When you enable InnoDB monitors for periodic output, InnoDB writes their output to the mysqld server standard error output (stderr). In this case, no output is sent to clients. When switched on, InnoDB monitors print data about every 15 seconds. Server output usually is directed to the error log (see Section 5.4.2, “The Error Log”). This data is useful in performance tuning. On Windows, start the server from a command prompt in a console window with the --console option if you want to direct the output to the window rather than to the error log.

InnoDB sends diagnostic output to stderr or to files rather than to stdout or fixed-size memory buffers, to avoid potential buffer overflows. As a side effect, the output of SHOW ENGINE INNODB STATUS is written to a status file in the MySQL data directory every fifteen seconds. The name of the file is innodb_status.pid, where pid is the server process ID. InnoDB removes the file for a normal shutdown. If abnormal shutdowns have occurred, instances of these status files may be present and must be removed manually. Before removing them, you might want to examine them to see whether they contain useful information about the cause of abnormal shutdowns. The innodb_status.pid file is created only if the configuration option innodb-status-file=1 is set.

InnoDB monitors should be enabled only when you actually want to see monitor information because output generation does result in some performance decrement. Also, if you enable monitor output by creating the associated table, your error log may become quite large if you forget to remove the table later.

Each monitor begins with a header containing a timestamp and the monitor name. For example:

=====================================
141016 15:41:44 INNODB MONITOR OUTPUT
=====================================

The header for the standard InnoDB Monitor (INNODB MONITOR OUTPUT) is also used for the Lock Monitor because the latter produces the same output with the addition of extra lock information.

Enabling an InnoDB monitor for periodic output involves using a CREATE TABLE statement to create a specially named InnoDB table that is associated with the monitor. For example, to enable the standard InnoDB Monitor, you would create an InnoDB table named innodb_monitor.

Using CREATE TABLE syntax is just a way to pass a command to the InnoDB engine through MySQL's SQL parser. The only things that matter are the table name and that it be an InnoDB table. The structure of the table is not relevant. If you shut down the server, the monitor does not restart automatically when you restart the server. Drop the monitor table and issue a new CREATE TABLE statement to start the monitor.

The PROCESS privilege is required to start or stop the InnoDB monitors.

Enabling the Standard InnoDB Monitor

To enable the standard InnoDB Monitor for periodic output, create the innodb_monitor table:

CREATE TABLE innodb_monitor (a INT) ENGINE=INNODB;

To disable the standard InnoDB Monitor, drop the table:

DROP TABLE innodb_monitor;

Obtaining Standard InnoDB Monitor Output On Demand

As an alternative to enabling the standard InnoDB Monitor for periodic output, you can obtain standard InnoDB Monitor output on demand using the SHOW ENGINE INNODB STATUS SQL statement, which fetches the output to your client program. If you are using the mysql interactive client, the output is more readable if you replace the usual semicolon statement terminator with \G:

mysql> SHOW ENGINE INNODB STATUS\G

SHOW ENGINE INNODB STATUS output also includes InnoDB Lock Monitor data if the InnoDB Lock Monitor is enabled for periodic output.

Enabling the InnoDB Lock Monitor

To enable the InnoDB Lock Monitor for periodic output, create the innodb_lock_monitor table:

CREATE TABLE innodb_lock_monitor (a INT) ENGINE=INNODB;

To disable the InnoDB Lock Monitor, drop the table:

DROP TABLE innodb_lock_monitor;

Enabling the InnoDB Tablespace Monitor

To enable the InnoDB Tablespace Monitor for periodic output, create the innodb_tablespace_monitor table:

CREATE TABLE innodb_tablespace_monitor (a INT) ENGINE=INNODB;

To disable the standard InnoDB Tablespace Monitor, drop the table:

DROP TABLE innodb_tablespace_monitor;

Enabling the InnoDB Table Monitor

To enable the InnoDB Table Monitor for periodic output, create the innodb_table_monitor table:

CREATE TABLE innodb_table_monitor (a INT) ENGINE=INNODB;

To disable the InnoDB Table Monitor, drop the table:

DROP TABLE innodb_table_monitor;