现象：
ADG无法切换：验证时就报错 ORA-16467
记录问题，顺便展现一次troubleshooting的心路历程。
具体查询：
在主库操作，
@primary
切换验证：
alter database switchover to demorac verify;
报错ORA-16467：
SQL> alter database switchover to demorac verify;
alter database switchover to demorac verify
*
ERROR at line 1:
ORA-16467: switchover target is not synchronized
主库alert告警日志：
ORA-16467 signalled during: alter database switchover to demorac verify...
主库传输链路并没有报错：
SQL> select error from v$archive_dest where dest_id= 2;
ERROR
-----------------------------------------------------------------
但是，如果去查v$archive_dest_status，就会发现问题，说有可解决的GAP：
SQL> select dest_id, status, gap_status from v$archive_dest_status where dest_id = 2;
DEST_ID STATUS GAP_STATUS
---------- --------- ------------------------
2 VALID RESOLVABLE GAP
但是去备库查询，
@standby,
ADG并没有任何延迟：
SQL> select * from v$dataguard_stats;
SOURCE_DBID SOURCE_DB_ NAME VALUE UNIT TIME_COMPUTED DATUM_TIME CON_ID
----------- ---------- ---------------------- ---------------------- ------------------------------ ------------------------------ ------------------------------ ----------
0 transport lag +00 00:00:00 day(2) to second(0) interval 05/11/2023 18:16:43 05/11/2023 18:16:41 0
0 apply lag +00 00:00:00 day(2) to second(0) interval 05/11/2023 18:16:43 05/11/2023 18:16:41 0
0 apply finish time day(2) to second(3) interval 05/11/2023 18:16:43 0
0 estimated startup time 18 second 05/11/2023 18:16:43 0
查MOS文档资料，有一个bug：
- Bug 33663444 - DataGuard: "alter database switchover verify" fails with ORA-16467 (Doc ID 33663444.8)
可是很快也排除掉：现象细节不完全匹配，另外这个bug正好在19.16已经修复：
[oracle@bogon ~]$ $ORACLE_HOME/OPatch/opatch lsinventory|grep 33663444
29353271, 29706141, 26352569, 33663444, 30476768, 30092280, 30843271
但这个文档给的一些解释中也得到了一些启发：
REDISCOVERY INFORMATION:
SELECT GAP_STATUS FROM V$ARCHIVE_DEST_STATUS with show unresolvable gap.
Workaround
temporarily re-enable the disabled redo thread
首先，我们查V$ARCHIVE_DEST_STATUS已经确认是resolvable gap，不匹配但是也有问题。
然后redo thread的re-enable，这个workaround让我去想到查询redo的thread，结果发现果然有些异常：
SQL> select * from v$log;
GROUP# THREAD# SEQUENCE# BYTES BLOCKSIZE MEMBERS ARC STATUS FIRST_CHANGE# FIRST_TIM NEXT_CHANGE# NEXT_TIME CON_ID
---------- ---------- ---------- ---------- ---------- ---------- --- ---------------- ------------- --------- ------------ --------- ----------
1 1 1132 209715200 512 1 NO CURRENT 44911942 11-MAY-23 9.2954E+18 0
2 1 1130 209715200 512 1 YES INACTIVE 44901958 11-MAY-23 44911931 11-MAY-23 0
3 1 1131 209715200 512 1 YES INACTIVE 44911931 11-MAY-23 44911942 11-MAY-23 0
4 2 0 104857600 512 1 YES UNUSED 0 0 0
5 2 0 104857600 512 1 YES UNUSED 0 0 0
我这里目前主库是单实例，而备库才是RAC，可是，为何主库的redo居然会有 thread#=2 的redo？
虽然都是unused，但出现在这里就很奇怪！
不知道谁动了这个环境，想不起来做过这样的测试。但可以肯定的是，完全可以把这个不该存在的thread删除掉！
直接删除会报错：
alter database drop logfile group 4;
alter database drop logfile group 5;
SQL> alter database drop logfile group 4;
alter database drop logfile group 4
*
ERROR at line 1:
ORA-01567: dropping log 4 would leave less than 2 log files for instance UNNAMED_INSTANCE_2 (thread 2)
ORA-00312: online log 4 thread 2: '/flash/fast_recovery_area/DEMO/onlinelog/o1_mf_4_kxn73zny_.log'
SQL> alter database drop logfile group 5;
alter database drop logfile group 5
*
ERROR at line 1:
ORA-01567: dropping log 5 would leave less than 2 log files for instance UNNAMED_INSTANCE_2 (thread 2)
ORA-00312: online log 5 thread 2: '/flash/fast_recovery_area/DEMO/onlinelog/o1_mf_5_kxn73zqd_.log'
删除不掉就尝试去先禁用掉这个没有用的thread 2，然后再次尝试删除：
alter database disable thread 2;
SQL> alter database disable thread 2;
Database altered.
SQL> select * from v$log;
GROUP# THREAD# SEQUENCE# BYTES BLOCKSIZE MEMBERS ARC STATUS FIRST_CHANGE# FIRST_TIM NEXT_CHANGE# NEXT_TIME CON_ID
---------- ---------- ---------- ---------- ---------- ---------- --- ---------------- ------------- --------- ------------ --------- ----------
1 1 1132 209715200 512 1 NO CURRENT 44911942 11-MAY-23 9.2954E+18 0
2 1 1130 209715200 512 1 YES INACTIVE 44901958 11-MAY-23 44911931 11-MAY-23 0
3 1 1131 209715200 512 1 YES INACTIVE 44911931 11-MAY-23 44911942 11-MAY-23 0
4 2 0 104857600 512 1 YES UNUSED 0 0 0
5 2 0 104857600 512 1 YES UNUSED 0 0 0
SQL> alter database drop logfile group 4;
Database altered.
SQL> alter database drop logfile group 5;
Database altered.
SQL>
SQL>
SQL> select * from v$log;
GROUP# THREAD# SEQUENCE# BYTES BLOCKSIZE MEMBERS ARC STATUS FIRST_CHANGE# FIRST_TIM NEXT_CHANGE# NEXT_TIME CON_ID
---------- ---------- ---------- ---------- ---------- ---------- --- ---------------- ------------- --------- ------------ --------- ----------
1 1 1132 209715200 512 1 NO CURRENT 44911942 11-MAY-23 9.2954E+18 0
2 1 1130 209715200 512 1 YES INACTIVE 44901958 11-MAY-23 44911931 11-MAY-23 0
3 1 1131 209715200 512 1 YES INACTIVE 44911931 11-MAY-23 44911942 11-MAY-23 0
删除成功！这次想来总该可以了吧？
SQL> alter database switchover to demorac verify;
alter database switchover to demorac verify
*
ERROR at line 1:
ORA-16467: switchover target is not synchronized
额，还是不行，但感觉再刺激刺激就OK了，继续尝试之前的十八般武艺（主要还是多切几次日志）：
SQL> select dest_id, status, gap_status from v$archive_dest_status where dest_id = 2;
DEST_ID STATUS GAP_STATUS
---------- --------- ------------------------
2 VALID RESOLVABLE GAP
SQL>
SQL> alter system switch logfile;
System altered.
SQL> select dest_id, status, gap_status from v$archive_dest_status where dest_id = 2;
DEST_ID STATUS GAP_STATUS
---------- --------- ------------------------
2 VALID RESOLVABLE GAP
SQL> alter system archive log current;
System altered.
SQL> select dest_id, status, gap_status from v$archive_dest_status where dest_id = 2;
DEST_ID STATUS GAP_STATUS
---------- --------- ------------------------
2 VALID RESOLVABLE GAP
SQL> alter system switch logfile;
alter system switch logfile;
System altered.
SQL>
System altered.
SQL>
SQL>
SQL>
SQL> alter system switch logfile;
System altered.
SQL> select dest_id, status, gap_status from v$archive_dest_status where dest_id = 2;
DEST_ID STATUS GAP_STATUS
---------- --------- ------------------------
2 VALID NO GAP
SQL>
哎呀，直接提示没有GAP了，赶紧尝试继续切换。。
SQL> alter database switchover to demorac verify;
alter database switchover to demorac verify
*
ERROR at line 1:
ORA-16475: succeeded with warnings, check alert log for more details
SQL> !date
Thu May 11 18:36:31 CST 2023
额？成功了，但是有警告，看告警日志又是一个新的ORA-16475 错误。
2023-05-11T18:36:14.906996+08:00
alter database switchover to demorac verify
2023-05-11T18:36:15.094516+08:00
SWITCHOVER VERIFY: Send VERIFY request to switchover target DEMORAC
SWITCHOVER VERIFY WARNING: switchover target temporary files are not the same with the primary. See switchover target's alert log for details.
ORA-16475 signalled during: alter database switchover to demorac verify...
哎呀，去看看备库的alert日志：
2023-05-11T18:41:29.407685+08:00
SWITCHOVER VERIFY BEGIN
SWITCHOVER VERIFY WARNING: primary database has 5 temporary files, this database has 4 temporary files. More temp files should be added to this database.
SWITCHOVER VERIFY COMPLETE
注意这个时间不太一致是因为两个机器时间不一样（差了5分钟），可以先不用管。
看问题本身是说临时文件，这无所谓，切换后可以自动创建。
快快来一把久违的切换吧！
--主库执行成功
alter database switchover to demorac;
--新主库demorac
alter database open;
--新备库demo
startup
recover managed standby database disconnect;
具体ADG切换参考：
嗯，终于OK了，也感觉肚子饿了，去点餐了。