增量检查点如何更新控制文件?

有同学在 T.askmaclean.com上提问关于增量检查点更新控制文件的问题：    

Know more about checkpoint checkpoint 分成很多种 full 、file、thread、parallel query、 object 、incremental 、logfile switch 每一种checkpoint 都有其自身的特性，例如Incremental Checkpoint会要求ckpt 每3s 更新一次controlfile 但是不更新datafile header， 而FULL CHECKPOINT要求立即完成(同步的) 且会同时更新 controlfile 和 datafile header。 Incremental Checkpoint会要求ckpt 每3s 更新一次controlfile >>我想问的时：如何查看此时控制文件中更新的SCN?除了DUMP控制文件，有没有命令查询?

      我希望通过以下演示说明该问题：    

SQL> select * from v$version;

BANNER
----------------------------------------------------------------
Oracle Database 10g Enterprise Edition Release 10.2.0.5.0 - 64bi
PL/SQL Release 10.2.0.5.0 - Production
CORE    10.2.0.5.0      Production
TNS for Linux: Version 10.2.0.5.0 - Production
NLSRTL Version 10.2.0.5.0 - Production

SQL> oradebug setmypid;
Statement processed.

SQL> oradebug dump controlf 4;
Statement processed.

SQL> oradebug tracefile_name ;
/s01/admin/G10R25/udump/g10r25_ora_4660.trc

另开一个窗口等待6s在做一次controlf DUMP

SQL>  exec dbms_lock.sleep(6);
oradebug setmypid;
oradebug dump controlf 4;
oradebug tracefile_name ;

PL/SQL procedure successfully completed.

SQL> Statement processed.
SQL> Statement processed.
SQL> /s01/admin/G10R25/udump/g10r25_ora_4663.trc

    比较以上获得的2个前后有6s间隔的CONTROLFILE DUMP 文件：    

[oracle@vrh8 udump]$ diff /s01/admin/G10R25/udump/g10r25_ora_4660.trc /s01/admin/G10R25/udump/g10r25_ora_4663.trc
1c1
< /s01/admin/G10R25/udump/g10r25_ora_4660.trc --- > /s01/admin/G10R25/udump/g10r25_ora_4663.trc
13c13
< Unix process pid: 4660, image: oracle@vrh8.oracle.com (TNS V1-V3) --- > Unix process pid: 4663, image: oracle@vrh8.oracle.com (TNS V1-V3)
15,18c15,19
< *** ACTION NAME:() 2012-07-22 07:59:08.215
< *** MODULE NAME:(sqlplus@vrh8.oracle.com (TNS V1-V3)) 2012-07-22 07:59:08.215
< *** SERVICE NAME:(SYS$USERS) 2012-07-22 07:59:08.215
< *** SESSION ID:(159.7) 2012-07-22 07:59:08.215 --- > *** 2012-07-22 07:59:31.779
> *** ACTION NAME:() 2012-07-22 07:59:31.779
> *** MODULE NAME:(sqlplus@vrh8.oracle.com (TNS V1-V3)) 2012-07-22 07:59:31.779
> *** SERVICE NAME:(SYS$USERS) 2012-07-22 07:59:31.779
> *** SESSION ID:(159.9) 2012-07-22 07:59:31.779
96,98c97,99
< THREAD #1 - status:0x2 flags:0x0 dirty:56
< low cache rba:(0x1a.3.0) on disk rba:(0x1a.121.0)
< on disk scn: 0x0000.013fe7a8 07/22/2012 07:59:02 --- > THREAD #1 - status:0x2 flags:0x0 dirty:57
> low cache rba:(0x1a.3.0) on disk rba:(0x1a.148.0)
> on disk scn: 0x0000.013fe7c2 07/22/2012 07:59:27
100,101c101,102
< heartbeat: 789262462 mount id: 2675014163
< Flashback log tail log# 15 thread# 1 seq 229 block 274 byte 0 --- > heartbeat: 789262470 mount id: 2675014163
> Flashback log tail log# 15 thread# 1 seq 229 block 275 byte 0
2490c2491
<   V$RMAN_STATUS: recid=140734752341296, stamp=140734752341288 --- >   V$RMAN_STATUS: recid=140733792718464, stamp=140733792718456
2501c2502
<   V$RMAN_STATUS: recid=140734752341296, stamp=140734752341288 --- >   V$RMAN_STATUS: recid=140733792718464, stamp=140733792718456
2511c2512
<   V$RMAN_STATUS: recid=140734752341296, stamp=140734752341288 --- >   V$RMAN_STATUS: recid=140733792718464, stamp=140733792718456
2521c2522
<   V$RMAN_STATUS: recid=140734752341296, stamp=140734752341288 --- >   V$RMAN_STATUS: recid=140733792718464, stamp=140733792718456
2531c2532
<   V$RMAN_STATUS: recid=140734752341296, stamp=140734752341288 --- >   V$RMAN_STATUS: recid=140733792718464, stamp=140733792718456

      排除部分V$RMAN_STATUS记录存在差异外，最主要的差别在于： CHECKPOINT PROGRESS RECORDS 这是因为 ckpt 每3s一次对controlfile做heartbeat 更新 CHECKPOINT PROGRESS RECORDS。 第一次 controlf dump：   *************************************************************************** CHECKPOINT PROGRESS RECORDS *************************************************************************** (size = 8180, compat size = 8180, section max = 11, section in-use = 0, last-recid= 0, old-recno = 0, last-recno = 0) (extent = 1, blkno = 2, numrecs = 11) THREAD #1 - status:0x2 flags:0x0 dirty:56 low cache rba:(0x1a.3.0) on disk rba:(0x1a.121.0) on disk scn: 0x0000.013fe7a8 07/22/2012 07:59:02 resetlogs scn: 0x0000.01394f1a 07/19/2012 07:27:21 heartbeat: 789262462 mount id: 2675014163 Flashback log tail log# 15 thread# 1 seq 229 block 274 byte 0 THREAD #2 - status:0x0 flags:0x0 dirty:0 low cache rba:(0x0.0.0) on disk rba:(0x0.0.0) on disk scn: 0x0000.00000000 01/01/1988 00:00:00 resetlogs scn: 0x0000.00000000 01/01/1988 00:00:00 heartbeat: 0 mount id: 0 Flashback log tail log# 0 thread# 0 seq 0 block 0 byte 0     第二次 controlf dump:   *************************************************************************** CHECKPOINT PROGRESS RECORDS *************************************************************************** (size = 8180, compat size = 8180, section max = 11, section in-use = 0, last-recid= 0, old-recno = 0, last-recno = 0) (extent = 1, blkno = 2, numrecs = 11) THREAD #1 - status:0x2 flags:0x0 dirty:57 low cache rba:(0x1a.3.0) on disk rba:(0x1a.148.0) on disk scn: 0x0000.013fe7c2 07/22/2012 07:59:27 resetlogs scn: 0x0000.01394f1a 07/19/2012 07:27:21 heartbeat: 789262470 mount id: 2675014163 Flashback log tail log# 15 thread# 1 seq 229 block 275 byte 0 THREAD #2 - status:0x0 flags:0x0 dirty:0 low cache rba:(0x0.0.0) on disk rba:(0x0.0.0) on disk scn: 0x0000.00000000 01/01/1988 00:00:00 resetlogs scn: 0x0000.00000000 01/01/1988 00:00:00   差异在于 on disk rba on disk scn heartbeat Flashback log tail log#   即实际CKPT每3s更新heartbeat控制文件一次，更新的内容是 on disk rba、on disk scn、heartbeat 如果启用了闪回日志的话那么还有Flashback log ， 而并不更新数据库当前的SCN(CURRENT SCN)。   如果你想查看ckpt每3s更新的 on disk scn的话可以参考 内部视图X$KCCCP--[K]ernel [C]ache [C]ontrolfile management [c]heckpoint [p]rogress