SET @@PERSIST_ONLY.back_log = 1000; 

Like PERSIST , PERSIST_ONLY  writes the variable setting to mysqld-auto.cnf . However, unlike PERSIST , PERSIST_ONLY  does not modify the global variable runtime value. This makes PERSIST_ONLY  suitable for configuring read-only system variables that can be set only at server startup. 

For more information about SET , see Section 13.7.6.1, “SET Syntax for Variable Assignment” . 

These RESET PERSIST  syntax options are available for removing persisted system variables: 

•To remove all persisted variables from mysqld-auto.cnf , use RESET PERSIST  without naming any system variable: 

RESET PERSIST; 

•To remove a specific persisted variable from mysqld-auto.cnf , name it in the statement: 

RESET PERSIST system_var_name ; 

This includes plugin system variables, even if the plugin is not currently installed. If the variable is not present in the file, an error occurs. 

•To remove a specific persisted variable from mysqld-auto.cnf , but produce a warning rather than an error if the variable is not present in the file, add an IF EXISTS  clause to the previous syntax: 

RESET PERSIST IF EXISTS system_var_name ; 

For more information about RESET PERSIST , see Section 13.7.8.7, “RESET PERSIST Statement” . 

Using SET  to persist a global system variable to a value of DEFAULT  or to its literal default value assigns the variable its default value and adds a setting for the variable to mysqld-auto.cnf . To remove the variable from the file, use RESET PERSIST . 

Some system variables cannot be persisted. See Section 5.1.9.4, “Nonpersistible and Persist-Restricted System Variables” . 

A system variable implemented by a plugin can be persisted if the plugin is installed when the SET statement is executed. Assignment of the persisted plugin variable takes effect for subsequent server restarts if the plugin is still installed. If the plugin is no longer installed, the plugin variable does not exist when the server reads the mysqld-auto.cnf  file. In this case, the server writes a warning to the error log and continues: 

## Obtaining Information About Persisted System Variables 

currently unknown variable ’ var_name ’ was read from the persisted config file 

The Performance Schema persisted_variables  table provides an SQL interface to the mysqld- auto.cnf  file, enabling its contents to be inspected at runtime using SELECT  statements. See Section 27.12.14.1, “Performance Schema persisted_variables Table” . 

The Performance Schema variables_info  table contains information showing when and by which user each system variable was most recently set. See Section 27.12.14.2, “Performance Schema variables_info Table” . 

RESET PERSIST  affects the contents of the persisted_variables  table because the table contents correspond to the contents of the mysqld-auto.cnf  file. On the other hand, because RESET PERSIST 

