最近开发总是反馈数据库通过DBLINK访问远程数据库失败的情况。数据库日志没有报错，所以我一直从两个方向考虑了：
1. 对方的数据库监听异常
2.网络有异常
但是都没找到什么证据。
昨晚又出现了该异常，于是立刻上去查看，现象如下：
1.数据库日志无报错
2.监听异常
C:\Users\Administrator>lsnrctl status
LSNRCTL for 64-bit Windows: Version 11.2.0.1.0 - Production on 13-AUG-2019 23:06:55
Copyright (c) 1991, 2010, Oracle. All rights reserved.
Connecting to (DESCRIPTION=(ADDRESS=(PROTOCOL=TCP)(HOST=192.168.69.101)(PORT=1521)))
TNS-12560: TNS:protocol adapter error
TNS-00530: Protocol adapter error
64-bit Windows Error: 55: Unknown error
3.telnet 自己端口不通
C:\Users\Administrator>telnet 192.168.69.101 1521
正在连接192.168.69.101...无法打开到主机的连接。 在端口 1521: 连接失败
4. 防火墙是关闭的
5.应用对于数据库的访问正常，外部PLSQL 也可以正常连接数据。
但是数据库服务器不能连接外部的TNS
===============================
我怀疑过是不是连接数过多，端口不足
以下是异常情况时：
netstat -n | find /C /I "established"
275
C:\Users\Administrator>netstat -nt |find "TCP" /c
18390
C:\Users\Administrator>netstat -nt |find "1521" /c
18332
C:\Users\Administrator>netstat -nt |find "69.101" /c
17723
后来自动恢复了，再次验证
C:\Users\Administrator>netstat -nat |find "TCP" /c
18453
C:\Users\Administrator>netstat -n | find /C /I "established"
292
1.监听log 没看到异常
如附件
2.昨晚的alert log 无异常。
Tue Aug 13 20:51:10 2019
Thread 1 cannot allocate new log, sequence 56202
Private strand flush not complete
Current log# 3 seq# 56201 mem# 0: D:\APP\ADMINISTRATOR\ORADATA\ORCL\REDO03.LOG
Thread 1 advanced to log sequence 56202 (LGWR switch)
Current log# 4 seq# 56202 mem# 0: D:\APP\ADMINISTRATOR\ORADATA\ORCL\REDO04.LOG
Tue Aug 13 21:51:09 2019
Thread 1 cannot allocate new log, sequence 56203
Private strand flush not complete
Current log# 4 seq# 56202 mem# 0: D:\APP\ADMINISTRATOR\ORADATA\ORCL\REDO04.LOG
Thread 1 advanced to log sequence 56203 (LGWR switch)
Current log# 1 seq# 56203 mem# 0: D:\APP\ADMINISTRATOR\ORADATA\ORCL\REDO01.LOG
Tue Aug 13 22:00:00 2019
Setting Resource Manager plan SCHEDULER[0x3004]:DEFAULT_MAINTENANCE_PLAN via scheduler window
Setting Resource Manager plan DEFAULT_MAINTENANCE_PLAN via parameter
Tue Aug 13 22:00:00 2019
Starting background process VKRM
Tue Aug 13 22:00:00 2019
VKRM started with pid=352, OS id=95488
Tue Aug 13 22:00:08 2019
Begin automatic SQL Tuning Advisor run for special tuning task "SYS_AUTO_SQL_TUNING_TASK"
Tue Aug 13 22:23:10 2019
End automatic SQL Tuning Advisor run for special tuning task "SYS_AUTO_SQL_TUNING_TASK"
Tue Aug 13 22:51:09 2019
Thread 1 cannot allocate new log, sequence 56204
Private strand flush not complete
Current log# 1 seq# 56203 mem# 0: D:\APP\ADMINISTRATOR\ORADATA\ORCL\REDO01.LOG
Thread 1 advanced to log sequence 56204 (LGWR switch)
Current log# 5 seq# 56204 mem# 0: D:\APP\ADMINISTRATOR\ORADATA\ORCL\REDO05.LOG
Tue Aug 13 23:51:09 2019
Thread 1 cannot allocate new log, sequence 56205
Private strand flush not complete
Current log# 5 seq# 56204 mem# 0: D:\APP\ADMINISTRATOR\ORADATA\ORCL\REDO05.LOG
Thread 1 advanced to log sequence 56205 (LGWR switch)
Current log# 2 seq# 56205 mem# 0: D:\APP\ADMINISTRATOR\ORADATA\ORCL\REDO02.LOG
Wed Aug 14 00:51:12 2019
Thread 1 cannot allocate new log, sequence 56206
Private strand flush not complete
Current log# 2 seq# 56205 mem# 0: D:\APP\ADMINISTRATOR\ORADATA\ORCL\REDO02.LOG
Thread 1 advanced to log sequence 56206 (LGWR switch)
Current log# 3 seq# 56206 mem# 0: D:\APP\ADMINISTRATOR\ORADATA\ORCL\REDO03.LOG
Wed Aug 14 01:00:07 2019
DM00 started with pid=176, OS id=356588, job SYSTEM.SYS_EXPORT_SCHEMA_01
评论
有用 0这个是处理问题的步骤，我想知道这个报错原因是什么？有没有办法杜绝。以前也出现过一次，当时重启服务器解决的。
另外，业务运行是正常的只是不能访问外部DBLINK ，而且过段时间问题还会自己恢复。所以业务人员不希望重启监听。
网上查到很多是环境变量导致的sqlplus 不能登录报类似的问题。但是这边sqlplus / as sysdba 是可以正常登录的，所以网上没找到相关案例。
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