Packet too Large - MySQL

A.2.9 Packet too large

A communication packet is a single SQL statement sent to the MySQL server or a single row that is sent to the client.

In MySQL 3.23, the largest possible packet is 16MB, due to limits in the client/server protocol. In MySQL 4.0.1 and up, the limit is 1GB.

When a MySQL client or the mysqld server receives a packet bigger than max_allowed_packet bytes, it issues a Packet too large error and closes the connection. With some clients, you may also get a Lost connection to MySQL server during query error if the communication packet is too large.

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. That is also the maximum value before MySQL 4.0. To set a larger value from 4.0 on, start mysql like this:

mysql> 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:

mysql> mysqld --max_allowed_packet=16M

Before MySQL 4.0, use this syntax instead:

mysql> mysqld --set-variable=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

Before MySQL 4.0, use this syntax instead:

[mysqld]
set-variable = max_allowed_packet=16M

It's safe to increase the value of this variable because the extra memory is allocated only when needed. For example, mysqld allocates more memory only when you issue a long query or when mysqld must return a large result row. The small default value of the variable is a precaution to catch incorrect packets between the client and server and also to ensure that you don't run out of memory by using large packets accidentally.

You can also get strange problems with large packets if you are using large BLOB values but have not given mysqld access to enough memory to handle the query. If you suspect this is the case, try adding ulimit -d 256000 to the beginning of the mysqld_safe script and restarting mysqld.

A.2.10 Communication Errors and Aborted Connections

The server error log can be a useful source of information about connection problems. See Section 4.8.1, "The Error Log." Starting with MySQL 3.23.40, if you start the server with the --warnings option (or --log-warnings from MySQL 4.0.3 on), you might find messages like this in your error log:

010301 14:38:23 Aborted connection 854 to db: 'users' user: 'josh'

If Aborted connections messages appear in the error log, the cause can be any of the following:

• The client program did not call mysql_close() before exiting.

• The client had been sleeping more than wait_timeout or interactive_timeout seconds without issuing any requests to the server. See Section 4.2.3, "Server System Variables."

• The client program ended abruptly in the middle of a data transfer.

When any of these things happen, the server increments the Aborted_clients status variable.

The server increments the Aborted_connects status variable when the following things happen:

• A client doesn't have privileges to connect to a database.

• A client uses an incorrect password.

• A connection packet doesn't contain the right information.

• It takes more than connect_timeout seconds to get a connect packet. See Section 4.2.3, "Server System Variables."

If these kinds of things happen, it might indicate that someone is trying to break into your server!

Other reasons for problems with aborted clients or aborted connections:

• Use of Ethernet protocol with Linux, both half and full duplex. Many Linux Ethernet drivers have this bug. You should test for this bug by transferring a huge file via FTP between the client and server machines. If a transfer goes in burst-pause-burst-pause mode, you are experiencing a Linux duplex syndrome. The only solution is switching the duplex mode for both your network card and hub/switch to either full duplex or to half duplex and testing the results to determine the best setting.

• Some problem with the thread library that causes interrupts on reads.

• Badly configured TCP/IP.

• Faulty Ethernets, hubs, switches, cables, and so forth. This can be diagnosed properly only by replacing hardware.

• The max_allowed_packet variable value is too small or queries require more memory than you have allocated for mysqld. See Section A.2.9, "Packet too large."

   

This chapter is from MySQL Administrator's Guide, by MySQL AB. (Sams, 2004, ISBN: 0672326345). Check it out at your favorite bookstore today. Buy this book now.

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