今天是2014-05-27，实在无聊顺便研究一下x$kccle的内容吧。例如以下所有是自己分析和实验结果，真实可靠。
1、怎样获得v$log的底层表？我们能够通过autotrace完毕查看如：
SQL> set autotrace trace exp
SQL> set linesize 200
SQL> select * from v$log;
Execution Plan
----------------------------------------------------------
Plan hash value: 2536105608
--------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 197 | 0 (0)| 00:00:01 |
| 1 | NESTED LOOPS | | 1 | 197 | 0 (0)| 00:00:01 |
|* 2 | FIXED TABLE FULL | X$KCCLE | 1 | 148 | 0 (0)| 00:00:01 |
|* 3 | FIXED TABLE FIXED INDEX| X$KCCRT (ind:1) | 1 | 49 | 0 (0)| 00:00:01 |
--------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter("LE"."LEDUP"<>0 AND "LE"."INST_ID"=USERENV('INSTANCE'))
3 - filter("RT"."INST_ID"=USERENV('INSTANCE') AND "LE"."LETHR"="RT"."RTNUM")
SQL>
能够知道当我们在查看v$log视图的时候，事实上就是查看的x$kccle视图字段信息。
查看该视图例如以下：
SQL> desc x$kccle
Name Null? Type
----------------------------------------- -------- ----------------------------
ADDR RAW(8)
INDX NUMBER
INST_ID NUMBER 实例号
LENUM NUMBER 相似group_number
LESIZ NUMBER logfile的size，是操作系统的blocks数目
LESEQ NUMBER v$log中的SEQUENCE#
LEHWS NUMBER
LEBSZ NUMBER block size（操作系统的块）
LENAB NUMBER
LEFLG NUMBER 表示日志的状态信息
LETHR NUMBER v$log中的THREAD#
LELFF NUMBER 下一个logfile
LELFB NUMBER 前一个logfile
LELOS VARCHAR2(16) low scn相似v$log FIRST_CHANGE#
LELOT VARCHAR2(20) low time相似v$log FIRST_TIME
LENXS VARCHAR2(16) nex scn 相似v$log中下一个logfile的low FIRST_CHANGE#同样
LENXT VARCHAR2(20) nex time相似v$log中下一个logfile的low FIRST_TIME 同样
LEPVS VARCHAR2(16) prev scn, 与上一个logfile的low scn
LEARF NUMBER archive link forward
LEARB NUMBER archive link backward
LEFNH NUMBER
LEFNT NUMBER
LEDUP NUMBER 等同于v$log中members
SQL> 以下对照一下：
SQL> select * from v$log;
GROUP# THREAD# SEQUENCE# BYTES MEMBERS ARC STATUS FIRST_CHANGE# FIRST_TIME
---------- ---------- ---------- ---------- ---------- --- ---------------- ------------- ------------
1 1 2944 104857600 1 YES INACTIVE 121424676 21-MAY-14
2 1 2945 104857600 1 NO CURRENT 121427428 21-MAY-14
3 1 2943 104857600 1 YES INACTIVE 121421889 21-MAY-14
SQL>
SQL> select inst_id,lenum,lesiz,leseq,lehws,lebsz,lebsz*lesiz log_size,lenab,leflg,lethr,lelff,lelfb,lelos from x$kccle;
INST_ID LENUM LESIZ LESEQ LEHWS LEBSZ LOG_SIZE LENAB LEFLG LETHR LELFF LELFB LELOS
---------- ---------- ---------- ---------- ---------- ---------- ---------- ---------- ---------- ---------- ---------- ---------- ----------------
1 1 204800 2944 2 512 104857600 190256 1 1 2 0 121424676
1 2 204800 2945 1 512 104857600 4294967295 8 1 3 1 121427428
1 3 204800 2943 2 512 104857600 190256 1 1 0 2 121421889
SQL>
SQL> select inst_id,lenum,leseq,lethr,lelot,lenxs,lenxt,lepvs,learf,learb,lefnh,lefnt,ledup from x$kccle;
INST_ID LENUM LESEQ LETHR LELOT LENXS LENXT LEPVS LEARF LEARB LEFNH LEFNT LEDUP
---------- ----- ----- ---------- -------------------- ---------------- -------------------- ---------------- ---------- ---------- ---------- ---------- ----------
1 1 2944 1 05/21/2014 11:26:42 121427428 05/21/2014 11:26:55 121421889 0 0 1 1 1
1 2 2945 1 05/21/2014 11:26:55 281474976710655 121424676 0 0 2 2 1
1 3 2943 1 05/21/2014 11:26:30 121424676 05/21/2014 11:26:42 121419052 0 0 3 3 1
SQL> 如今单独关注一下leflg字段，当该值为8说明是正在使用的logfile group：
dump出控制文件里的log file信息例如以下：
SQL> select * from v$log;
GROUP# THREAD# SEQUENCE# BYTES BLOCKSIZE MEMBERS ARC STATUS FIRST_CHANGE# FIRST_TIM NEXT_CHANGE# NEXT_TIME
---------- ---------- ---------- ---------- ---------- ---------- --- ---------------- ------------- --------- ------------ ---------
1 1 19 52428800 512 2 NO CURRENT 1013368 27-MAY-14 2.8147E+14
2 1 17 52428800 512 2 NO ACTIVE 1013348 27-MAY-14 1013365 27-MAY-14
3 1 18 52428800 512 2 NO ACTIVE 1013365 27-MAY-14 1013368 27-MAY-14
SQL> oradebug setmypid
Statement processed.
SQL> oradebug dump controlf 12
Statement processed.
注意：也能够使用alter system set events ‘immediate trace name controlf level 12';完毕控制文件的dump操作：
LOG FILE RECORDS
***************************************************************************
(size = 72, compat size = 72, section max = 16, section in-use = 3,
last-recid= 17, old-recno = 0, last-recno = 0)
(extent = 1, blkno = 10, numrecs = 16)
LOG FILE #1:
name #5: /u01/app/oracle/oradata/RHYS/onlinelog/o1_mf_1_9r1z70w0_.log
name #6: /u01/app/oracle/fast_recovery_area/RHYS/onlinelog/o1_mf_1_9r1z70z2_.log
Thread 1 redo log links: forward: 2 backward: 0
siz: 0x19000 seq: 0x00000013 hws: 0x1 bsz: 512 nab: 0xffffffff flg: 0x8 dup: 2
Archive links: fwrd: 0 back: 0 Prev scn: 0x0000.000f7675
Low scn: 0x0000.000f7678 05/27/2014 17:54:08
Next scn: 0xffff.ffffffff 01/01/1988 00:00:00
LOG FILE #2:
name #3: /u01/app/oracle/oradata/RHYS/onlinelog/o1_mf_2_9r1z726s_.log
name #4: /u01/app/oracle/fast_recovery_area/RHYS/onlinelog/o1_mf_2_9r1z7291_.log
Thread 1 redo log links: forward: 3 backward: 1
siz: 0x19000 seq: 0x00000011 hws: 0x2 bsz: 512 nab: 0x14 flg: 0x0 dup: 2
Archive links: fwrd: 0 back: 0 Prev scn: 0x0000.000f75ec
Low scn: 0x0000.000f7664 05/27/2014 17:54:01
Next scn: 0x0000.000f7675 05/27/2014 17:54:05
LOG FILE #3:
name #1: /u01/app/oracle/oradata/RHYS/onlinelog/o1_mf_3_9r1z73jf_.log
name #2: /u01/app/oracle/fast_recovery_area/RHYS/onlinelog/o1_mf_3_9r1z73lp_.log
Thread 1 redo log links: forward: 0 backward: 2
siz: 0x19000 seq: 0x00000012 hws: 0x2 bsz: 512 nab: 0x4 flg: 0x0 dup: 2
Archive links: fwrd: 0 back: 0 Prev scn: 0x0000.000f7664
Low scn: 0x0000.000f7675 05/27/2014 17:54:05
Next scn: 0x0000.000f7678 05/27/2014 17:54:08
相应一下：
SQL> select inst_id,lenum,leseq,lethr,lelot,lenxs,lenxt,lepvs,learf,learb,lefnh,lefnt,ledup from x$kccle;
INST_ID LENUM LESEQ LETHR LELOT LENXS LENXT LEPVS LEARF LEARB LEFNH LEFNT LEDUP
---------- ----- ----- ---------- -------------------- ---------------- -------------------- ---------------- ---------- ---------- ---------- ---------- ----------
1 1 19 1 05/27/2014 17:54:08 281474976710655 1013365 0 0 5 6 2
1 2 17 1 05/27/2014 17:54:01 1013365 05/27/2014 17:54:05 1013228 0 0 3 4 2
1 3 18 1 05/27/2014 17:54:05 1013368 05/27/2014 17:54:08 1013348 0 0 1 2 2
SQL> select inst_id,lenum,lesiz,leseq,lehws,lebsz,lebsz*lesiz log_size,lenab,leflg,lethr,lelff,lelfb,lelos from x$kccle;
INST_ID LENUM LESIZ LESEQ LEHWS LEBSZ LOG_SIZE LENAB LEFLG LETHR LELFF LELFB LELOS
---------- ----- ---------- ----- ---------- ---------- ---------- ---------- ---------- ---------- ---------- ---------- ----------------
1 1 102400 19 1 512 52428800 4294967295 8 1 2 0 1013368
1 2 102400 17 2 512 52428800 20 0 1 3 1 1013348
1 3 102400 18 2 512 52428800 4 0 1 0 2 1013365
SQL> 拿group3做一个分析：例如以下：
LOG FILE #3:（代表第三个group）
name #1: /u01/app/oracle/oradata/RHYS/onlinelog/o1_mf_3_9r1z73jf_.log（第一个日志文件）
name #2: /u01/app/oracle/fast_recovery_area/RHYS/onlinelog/o1_mf_3_9r1z73lp_.log（第二个日志文件）
Thread 1（线程1x$kccle.lethr） redo log links: forward: 0（相应x$kccle的LELFF为0) backward: 2(相应x$kccle的lelfb为2)
siz: 0x19000(相应x$kccle的lesiz转为10进制正好是102400）： seq: 0x00000012（转为10进制为18相应x$kccle的leseq）： hws: 0x2（相应x$kccle的lehws为2） bsz: 512（数据块相应x$kccle的lebsz）
nab: 0x4（相应lenab为4） flg: 0x0（相应x$kccle的leflg） dup: 2（相应x$kccle的ledup）
Archive links: fwrd: 0（前一个链表x$kccle的learf) back: 0 (相应x$kccle的learb）Prev scn: 0x0000.000f7664（转为10进制scn为1013348。相应lepvs)
Low scn: 0x0000.000f7675 05/27/2014 17:54:05(相应x$kccle的lelos）
Next scn: 0x0000.000f7678 05/27/2014 17:54:08（相应x$kccle的lenxt）
由此可知x$kccle正式抓取的是控制文件里的内容，这也是为什么数据块在mount状态下。能够查看v$log的原因了。