err you might see: Packet too large (73904) To fix you just have to start up mysql with the option -O max_allowed_packet=maxsize You would just replace maxsize with the max size you want to insert, the default is 65536 mysql手冊上說 Both the client and the server have their own max_allowed_packet variable, so if you want to handle big packets, you must increase this variable both in the client and in the server. If you are using the mysql client program, its default max_allowed_packet variable is 16MB. To set a larger value, start mysql like this: shell> mysql --max_allowed_packet=32M That sets the packet size to 32MB. The server's default max_allowed_packet value is 1MB. You can increase this if the server needs to handle big queries (for example, if you are working with big BLOB columns). For example, to set the variable to 16MB, start the server like this: shell> mysqld --max_allowed_packet=16M You can also use an option file to set max_allowed_packet. For example, to set the size for the server to 16MB, add the following lines in an option file: [mysqld]max_allowed_packet=16M 使用mysql作數據庫還原的時候，因爲有些數據很大，會出現這樣的錯誤：The MySQL Server returned this Error:MySQL Error Nr.2006-MySQL server has gone away。個人一個150mb的備份還原的時候就出現了這錯誤。解決的方法就是找到mysql安裝目錄，找到my.ini文件，在文件的最後添 加：max_allowed_packet = 10M(也能夠設置本身須要的大小)。 max_allowed_packet 參數的做用是，用來控制其通訊緩衝區的最大長度。