select @@global.myisam_mmap_size; @@global.myisam_mmap_size 18446744073709551615 select @@session.myisam_mmap_size; ERROR HY000: Variable 'myisam_mmap_size' is a GLOBAL variable show global variables like 'myisam_mmap_size'; Variable_name Value myisam_mmap_size 18446744073709551615 show session variables like 'myisam_mmap_size'; Variable_name Value myisam_mmap_size 18446744073709551615 select * from information_schema.global_variables where variable_name='myisam_mmap_size'; VARIABLE_NAME VARIABLE_VALUE MYISAM_MMAP_SIZE 18446744073709551615 select * from information_schema.session_variables where variable_name='myisam_mmap_size'; VARIABLE_NAME VARIABLE_VALUE MYISAM_MMAP_SIZE 18446744073709551615 set global myisam_mmap_size=1; ERROR HY000: Variable 'myisam_mmap_size' is a read only variable set session myisam_mmap_size=1; ERROR HY000: Variable 'myisam_mmap_size' is a read only variable