Hi.
I'm currently trying to substitute the existing controller node with a new server.
After I install the 'Minimal deployment for Queens' on the new server, I connect all nodes.

However, when I made an instance on Horizon, there were some error messages, as below link:


And, when I typed 'openstack flavor list', the result was as follows:
# openstack flavor list
Unexpected API Error. Please report this at http://bugs.launchpad.net/nova/ and attach the Nova API log if possible.
<class 'sqlalchemy.exc.OperationalError'> (HTTP 500) (Request-ID: req-666d4e30-081a-429f-88ae-763e62990770)

Moreover, I found something regarding this problem in the Nova log:

2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi [req-666d4e30-081a-429f-88ae-763e62990770 bb1e571e4d64462bac80654b153a88c3 96ad10a59d114042b8f1ee82c438649a - default default] Unexpected exception in API method: OperationalError: (pymysql.err.OperationalError) (1040, u'Too many connections')
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi Traceback (most recent call last):
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi   File "/usr/lib/python2.7/dist-packages/nova/api/openstack/wsgi.py", line 788, in wrapped
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi     return f(*args, **kwargs)
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi   File "/usr/lib/python2.7/dist-packages/nova/api/openstack/compute/flavors.py", line 50, in detail
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi     limited_flavors = self._get_flavors(req)
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi   File "/usr/lib/python2.7/dist-packages/nova/api/openstack/compute/flavors.py", line 117, in _get_flavors
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi     limit=limit, marker=marker)
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi   File "/usr/lib/python2.7/dist-packages/oslo_versionedobjects/base.py", line 184, in wrapper
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi     result = fn(cls, context, *args, **kwargs)
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi   File "/usr/lib/python2.7/dist-packages/nova/objects/flavor.py", line 650, in get_all
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi     marker=marker)
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi   File "/usr/lib/python2.7/dist-packages/oslo_db/sqlalchemy/enginefacade.py", line 985, in wrapper
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi     with self._transaction_scope(context):
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi   File "/usr/lib/python2.7/contextlib.py", line 17, in __enter__
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi     return self.gen.next()
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi   File "/usr/lib/python2.7/dist-packages/oslo_db/sqlalchemy/enginefacade.py", line 1035, in _transaction_scope
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi     context=context) as resource:
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi   File "/usr/lib/python2.7/contextlib.py", line 17, in __enter__
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi     return self.gen.next()
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi   File "/usr/lib/python2.7/dist-packages/oslo_db/sqlalchemy/enginefacade.py", line 638, in _session
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi     bind=self.connection, mode=self.mode)
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi   File "/usr/lib/python2.7/dist-packages/oslo_db/sqlalchemy/enginefacade.py", line 403, in _create_session
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi     self._start()
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi   File "/usr/lib/python2.7/dist-packages/oslo_db/sqlalchemy/enginefacade.py", line 489, in _start
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi     engine_args, maker_args)
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi   File "/usr/lib/python2.7/dist-packages/oslo_db/sqlalchemy/enginefacade.py", line 513, in _setup_for_connection
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi     sql_connection=sql_connection, **engine_kwargs)
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi   File "/usr/lib/python2.7/dist-packages/debtcollector/renames.py", line 45, in wrapper
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi     return f(*args, **kwargs)
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi   File "/usr/lib/python2.7/dist-packages/oslo_db/sqlalchemy/engines.py", line 184, in create_engine
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi     test_conn = _test_connection(engine, max_retries, retry_interval)
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi   File "/usr/lib/python2.7/dist-packages/oslo_db/sqlalchemy/engines.py", line 362, in _test_connection
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi     return engine.connect()
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi   File "/usr/lib/python2.7/dist-packages/sqlalchemy/engine/base.py", line 2091, in connect
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi     return self._connection_cls(self, **kwargs)
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi   File "/usr/lib/python2.7/dist-packages/sqlalchemy/engine/base.py", line 90, in __init__
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi     if connection is not None else engine.raw_connection()
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi   File "/usr/lib/python2.7/dist-packages/sqlalchemy/engine/base.py", line 2177, in raw_connection
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi     self.pool.unique_connection, _connection)
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi   File "/usr/lib/python2.7/dist-packages/sqlalchemy/engine/base.py", line 2151, in _wrap_pool_connect
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi     e, dialect, self)
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi   File "/usr/lib/python2.7/dist-packages/sqlalchemy/engine/base.py", line 1461, in _handle_dbapi_exception_noconnection
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi     util.raise_from_cause(newraise, exc_info)
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi   File "/usr/lib/python2.7/dist-packages/sqlalchemy/util/compat.py", line 203, in raise_from_cause
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi     reraise(type(exception), exception, tb=exc_tb, cause=cause)
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi   File "/usr/lib/python2.7/dist-packages/sqlalchemy/engine/base.py", line 2147, in _wrap_pool_connect
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi     return fn()
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi   File "/usr/lib/python2.7/dist-packages/sqlalchemy/pool.py", line 328, in unique_connection
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi     return _ConnectionFairy._checkout(self)
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi   File "/usr/lib/python2.7/dist-packages/sqlalchemy/engine/base.py", line 2151, in _wrap_pool_connect
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi     e, dialect, self)
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi   File "/usr/lib/python2.7/dist-packages/sqlalchemy/engine/base.py", line 1461, in _handle_dbapi_exception_noconnection
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi     util.raise_from_cause(newraise, exc_info)
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi   File "/usr/lib/python2.7/dist-packages/sqlalchemy/util/compat.py", line 203, in raise_from_cause
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi     reraise(type(exception), exception, tb=exc_tb, cause=cause)
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi   File "/usr/lib/python2.7/dist-packages/sqlalchemy/engine/base.py", line 2147, in _wrap_pool_connect
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi     return fn()
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi   File "/usr/lib/python2.7/dist-packages/sqlalchemy/pool.py", line 328, in unique_connection
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi     return _ConnectionFairy._checkout(self)
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi   File "/usr/lib/python2.7/dist-packages/sqlalchemy/pool.py", line 766, in _checkout
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi     fairy = _ConnectionRecord.checkout(pool)
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi   File "/usr/lib/python2.7/dist-packages/sqlalchemy/pool.py", line 516, in checkout
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi     rec = pool._do_get()
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi   File "/usr/lib/python2.7/dist-packages/sqlalchemy/pool.py", line 1138, in _do_get
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi     self._dec_overflow()
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi   File "/usr/lib/python2.7/dist-packages/sqlalchemy/util/langhelpers.py", line 66, in __exit__
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi     compat.reraise(exc_type, exc_value, exc_tb)
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi   File "/usr/lib/python2.7/dist-packages/sqlalchemy/pool.py", line 1135, in _do_get
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi     return self._create_connection()
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi   File "/usr/lib/python2.7/dist-packages/sqlalchemy/pool.py", line 333, in _create_connection
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi     return _ConnectionRecord(self)
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi   File "/usr/lib/python2.7/dist-packages/sqlalchemy/pool.py", line 461, in __init__
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi     self.__connect(first_connect_check=True)
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi   File "/usr/lib/python2.7/dist-packages/sqlalchemy/pool.py", line 651, in __connect
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi     connection = pool._invoke_creator(self)
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi   File "/usr/lib/python2.7/dist-packages/sqlalchemy/engine/strategies.py", line 105, in connect
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi     return dialect.connect(*cargs, **cparams)
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi   File "/usr/lib/python2.7/dist-packages/sqlalchemy/engine/default.py", line 393, in connect
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi     return self.dbapi.connect(*cargs, **cparams)
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi   File "/usr/lib/python2.7/dist-packages/pymysql/__init__.py", line 90, in Connect
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi     return Connection(*args, **kwargs)
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi   File "/usr/lib/python2.7/dist-packages/pymysql/connections.py", line 699, in __init__
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi     self.connect()
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi   File "/usr/lib/python2.7/dist-packages/pymysql/connections.py", line 936, in connect
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi     self._request_authentication()
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi   File "/usr/lib/python2.7/dist-packages/pymysql/connections.py", line 1156, in _request_authentication
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi     auth_packet = self._read_packet()
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi   File "/usr/lib/python2.7/dist-packages/pymysql/connections.py", line 1018, in _read_packet
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi     packet.check_error()
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi   File "/usr/lib/python2.7/dist-packages/pymysql/connections.py", line 384, in check_error
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi     err.raise_mysql_exception(self._data)
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi   File "/usr/lib/python2.7/dist-packages/pymysql/err.py", line 107, in raise_mysql_exception
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi     raise errorclass(errno, errval)
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi OperationalError: (pymysql.err.OperationalError) (1040, u'Too many connections')
2018-12-29 00:58:52.928 3765 ERROR nova.api.openstack.wsgi
2018-12-29 00:58:52.930 3765 INFO nova.api.openstack.wsgi [req-666d4e30-081a-429f-88ae-763e62990770 bb1e571e4d64462bac80654b153a88c3 96ad10a59d114042b8f1ee82c438649a - default default] HTTP exception thrown: Unexpected API Error. Please report this at http://bugs.launchpad.net/nova/ and attach the Nova API log if possible.
<class 'sqlalchemy.exc.OperationalError'>
2018-12-29 00:58:52.931 3765 INFO nova.osapi_compute.wsgi.server [req-666d4e30-081a-429f-88ae-763e62990770 bb1e571e4d64462bac80654b153a88c3 96ad10a59d114042b8f1ee82c438649a - default default] 10.150.21.195 "GET /v2.1/flavors/detail HTTP/1.1" status: 500 len: 638 time: 0.1569352

I already did googling with 'Too many connections' but, there were only results about 'max_connections' of the mysql config file.
I also set the value as 4096 and I think it is enough.

Now, I do not know what to look for to resolve this problem.
Please give me any clue.

Thanks!
Regards,