Issue with logging slow queries to TABLE, variable log_output is set to FILE during mysqld run

[Mariusz Zalewski]

Hello,

I have an issue with logging slow queries to the TABLE on one of MySQL 5.7 instances. The configuration seems to be OK on this instance - it worked for months. I administrate other instances with such settings without the issue described below.

When I start mysqld configuration file (Centos /etc/my.cnf) has these parameters:
log_output=TABLE
log-slow-admin-statements
slow-query-log
long_query_time=1

So mysqld should log into mysql.slow_log table every query that takes more than 1 second.

After restarting mysqld I'm checking manually what is the value for log_output variable:
[mysql]> show variables like '%log_output%';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| log_output | TABLE |
+---------------+-------+
1 row in set (0.00 sec)


I can even generate query that takes more than 1 seconds and it is logged correctly to mysql.slow_log table

But in less than a minute after mysqld started the log_output variable mysteriously (for me) changes the value to FILE and slow queries are not logged to TABLE.

Tried to recreate mysql.slow_log table, tried to start mysqld with "-log-output=TABLE" parameter - did not solve the issue. Mysqld has a log_output variable set to TABLE for a few moments and then changes that to FILE.

Any suggestion? What can be the source of the issue? How to investigate the problem and solve.

Best regards,