相信近几个月好些DBA一定都被SCN compat(兼容级别)在2019年6月23日自动从1直接跳级到3的问题搞的紧张兮兮， 现在这个特殊日期已经过去几天，不知道是不是觉的风平浪静有些失望， 不要以为这事已过去，恰恰是问题才刚刚开始，SCN已经rollover的有可能会更快速的使用，更多原理以前有写过《 预警：2019年ORACLE SCN 兼容性特性( Compatibility)自动改变的影响》。
最近应该都开始检查是否SCN Compat是否已自动变为3， 在auto rollover未禁用的情况下，还是有些情况下SCN compat当前并没有改变，下面列几种情况。
检查脚本 scn_compat_check.sql:
set serveroutput on
declare
rsl number;
headroom_in_scn number;
headroom_in_sec number;
CUR_SCN_COMPAT number;
MAX_SCN_COMPAT number;
auto_rollover_ts date;
target_compat number ;
is_enabled boolean;
VERSION varchar2(100);
is_rolloverd boolean;
db_role varchar2(100);
started_ts date;
begin
select banner into VERSION from v$version where rownum=1;
select DATABASE_ROLE into db_role from v$database;
select startup_time into started_ts from v$instance;
dbms_scn.GETCURRENTSCNPARAMS(rsl,headroom_in_scn,HEADROOM_IN_SEC,CUR_SCN_COMPAT,MAX_SCN_COMPAT);
dbms_output.put_line('Current datatime:'||to_char(sysdate,'RRRRmmdd hh24:mi:ss'));
dbms_output.put_line('Oracle Version:'||VERSION);
dbms_output.put_line('Database role:' ||db_role);
dbms_output.put_line('Instance starttime: '||to_char(started_ts,'RRRRmmdd hh24:mi:ss'));
dbms_output.put_line('RSL=' ||rsl);
dbms_output.put_line('headroom_in_scn=' || headroom_in_scn);
dbms_output.put_line('headroom_in_sec=' || headroom_in_sec);
dbms_output.put_line('CUR_SCN_COMPAT=' ||CUR_SCN_COMPAT);
dbms_output.put_line('MAX_SCN_COMPAT=' ||MAX_SCN_COMPAT);
dbms_scn.getscnautorolloverparams (auto_rollover_ts, target_compat, is_enabled);
dbms_output.put_line( 'auto_rollover_ts='|| to_char(auto_rollover_ts, 'YYYY-MM-DD' ));
dbms_output.put_line( 'target_compat='||target_compat);
if(is_enabled) then
dbms_output.put_line(' Auto_rollover is enabled!' );
if CUR_SCN_COMPAT = target_compat then
dbms_output.put_line('SCN compat had Auto rollover !' );
end if;
if CUR_SCN_COMPAT < target_compat and sysdate > auto_rollover_ts then
dbms_output.put_line('SCN compat No Auto_rollover !' );
-- standby or read-only database
if started_ts<auto_rollover_ts and db_role='PHYSICAL STANDBY' then
dbms_output.put_line('Tip: Restart Instance SCN compat will rollover automatic.' );
end if;
end if ;
else
dbms_output.put_line( ' Auto_rollover is disabled!');
end if;
end;
/
1， online redo log never switch until 20190623
SQL> @scn_compat_check.sql Current datatime:20190624 18:30:32 Oracle Version:Oracle Database 12c Enterprise Edition Release 12.2.0.1.0 - 64bit Production RSL=16577505394688 headroom_in_scn=124244680842 headroom_in_sec=7583293 CUR_SCN_COMPAT=1 MAX_SCN_COMPAT=3 auto_rollover_ts=2019-06-23 target_compat=3 Auto_rollover is enabled! SCN compat No Auto_rollover !
第二天再查时
SQL> @scn_compat_check.sql Current datatime:20190625 11:05:40 Oracle Version:Oracle Database 12c Enterprise Edition Release 12.2.0.1.0 - 64bit Production RSL=35501998735360 headroom_in_scn=19048737747293 headroom_in_sec=193773780 CUR_SCN_COMPAT=3 MAX_SCN_COMPAT=3 auto_rollover_ts=2019-06-23 target_compat=3 Auto_rollover is enabled! SCN compat had Auto rollover ! PL/SQL procedure successfully completed. SQL> @log Show redo log layout from V$LOG and V$LOGFILE... GROUP# THREAD# SEQUENCE# BYTES BLOCKSIZE MEMBERS ARC STATUS FIRST_CHANGE# FIRST_TIME ---------- ---------- ---------- ---------- ---------- ---------- --- ---------------- ------------------ ----------------- 1 1 69 4294967296 512 1 NO INACTIVE 16451560456642 20190612 11:36:44 2 1 70 4294967296 512 1 NO ACTIVE 16453257001681 20190615 09:09:43 3 1 71 4294967296 512 1 NO CURRENT 16453260987629 20190625 11:04:02 <<<< 4 1 68 4294967296 512 1 NO INACTIVE 16402315721466 20190607 01:00:33 5 2 5 4294967296 512 1 NO CURRENT 16402316741755 20190609 20:00:09 6 2 2 4294967296 512 1 NO INACTIVE 15521625611890 20190416 10:59:46 7 2 3 4294967296 512 1 NO INACTIVE 15521625904274 20190416 17:48:10 8 2 4 4294967296 512 1 NO INACTIVE 16402306508842 20190513 05:00:36
DB alert log
2019-06-25 11:04:02.353000 +08:00 Thread 1 advanced to log sequence 71 (LGWR switch) Current log# 3 seq# 71 mem# 0: +DATADG/SPDB/ONLINELOG/group_3.260.1004432869 Database SCN compatibility auto-rollover - control file update SCN compatibility changed from 1 to 3 (auto-rollover) 2019-06-25 11:05:48.487000 +08:00
Note:
这是一套比较空闲的数据库，所以online redo log 也一致未切换，直到06/25日志切换时才自动SCN compatibility changed from 1 to 3。后来经测试的确可以手动切换online redo , SQL : alter system switch logfile，触发没有自动rollover的实例改变。
2, Physical Standby database or open read-only
SQL> @scn_compat_check.sql Current datatime:20190626 14:47:10 Oracle Version:Oracle Database 12c Enterprise Edition Release 12.2.0.1.0 - 64bit Production Database role:PHYSICAL STANDBY Instance starttime: 20190621 11:24:10 RSL=16580116971520 headroom_in_scn=117142463378 headroom_in_sec=7149808 CUR_SCN_COMPAT=1 MAX_SCN_COMPAT=3 auto_rollover_ts=2019-06-23 target_compat=3 Auto_rollover is enabled! SCN compat No Auto_rollover ! Tip: Restart Instance SCN compat will rollover automatic. SQL> alter database recover managed standby database cancel; Database altered. SQL> shutdown immediate Database closed. Database dismounted. ORACLE instance shut down. SQL> startup ORACLE instance started. Total System Global Area 6.4425E+10 bytes Fixed Size 29967376 bytes Variable Size 3.2078E+10 bytes Database Buffers 3.2212E+10 bytes Redo Buffers 104247296 bytes Database mounted. Database opened. SQL> @sch_compat_check.sql Current datatime:20190626 14:49:09 Oracle Version:Oracle Database 12c Enterprise Edition Release 12.2.0.1.0 - 64bit Production Database role:PHYSICAL STANDBY Instance starttime: 20190626 14:48:36 RSL=35511810359296 headroom_in_scn=19048835508702 headroom_in_sec=193774775 CUR_SCN_COMPAT=3 MAX_SCN_COMPAT=3 auto_rollover_ts=2019-06-23 target_compat=3 Auto_rollover is enabled! SCN compat had Auto rollover ! PL/SQL procedure successfully completed.
Note:
对于ADG环境，如果实例是06/23以前启动的，需要重启一下实例，然后SCN compat会自动rollover到3，(open read-only 环境如果存在该现象同样重启instance可以解决)。