A.2. Errores comunes al usar programas MySQL

If you get a Too many connections error when you try to connect to the mysqld server, this means that all available connections are in use by other clients.

The number of connections allowed is controlled by the max_connections system variable. Its default value is 100. If you need to support more connections, you should restart mysqld with a larger value for this variable.

mysqld actually allows max_connections+1 clients to connect. The extra connection is reserved for use by accounts that have the SUPER privilege. By granting the SUPER privilege to administrators and not to normal users (who should not need it), an administrator can connect to the server and use SHOW PROCESSLIST to diagnose problems even if the maximum number of unprivileged clients are connected. See Sección 13.5.4.16, “Sintaxis de SHOW PROCESSLIST”.

The maximum number of connections MySQL can support depends on the quality of the thread library on a given platform. Linux or Solaris should be able to support 500-1000 simultaneous connections, depending on how much RAM you have and what your clients are doing. Static Linux binaries provided by MySQL AB can support up to 4000 connections.

If you issue a query using the mysql client program and receive an error like the following one, it means that mysql does not have enough memory to store the entire query result:

mysql: Out of memory at line 42, 'malloc.c'
mysql: needed 8136 byte (8k), memory in use: 12481367 bytes (12189k)
ERROR 2008: MySQL client ran out of memory

To remedy the problem, first check whether your query is correct. Is it reasonable that it should return so many rows? If not, correct the query and try again. Otherwise, you can invoke mysql with the --quick option. This causes it to use the mysql_use_result() C API function to retrieve the result set, which places less of a load on the client (but more on the server).

This section also covers the related Lost connection to server during query error.

The most common reason for the MySQL server has gone away error is that the server timed out and closed the connection. In this case, you normally get one of the following error codes (which one you get is operating system-dependent):

By default, the server closes the connection after eight hours if nothing has happened. You can change the time limit by setting the wait_timeout variable when you start mysqld. See Sección 5.3.3, “Variables de sistema del servidor”.

If you have a script, you just have to issue the query again for the client to do an automatic reconnection. This assumes that you have automatic reconnection in the client enabled (which is the default for the mysql command-line client).

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

You can check whether the MySQL server died and restarted by executing mysqladmin version and examining the server's uptime. If the client connection was broken because mysqld crashed and restarted, you should concentrate on finding the reason for the crash. Start by checking whether issuing the query again kills the server again. See Sección A.4.2, “Qué hacer si MySQL sigue fallando (crashing)”.

You can get more information about the lost connections by starting mysqld with the --log-warnings=2 option. This logs some of the disconnected errors in the hostname.err file. See Sección 5.10.1, “El registro de errroes (Error Log)”.

If you want to create a bug report regarding this problem, be sure that you include the following information:

See also See Sección A.2.10, “Errores de comunicación y conexiones abortadas”.

See Sección 1.6.1.2, “Hacer preguntas y reportar bugs”.

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.

The server error log can be a useful source of information about connection problems. See Sección 5.10.1, “El registro de errroes (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:

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:

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:

See also See Sección A.2.8, “MySQL server has gone away”.

There are several ways a full-table error can occur:

If you get an error of the following type for some queries, it means that MySQL cannot create a temporary file for the result set in the temporary directory:

Can't create/write to file '\\sqla3fe_0.ism'.

The preceding error is a typical message for Windows; the Unix message is similar.

One fix is to start mysqld with the --tmpdir option or to add the option to the [mysqld] section of your option file. For example, to specify a directory of C:\temp, use these lines:

[mysqld]
tmpdir=C:/temp

The C:\temp directory must exist and have sufficient space for the MySQL server to write to. See Sección 4.3.2, “Usar ficheros de opciones”.

Another cause of this error can be permissions issues. Make sure that the MySQL server can write to the tmpdir directory.

Check also the error code that you get with perror. One reason the server cannot write to a table is that the filesystem is full:

shell> perror 28
Error code  28:  No space left on device

If you get Commands out of sync; you can't run this command now in your client code, you are calling client functions in the wrong order.

This can happen, for example, if you are using mysql_use_result() and try to execute a new query before you have called mysql_free_result(). It can also happen if you try to execute two queries that return data without calling mysql_use_result() or mysql_store_result() in between.

If you get the following error, it means that when mysqld was started or when it reloaded the grant tables, it found an account in the user table that had an invalid password.

Found wrong password for user 'some_user'@'some_host'; ignoring user