最近监控系统的数据库监控总是延迟，查看日志报错：

2015-08-27 10:20:19,534 ERROR Traceback (most recent call last):
  File "oracle_mon.py",line 306,in <module>
    update_status(ip,'stop',timer.end_date)
  File "oracle_mon.py",line 285,in update_status
    pg.db.update("oracle_mon",where=where,status=status,stat_date=status_date)
  File "/usr/lib/python2.7/site-packages/web/db.py",line 881,in update
    db_cursor = self._db_cursor()
  File "/usr/lib/python2.7/site-packages/web/db.py",line 566,in _db_cursor
    return self.ctx.db.cursor()
  File "/usr/lib/python2.7/site-packages/web/db.py",line 507,in _getctx
    self._load_context(self._ctx)
  File "/usr/lib/python2.7/site-packages/web/db.py",line 516,in _load_context
    ctx.db = self._connect_with_pooling(self.keywords)
  File "/usr/lib/python2.7/site-packages/web/db.py",line 970,in _connect_with_pooling
    conn = DB._connect_with_pooling(self,keywords)
  File "/usr/lib/python2.7/site-packages/web/db.py",line 563,in _connect_with_pooling
    return self._pooleddb.connection()
  File "/usr/lib/python2.7/site-packages/DBUtils-1.1-py2.7.egg/DBUtils/PooledDB.py",line 331,in connection
    con = self.steady_connection()
  File "/usr/lib/python2.7/site-packages/DBUtils-1.1-py2.7.egg/DBUtils/PooledDB.py",line 279,in steady_connection
    *self._args,**self._kwargs)
  File "/usr/lib/python2.7/site-packages/DBUtils-1.1-py2.7.egg/DBUtils/SteadyDB.py",line 134,in connect
    failures,ping,closeable,*args,**kwargs)
  File "/usr/lib/python2.7/site-packages/DBUtils-1.1-py2.7.egg/DBUtils/SteadyDB.py",line 186,in __init__
    self._store(self._create())
  File "/usr/lib/python2.7/site-packages/DBUtils-1.1-py2.7.egg/DBUtils/SteadyDB.py",line 190,in _create
    con = self._creator(*self._args,**self._kwargs)
  File "/usr/lib64/python2.7/site-packages/psycopg2/__init__.py",line 179,in connect
    connection_factory=connection_factory,async=async)
OperationalError: 致命错误:  已保留的连接位置为执行非复制请求的超级用户预留

查到数据库使用的活动连接数为90左右，而pg的默认最大连接数MAX_CONNECTION=100，怀疑是数据库连接数过多导致报错。

事实果然如此，老数据库的MAX_CONNECTION=200，并没有使用默认的100。

解决办法：修改配置文件（postgresql.conf）中的默认最大连接数MAX_CONNECTION=200，重启数据库即可。