关于oracle db 11gR2版本号上的_external_scn_rejection_threshold_hours參数和scn headroom补丁问题
来自于：
Installing, Executing and Interpreting output from the "scnhealthcheck.sql" script (文档 ID 1393363.1)
第一点：
In addition to the above result the script output may advise to set the hidden parameter "_external_scn_rejection_threshold_hours" on some Oracle versions. The following text gives more information about setting this parameter:
Set _external_scn_rejection_threshold_hours=24 after apply
Oracle recommends setting this parameter to the value 24 in 10g and 11.1 releases - it does not need to be set in 11.2 or later releases. The parameter is static and so must be set in the init.ora or spfile used to start the instance.
eg:
In init.ora:
# Set threshold on dd/mon/yyyy - See MOS Document 1393363.1
_external_scn_rejection_threshold_hours = 24
In the spfile:
alter system set "_external_scn_rejection_threshold_hours" = 24
comment='Set threshold on dd/mon/yyyy - See MOS Document 1393363.1'
scope=spfile ;
第二点：
来自于：
Installing, Executing and Interpreting output from the "scnhealthcheck.sql" script (文档 ID 1393363.1)
第一点：
In addition to the above result the script output may advise to set the hidden parameter "_external_scn_rejection_threshold_hours" on some Oracle versions. The following text gives more information about setting this parameter:
Set _external_scn_rejection_threshold_hours=24 after apply
Oracle recommends setting this parameter to the value 24 in 10g and 11.1 releases - it does not need to be set in 11.2 or later releases. The parameter is static and so must be set in the init.ora or spfile used to start the instance.
eg:
In init.ora:
# Set threshold on dd/mon/yyyy - See MOS Document 1393363.1
_external_scn_rejection_threshold_hours = 24
In the spfile:
alter system set "_external_scn_rejection_threshold_hours" = 24
comment='Set threshold on dd/mon/yyyy - See MOS Document 1393363.1'
scope=spfile ;
从以上能够看出，在 11.2及其更高的版本号上，是无需设置_external_scn_rejection_threshold_hours參数的。
实际上。在11.2.0.4的db软件中，该參数已经出如今隐含參数中了。
第二点：
Please note that there are no SCN hardening fixes for the following versions. The recommendation for these versions is to upgrade to a newer release: All versions up to and including 9.2.0.7 Versions 10.1.0.2 to 10.1.0.4 inclusive Versions 10.2.0.1 and 10.2.0.2 Version 11.1.0.6 Version 11.2.0.1从这句话能够看出：以上版本号的oracle数据库软件没有修复scn headroom 的补丁。请升级到更新的版本号。这一招够狠的。
Versions 11.2.0.4 and 12.1.0.1 onwards already include the main SCN fixes but the "scnhealthcheck" script described above can still be used on those releases to check the current headroom.从这句话能够看出，在11.2.0.4及其12.1.0.1版本号上，已经包含了对scn问题的修复（fixes）,说白了。11.2.0.4及其12.1.0.1的程序版本号已经修复了scn headroom问题。