谈谈Oracle数据库启动时禁用smon事务恢复
数据库在异常宕机之后，数据库再次启动时，smon会进行crash recover之后再进行tx recover，这在alert日志可以清晰的看到smon的操作过程。
但事情发展往往不如人意，试想一下当回滚段出现异常时，数据库势必不能启动，这时候我们往往需要借助一些异常手段，将数据库open。
Oracle的一些内部事件允许设置debug_mode，以便允许在AUM模式下，手工进行回滚段的处理：
alter session set "_smu_debug_mode"=4;
alter rollback segment "_SYSSMU7$" ONLINE;
或者使用隐含参数offline_rollback_segments进行指定回滚段处理：
_offline_rollback_segments
drop rollback segment ‘xxx’
如果回滚段正常，某些恢复可能需要很长的时间，在此期间的一些异常可能导致SMON Crash，进而数据库Instance崩溃。这时候我们可以引进一些事件进行诊断。
如：
禁用smon恢复
event = '10513 trace name context forever,level 2'
跟踪smon进程
event = '10500 trace name context forever, level 1'

测试案例。
不设置event 系统为初始状态
SQL> show parameter event

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
event
并产生大批量脏数据
SQL> insert into testsmon1 select * from testsmon1;

81920 rows created
数据库重启，注意这里使用的是force选项
SQL> startup force
ORACLE instance started.

Total System Global Area  838860800 bytes
Fixed Size                  1262164 bytes
Variable Size             494931372 bytes
Database Buffers          327155712 bytes
Redo Buffers               15511552 bytes
Database mounted.
Database opened.
我们在alert日志里可以看到

ALTER DATABASE OPEN
Fri Dec 17 16:30:49 2010
Beginning crash recovery of 1 threads
parallel recovery started with 3 processes
Fri Dec 17 16:30:49 2010
Started redo scan
Fri Dec 17 16:30:49 2010
Completed redo scan
28142 redo blocks read, 1885 data blocks need recovery
Fri Dec 17 16:30:49 2010
Started redo application at
Thread 1: logseq 66211, block 3
Fri Dec 17 16:30:49 2010
Recovery of Online Redo Log: Thread 1 Group 5 Seq 66211 Reading mem 0
  Mem# 0: /oradata/mcstar/mcstar/redo05.log
Fri Dec 17 16:30:49 2010
Completed redo application
Fri Dec 17 16:30:49 2010
db_recovery_file_dest_size of 10240 MB is 79.77% used. This is a
user-specified limit on the amount of space that will be used by this
database for recovery-related files, and does not reflect the amount of
space available in the underlying filesystem or ASM diskgroup.
Fri Dec 17 16:30:50 2010
Completed crash recovery at
Thread 1: logseq 66211, block 28145, scn 10995135133737
1885 data blocks read, 1885 data blocks written, 28142 redo blocks read
Fri Dec 17 16:30:50 2010
LGWR: STARTING ARCH PROCESSES
ARC0 started with pid=29, OS id=16094
Fri Dec 17 16:30:50 2010
ARC0: Archival started
ARC1: Archival started
LGWR: STARTING ARCH PROCESSES COMPLETE
ARC1 started with pid=30, OS id=16111
Fri Dec 17 16:30:50 2010
Thread 1 advanced to log sequence 66212
Thread 1 opened at log sequence 66212
  Current log# 4 seq# 66212 mem# 0: /oradata/mcstar/mcstar/redo04.log
Successful open of redo thread 1
Fri Dec 17 16:30:50 2010
MTTR advisory is disabled because FAST_START_MTTR_TARGET is not set
Fri Dec 17 16:30:50 2010
ARC1: Becoming the 'no FAL' ARCH
ARC1: Becoming the 'no SRL' ARCH
Fri Dec 17 16:30:50 2010
SMON: enabling cache recovery
Fri Dec 17 16:30:50 2010
ARC0: Becoming the heartbeat ARCH
Fri Dec 17 16:30:50 2010
Successfully onlined Undo Tablespace 12.
Fri Dec 17 16: