MySQL 5.7 Reference Manual Including MySQL NDB Cluster 7.5 and NDB Cluster 7.6

5.1.7 Server System Variables

The MySQL server maintains many system variables that affect its operation. Most system variables can be set at server startup using options on the command line or in an option file. Most of them can be changed dynamically at runtime using the SET statement, which enables you to modify operation of the server without having to stop and restart it. Some variables are read-only, and their values are determined by the system environment, by how MySQL is installed on the system, or possibly by the options used to compile MySQL. Most system variables have a default value, but there are exceptions, including read-only variables. You can also use system variable values in expressions.

At runtime, setting a global system variable value requires the SUPER privilege. Setting a session system variable value normally requires no special privileges and can be done by any user, although there are exceptions. For more information, see Section, “System Variable Privileges”

There are several ways to see the names and values of system variables:

This section provides a description of each system variable. For a system variable summary table, see Section 5.1.4, “Server System Variable Reference”. For more information about manipulation of system variables, see Section 5.1.8, “Using System Variables”.

For additional system variable information, see these sections:


Some of the following variable descriptions refer to enabling or disabling a variable. These variables can be enabled with the SET statement by setting them to ON or 1, or disabled by setting them to OFF or 0. Boolean variables can be set at startup to the values ON, TRUE, OFF, and FALSE (not case-sensitive), as well as 1 and 0. See Section, “Program Option Modifiers”.

Some system variables control the size of buffers or caches. For a given buffer, the server might need to allocate internal data structures. These structures typically are allocated from the total memory allocated to the buffer, and the amount of space required might be platform dependent. This means that when you assign a value to a system variable that controls a buffer size, the amount of space actually available might differ from the value assigned. In some cases, the amount might be less than the value assigned. It is also possible for the server to adjust a value upward. For example, if you assign a value of 0 to a variable for which the minimal value is 1024, the server sets the value to 1024.

Values for buffer sizes, lengths, and stack sizes are given in bytes unless otherwise specified.


Some system variable descriptions include a block size, in which case a value that is not an integer multiple of the stated block size is rounded down to the next lower multiple of the block size before being stored by the server, that is to FLOOR(value) * block_size.

Example: Suppose that the block size for a given variable is given as 4096, and you set the value of the variable to 100000 (we assume that the variable's maximum value is greater than this number). Since 100000 / 4096 = 24.4140625, the server automatically lowers the value to 98304 (24 * 4096) before storing it.

In some cases, the stated maximum for a variable is the maximum allowed by the MySQL parser, but is not an exact multiple of the block size. In such cases, the effective maximum is the next lower multiple of the block size.

Example: A system variable's maxmum value is shown as 4294967295 (232-1), and its block size is 1024. 4294967295 / 1024 = 4194303.9990234375, so if you set this variable to its stated maximum, the value actually stored is 4194303 * 1024 = 4294966272.

Some system variables take file name values. Unless otherwise specified, the default file location is the data directory if the value is a relative path name. To specify the location explicitly, use an absolute path name. Suppose that the data directory is /var/mysql/data. If a file-valued variable is given as a relative path name, it is located under /var/mysql/data. If the value is an absolute path name, its location is as given by the path name.