实例恢复的深入解析
| 什么时候会产生实例恢复呢？当你数据库服务器异常断电，重启数据库就会发生实例恢复。实例恢复是由数据库自动完成的，无须DBA的干涉。当然这里有个前提条件：数据文件、在线日志文件、控制文件不得有损坏。 我们用实验来分析一下实例恢复的整个过程吧！ 1、在关闭数据库前，我们先看一下几个检查点的SCN SQL> select checkpoint_change# from v$database; CHECKPOINT_CHANGE# ------------------ 1455180 --控制文件中保存的数据库检查点SCN号实际上在所有数据文件头部中最小的检查点SCN SQL> select file#,checkpoint_change# from v$datafile; FILE# CHECKPOINT_CHANGE# ---------- ------------------ 1 1455180 2 1455180 3 1455180 4 1455180 5 1455180 6 1455180 --控制文件中保存的数据文件检查点SCN:当一个检查点动作完成之后，Oracle就把每个数据文件的scn单独存放在控制文件中 SQL> select file#,checkpoint_change# from v$datafile_header; FILE# CHECKPOINT_CHANGE# ---------- ------------------ 1 1455180 2 1455180 3 1455180 4 1455180 5 1455180 6 1455180 --每个数据文件的文件头中的检查点SCN 这三个检查点的SCN一致，接下来模拟异常断电，重启机器 2、此命令可以模拟异常断电 SQL> shutdown abort; ORACLE instance shut down. 3、监控告警日志 [oracle@guoyj trace]$ tail -f alert_bxocp.log Starting background process VKRM Tue Dec 11 22:54:41 2012 VKRM started with pid=24, OS id=12500 Tue Dec 11 22:58:11 2012 Shutting down instance (abort) License high water mark = 3 USER (ospid: 12479): terminating the instance Instance terminated by USER, pid = 12479 Tue Dec 11 22:58:12 2012 Instance shutdown complete 4、数据库启动到MOUNT状 SQL> shutdown abort; ORACLE instance shut down. SQL> startup mount; ORACLE instance started. Total System Global Area 839282688 bytes Fixed Size 2233000 bytes Variable Size 524291416 bytes Database Buffers 310378496 bytes Redo Buffers 2379776 bytes Database mounted. 5、再确定一下这个时间的检查点SCN SQL> select checkpoint_change# from v$database; CHECKPOINT_CHANGE# ------------------ 1455180 SQL> SQL> select file#,checkpoint_change# from v$datafile; FILE# CHECKPOINT_CHANGE# ---------- ------------------ 1 1455180 2 1455180 3 1455180 4 1455180 5 1455180 6 1455180 6 rows selected. SQL> select file#,checkpoint_change# from v$datafile_header; FILE# CHECKPOINT_CHANGE# ---------- ------------------ 1 1455180 2 1455180 3 1455180 4 1455180 5 1455180 6 1455180 发现与异常断电前的检查点的SCN一致，这里一致无须介质恢复。 先不着急open数据库，我们做一些dump 6、dump的控制文件 alter session set events 'immediate trace name CONTROLF level 12'; 取部分内容： *************************************************************************** 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) 12/07/2012 10:36:14 DB Name "BXOCP" Database flags = 0x00404000 0x00001000 Controlfile Creation Timestamp 12/07/2012 10:36:15 Incmplt recovery scn: 0x0000.00000000 Resetlogs scn: 0x0000.000f30dc Resetlogs Timestamp 12/07/2012 10:36:16 Prior resetlogs scn: 0x0000.00000001 Prior resetlogs Timestamp 09/17/2011 09:46:04 Redo Version: compatible=0xb200000 #Data files = 6, #Online files = 6 Threads: #Enabled=1, #Open=1, Head=1, Tail=1 *************************************************************************** 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:55 on disk scn: 0x0000.0016359c 12/11/2012 22:57:42 resetlogs scn: 0x0000.000f30dc 12/07/2012 10:36:16 heartbeat: 801789080 mount id: 848836772 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 *************************************************************************** DATA FILE RECORDS *************************************************************************** (size = 520, compat size = 520, section max = 100, section in-use = 6, last-recid= 43, old-recno = 0, last-recno = 0) (extent = 1, blkno = 11, numrecs = 100) DATA FILE #1: name #7: /oradata/bxocp/system01.dbf creation size=0 block size=8192 status=0xe head=7 tail=7 dup=1 tablespace 0, index=1 krfil=1 prev_file=0 unrecoverable scn: 0x0000.00000000 01/01/1988 00:00:00 Creation Checkpointed at scn: 0x0000.00000007 09/17/2011 09:46:08 thread:0 rba:(0x0.0.0) 7、dump数据文件头 alter session set events 'immediate trace name file_hdrs level 10'; 显示数据文件头的部分内容： V10 STYLE FILE HEADER: Compatibility Vsn = 186646528=0xb200000 Db ID=848459038=0x3292751e, Db Name='BXOCP' Activation ID=0=0x0 Control Seq=2099=0x833, File size=79360=0x13600 File Number=2, Blksiz=8192, File Type=3 DATA Tablespace #1 - SYSAUX rel_fn:2 Creation at scn: 0x0000.0000088c 09/17/2011 09:46:16 Backup taken at scn: 0x0000.00000000 01/01/1988 00:00:00 thread:0 reset logs count:0x2fc45da0 scn: 0x0000.000f30dc prev reset logs count:0x2d6c775c scn: 0x0000.00000001 recovered at 12/11/2012 22:54:36 status:0x4 root dba:0x00000000 chkpt cnt: 121 ctl cnt:120 begin-hot-backup file size: 0 注意： 从控制文件中得到重做日志恢复起始地址： low cache rba:(0x13.3.0)：19号日志，第3个块，第0个字节开始恢复 从数据文件头部得到重做日志恢复起始地址： thread:1 rba:(0x13.2.10) ：9号日志，第2号块，第16个字节开始恢复 8、最后我们打开数据库，然后监控告警日志alert_bxocp.log日志，看是怎么恢复的 [oracle@guoyj trace]$ tail -f alert_bxocp.log alter database open Beginning crash recovery of 1 threads Started redo scan Completed redo scan read 81 KB redo, 55 data blocks need recovery Started redo application at Recovery of Online Redo Log: Thread 1 Group 1 Seq 19 Reading mem 0 Mem# 0: /oradata/bxocp/redo01.log Completed redo application of 0.06MB Completed crash recovery at 55 data blocks read, 55 data blocks written, 81 redo k-bytes read Tue Dec 11 23:46:42 2012 Thread 1 advanced to log sequence 20 (thread open) Thread 1 opened at log sequence 20 Current log# 2 seq# 20 mem# 0: /oradata/bxocp/redo02.log Successful open of redo thread 1 MTTR advisory is disabled because FAST_START_MTTR_TARGET is not set [12867] Successfully onlined Undo Tablespace 2. Undo initialization finished serial:0 start:20725234 end:20725294 diff:60 (0 seconds) Verifying file header compatibility for 11g tablespace encryption.. Verifying 11g file header compatibility for tablespace encryption completed Tue Dec 11 23:46:42 2012 SMON: enabling cache recovery SMON: enabling tx recovery Database Characterset is ZHS16GBK No Resource Manager plan active replication_dependency_tracking turned off (no async multimaster replication found) Starting background process QMNC Tue Dec 11 23:46:43 2012 QMNC started with pid=21, OS id=13839 Completed: alter database open Tue Dec 11 23:46:44 2012 Starting background process CJQ0 Tue Dec 11 23:46:44 2012 CJQ0 started with pid=22, OS id=13851 Setting Resource Manager plan SCHEDULER[0x318A]:DEFAULT_MAINTENANCE_PLAN via scheduler window Setting Resource Manager plan DEFAULT_MAINTENANCE_PLAN via parameter Tue Dec 11 23:46:47 2012 Starting background process VKRM Tue Dec 11 23:46:47 2012 VKRM started with pid=23, OS id=13857 9、可以看出，实例恢复的起始的重做日志是以控制文件中的low cache rba:(0x13.3.0)：19号日志，第3个块，第0个字节开始恢复，而不是从文件头的thread:1 rba:(0x13.2.10) --重做日志的地址0x13.2.10－> 19号日志，第2号块，第16个字节开始恢复 10、最后总结一下实例恢复 (1)数据文件、在线日志文件、控制文件不得有损坏 (2)数据库自动恢复，无需DBA干涉 (3)恢复只需在线日志文件，无需归档日志 (4)数据库在open的时候开始实例恢复 实际上我做的这个实例恢实验的还没有写完整， 还有最后一步回滚！这个就留给你们思考！ 实例恢复三步：前滚--->打开库---->后滚(也叫回滚) 其实:On Disk RBA不是Instance Recovery的终点！！！ |