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
Database checkpoint: Thread=1 scn: 0x0000.0016344c --数据库检查点SCN＝16344c转成10进制为1455180
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
low cache rba:(0x13.3.0) on disk rba:(0x13.a6.0)
-- low cache rba:(0x13.3.0)实例恢复的起点：19号日志，第3个块，第0个字节
--on disk rba:(0x13.a6.0)：实例恢复的终点：19号日志，第166个块，第0个字节
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
Checkpoint cnt:121 scn: 0x0000.0016344c 12/11/2012 22:54:36
--控制文件中保存的数据文件检查点SCN＝16344c转成10进制为1455180
Stop scn: 0xffff.ffffffff 12/11/2012 22:53:05
--结束的SCN填无穷大，说明是异常关机的，重启数据库必须做实例恢复
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
Checkpointed at scn: 0x0000.0016344c 12/11/2012 22:54:36
--数据文件的文件头中的检查点SCN＝16344c转成10进制为1455180
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
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
Thread 1: logseq 19, block 3 --实例恢复开始的重做日志：19号日志第3个块
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
Thread 1: logseq 19, block 166, scn 1475516 --实例恢复结束点的重做日志：19号日志第166个块
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)
