Dovecot and MySQL aborted connections.

[Gerald Galster]

> We also spotted these sql connections getting aborted, upon upgrading MySQL from 5.6 to 5.7. (Going back to 5.6 we don't see them!)

I have a mailserver running MySQL 5.6.44 which is not very busy that logs these warnings.
Another busy one running MySQL 5.6.45 does not log any warnings.

MySQL interactive-/wait_timeout is 28800 (8 hours), which is more than enough time to wait for a query. Connections are closed way earlier.

dovecot and postfix use the same database and mysql user, so I created a new user for dovecot.
syslog shows a few communication packet errors but these are all from postfix, none from dovecot so far.

It seems connections are closed after 60s of inactivity (vmail is postfix, vmail2 is dovecot):

mysql> select * from INFORMATION_SCHEMA.PROCESSLIST where USER like 'vmail%';
+-----+--------+-----------------+-------+---------+------+-------+------+---------+-----------+---------------+------+
| ID  | USER   | HOST            | DB    | COMMAND | TIME | STATE | INFO | TIME_MS | ROWS_SENT | ROWS_EXAMINED | TID  |
+-----+--------+-----------------+-------+---------+------+-------+------+---------+-----------+---------------+------+
| 147 | vmail  | localhost       | vmail | Sleep   |    1 |       | NULL |     566 |         0 |             0 | 6664 |
| 148 | vmail  | localhost       | vmail | Sleep   |    1 |       | NULL |     564 |         0 |             0 | 6069 |
| 149 | vmail  | localhost       | vmail | Sleep   |    1 |       | NULL |     566 |         0 |             0 | 6037 |
| 151 | vmail2 | 127.0.0.2:35058 | vmail | Sleep   |   51 |       | NULL |   50872 |         1 |             1 | 6072 |
| 152 | vmail2 | 127.0.0.2:35060 | vmail | Sleep   |   60 |       | NULL |   59324 |         1 |             1 | 6025 |
| 150 | vmail  | localhost       | vmail | Sleep   |    1 |       | NULL |     565 |         0 |             0 | 6071 |
+-----+--------+-----------------+-------+---------+------+-------+------+---------+-----------+---------------+------+
6 rows in set (0.00 sec)

mysql> select * from INFORMATION_SCHEMA.PROCESSLIST where USER like 'vmail%';
+-----+--------+-----------------+-------+---------+------+-------+------+---------+-----------+---------------+------+
| ID  | USER   | HOST            | DB    | COMMAND | TIME | STATE | INFO | TIME_MS | ROWS_SENT | ROWS_EXAMINED | TID  |
+-----+--------+-----------------+-------+---------+------+-------+------+---------+-----------+---------------+------+
| 147 | vmail  | localhost       | vmail | Sleep   |    2 |       | NULL |    1775 |         0 |             0 | 6664 |
| 148 | vmail  | localhost       | vmail | Sleep   |    2 |       | NULL |    1772 |         0 |             0 | 6069 |
| 149 | vmail  | localhost       | vmail | Sleep   |    2 |       | NULL |    1774 |         0 |             0 | 6037 |
| 151 | vmail2 | 127.0.0.2:35058 | vmail | Sleep   |   52 |       | NULL |   52080 |         1 |             1 | 6072 |
| 150 | vmail  | localhost       | vmail | Sleep   |    2 |       | NULL |    1773 |         0 |             0 | 6071 |
+-----+--------+-----------------+-------+---------+------+-------+------+---------+-----------+---------------+------+
5 rows in set (0.00 sec)

Only vmail (postfix) has dropped connections (you need performance schema enabled):

mysql> SELECT ess.user, ess.host, (a.total_connections - a.current_connections) - ess.count_star as not_closed, ((a.total_connections - a.current_connections) - ess.count_star) * 100 / (a.total_connections - a.current_connections) as pct_not_closed FROM performance_schema.events_statements_summary_by_account_by_event_name ess JOIN performance_schema.accounts a on (ess.user = a.user and ess.host = a.host)  WHERE ess.event_name = 'statement/com/quit' AND (a.total_connections - a.current_connections) > ess.count_star;
+-------+-----------+------------+----------------+
| user  | host      | not_closed | pct_not_closed |
+-------+-----------+------------+----------------+
| vmail | localhost |         11 |        15.4930 |
+-------+-----------+------------+----------------+
1 row in set (0.02 sec)


My setup is different concerning quotas, they are not stored in MySQL. So if you don't use postfix with the same user/database the source for your warnings might be quota. For me it seems to be related to postfix.

Don't know if this helps, there is a new option in 5.7
https://dev.mysql.com/doc/refman/5.7/en/x-plugin-options-system-variables.html#sysvar_mysqlx_idle_worker_thread_timeout

Gerald


> Turning on mysql general query logging we can see it is Dovecot's mysql connections that inquire about or update quota usage in particular:
> 
> 
> *** /logs//mysql.log ***
> 2019-10-30T10:52:22.624690-07:00	    2 Connect	dovecot at localhost on npomail using Socket
> 
> 2019-10-30T10:52:40.019780-07:00	    2 Query	SELECT bytes FROM quota2 WHERE username = 'a at bla'
> 2019-10-30T10:52:40.020948-07:00	    2 Query	SELECT messages FROM quota2 WHERE username = 'a at bla'
> 
> 2019-10-30T10:53:40.113374-07:00 2 [Note] Aborted connection 2 to db: 'npomail' user: 'dovecot' host: 'localhost' (Got an error reading communication packets)
> 
> 
> Increasing mysqld's net_read_timeout and net_write_timeout values to 3600 does not stop the aborted connections.
> 
> It seems asymptomatic. If the connection to mysql is still there to do quota operations, Dovecot seems to use it. If it's been aborted by the server, Dovecot seems to just (re)connect. . . But I'd love to help find the root or file a bug report or learn what it is we've configured wrong. . .
> 
> Benjamin