Oracle 数据库的scn是系统识别号，递增的,它的保存位数为48位，就是说最大值是2的48次方. 系统每秒最大增加16times;1024数。
Oracle 数据库的scn是系统识别号，递增的,它的保存位数为48位，就是说最大值是2的48次方.
系统每秒最大增加16×1024数。
我们的数据库有很多，相互之间中dblink关联的很多。
而如使用A表使用dblink连接B库，A库的SCN的100，B库的SCN是300.在使用dblink连接操作一次后，A库的SCN立即变成300.两个库的SCN值会变成一致，，取的是两个数据库的SCN的最大值。
这就是导致了数据库SCN不在遵守每秒16×1024这个最大值的限制。
一个数据库群中，使用dblink相互关联后，所有的数据库SCN号变成SCN号最大的值。
问题来了。。。
数据库群中的版本会是各种各样的。有10g,也有11g
如果某一个版本的库有问题，SCN爆涨，会导致所有的数据库SCN暴涨。
如果SCN号增加到2的48次方，数据库将宕机，能不能再起得来都不知道。
在这之前，alert.log会有这样的错误提示：
************************************************************
Warning: The SCN headroom for this database is only 38 days!
************************************************************
说scn只能用38天了。
不幸的是，oracle 在2012年发布了一个补丁包，会导致这个bug出现。
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