昨天有人問到重新創建控制文件后Oracle如何知道該從那么日志開始恢復,這里牽涉到一些概念,我這里一并做答了. 首先,我們知道重新創建了控制文件后所有的歸檔信息都無法在控制文件里面找到了,那么oracle怎么判定從哪個日志開始恢復呢. 我們做一個controlfile dump來看一下*** 2005-07-27 10:52:25.931
*** SERVICE NAME:() 2005-07-27 10:52:25.931
*** session ID:(159.5) 2005-07-27 10:52:25.931
DUMP OF CONTROL FILES, Seq # 439 = 0x1b7
V10 STYLE FILE HEADER:
Compatibility Vsn = 169869568=0xa200100
Db ID=956585232=0x39045510, Db Name='DBTEST'
Activation ID=0=0x0
Control Seq=439=0x1b7, File size=450=0x1c2
File Number=0, Blksiz=16384, File Type=4 BACKUP CONTROL***************************************************************************
DATABASE ENTRY
***************************************************************************
(size = 316, compat size = 316, section max = 1, section in-use = 1,
last-recid= 0, old-recno = 0, last-recno = 0)
(extent = 1, blkno = 1, numrecs = 1)
07/27/2005 10:39:06
DB Name "DBTEST"
Database flags = 0x00400107 0x00001000
Controlfile Creation Timestamp 07/27/2005 10:39:07
Incmplt recovery scn: 0x0000.000a75f8
Resetlogs scn: 0x0000.0006ce7b Resetlogs Timestamp 07/15/2005 13:33:07
PRior resetlogs scn: 0x0000.00000001 Prior resetlogs Timestamp 06/30/2005 19:09:40
Redo Version: compatible=0xa200100
#Data files = 4, #Online files = 4
Database checkpoint: Thread=0 scn: 0x0000.00000000
Threads: #Enabled=1, #Open=0, Head=0, Tail=0 省略一些內容......***************************************************************************
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: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: 564752585 mount id: 957645365 可以看到重新創建控制文件后CHECKPOINT PROGRESS RECORDS這一塊內容都被清零了***************************************************************************
ARCHIVED LOG RECORDS
***************************************************************************
(size = 584, compat size = 584, section max = 308, section in-use = 0,
last-recid= 0, old-recno = 0, last-recno = 0)
(extent = 1, blkno = 97, numrecs = 308)
歸檔信息也被清除了 我們試一下recover databaseSQL> recover database using backup controlfile;
ORA-00279: change 685560 generated at 07/25/2005 16:12:30 needed for thread 1
ORA-00289: suggestion : /opt/oracle/archive/dbtest/1_7_563722387.dbf
ORA-00280: change 685560 for thread 1 is in sequence #7Specify log: {<RET>=suggested filename AUTO CANCEL} 當我們進行recover時oracle提示需要1_7_563722387.dbf這個歸檔,scn為685560 ,但是控制文件里面并沒有任何關于歸檔的記載.答案在數據文件頭里面,來做一個datafile header dump ALTER SESSION SET EVENTS 'immediate trace name file_hdrs level 3; 可以在datafile header dump里面發現Tablespace #0 - SYSTEM rel_fn:1
Creation at scn: 0x0000.00000009 06/30/2005 19:10:11
Backup taken at scn: 0x0000.00000000 01/01/1988 00:00:00 thread:0
reset logs count:0x2199b893 scn: 0x0000.0006ce7b reset logs terminal rcv data:0x0 scn: 0x0000.00000000
prev reset logs count:0x2184ef74 scn: 0x0000.00000001 prev reset logs terminal rcv data:0x0 scn: 0x0000.00000000
recovered at 07/27/2005 11:25:03
status:0x2000 root dba:0x00400179 chkpt cnt: 55 ctl cnt:54
begin-hot-backup file size: 0
Checkpointed at scn: 0x0000.000a75f8 07/25/2005 16:12:30
thread:1 rba:(0x7.384.10) 這里的rba代表了redo block addres,分為3段 分別由 the log file sequence number (4 bytes) the log file block number (4 bytes) the byte offset into the block at which the redo record starts (2 bytes) 組成,從這里我們也可以看出datafile#1需要從sequence 7的日志文件開始恢復,所以oracle可以知道歸檔名字是"1_7_563722387.dbf",至于這里的563722387則代表了reset logs count,由于10g默認的log_archive_format為 %t_%s_%r.arc,所以這里多了一個563722387,在datafile header dump里面可以發現. reset logs count:0x2199b893 oracle綜合這一系列信息得出了需要歸檔文件.
