环境：11.2.0.4.0 升 11.2.0.4.8 后测试
背景：oracle 的db_link会导致实例间SCN同步，SCN增长速度过快则会产生错误；
方案：oracle官方推荐升级版本，但升级之后该问题依然存在。
新创建orcl11g库
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SQL> alter user scott identified by tiger account unlock;
SQL> grant create synonym to scott;
SQL> grant create public database link,create database link to scott;
[oracle@red12 dbs]$ export ORACLE_SID=orcl11g
[oracle@red12 dbs]$ sqlplus scott/tiger
create public database link red12 connect to scott identified by tiger using '(DESCRIPTION =
(ADDRESS = (PROTOCOL = TCP)(HOST = 172.17.201.12)(PORT = 1521))
(CONNECT_DATA =
(SERVER = DEDICATED)
(SERVICE_NAME = hyzf)
)
)';
hyzf 库SCN
------------------------------------
[oracle@red12 scripts]$ export ORACLE_SID=hyzf1
[oracle@red12 scripts]$ sqlplus / as sysdba
SQL>
SQL> @SCNCHECK.txt;
--------------------------------------------------------------
ScnHealthCheck
--------------------------------------------------------------
Current Date: 2016/10/28 03:53:44
Current SCN: 5118400
SCN Headroom: 10722.16
Version: 11.2.0.4.0
--------------------------------------------------------------
Result: A - SCN Headroom is good
Apply the latest recommended patches
based on your maintenance schedule
For further information review MOS document id 1393363.1
--------------------------------------------------------------
SQL>
新建库SCN
---------------------------------
[oracle@red12 dbs]$ export ORACLE_SID=orcl11g
SQL> @SCNCHECK.txt;
--------------------------------------------------------------
ScnHealthCheck
--------------------------------------------------------------
Current Date: 2016/10/28 03:53:00
Current SCN: 972801
SCN Headroom: 10722.16
Version: 11.2.0.4.0
--------------------------------------------------------------
Result: A - SCN Headroom is good
Apply the latest recommended patches
based on your maintenance schedule
For further information review MOS document id 1393363.1
--------------------------------------------------------------
等待一段时间再次查看，没有什么变化
SQL> /
--------------------------------------------------------------
ScnHealthCheck
--------------------------------------------------------------
Current Date: 2016/10/28 03:55:15
Current SCN: 973120
SCN Headroom: 10722.16
Version: 11.2.0.4.0
--------------------------------------------------------------
Result: A - SCN Headroom is good
Apply the latest recommended patches
based on your maintenance schedule
For further information review MOS document id 1393363.1
--------------------------------------------------------------
调用db_link后立即查看
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SQL> select * from emp@red12;
[oracle@red12 scripts]$ sqlplus / as sysdba
SQL> @SCNCHECK.txt;
--------------------------------------------------------------
ScnHealthCheck
--------------------------------------------------------------
Current Date: 2016/10/28 03:55:53
Current SCN: 5118552
SCN Headroom: 10722.16
Version: 11.2.0.4.0
--------------------------------------------------------------
Result: A - SCN Headroom is good
Apply the latest recommended patches
based on your maintenance schedule
For further information review MOS document id 1393363.1
--------------------------------------------------------------
总结：11.2.0.4 升 11.2.0.4.8之后，db_link对SCN同步的影响并没有消除；只创建db_link而不使用，则库之间的SCN不会同步；SCN从db_link调用时开始同步。