不管是Instance Recovery还是Crash Recovery，都由2个部分组成：cache recovery继以transaction recovery。
根据官方文档的介绍，Cache Recovery也叫Rolling Forward，就是我们常说的前滚；而Transaction Recovery也叫Rolling Back，就是我们常说的回滚。前滚和回滚贯穿Oracle恢复的基本概念，是我们入门必要学习的知识，在次不多介绍。
有文事者，必济之以武略。理论学得再好，不实践也无用。所幸Crash Recovery是很容易做成的实验，我们不妨来看一看:
SQL> shutdown abort;
ORACLE instance shut down.
SQL> startup mount;
ORACLE instance started.
Total System Global Area 1065353216 bytes
Fixed Size 2089336 bytes
Variable Size 486542984 bytes
Database Buffers 570425344 bytes
Redo Buffers 6295552 bytes
Database mounted.
SQL> alter database open;
Crash Recovery将从alter database open开始，我们来观察其日志
==================alert.log====================
alter database open
Tue Jun 14 18:19:53 2011
Beginning crash recovery of 1 threads
parallel recovery started with 2 processes
Tue Jun 14 18:19:53 2011
Started redo scan
Tue Jun 14 18:19:53 2011
Completed redo scan
0 redo blocks read, 0 data blocks need recovery
Tue Jun 14 18:19:53 2011
Started redo application at
Thread 1: logseq 1004, block 1124, scn 17136185
Tue Jun 14 18:19:53 2011
Recovery of Online Redo Log: Thread 1 Group 2 Seq 1004 Reading mem 0
Mem# 0: /flashcard/oradata/G10R2/onlinelog/o1_mf_2_6v34jokt_.log
Mem# 1: /s01/flash_recovery_area/G10R2/onlinelog/o1_mf_2_6v34jotq_.log
Tue Jun 14 18:19:53 2011
Completed redo application
Tue Jun 14 18:19:53 2011
Completed crash recovery at
Thread 1: logseq 1004, block 1124, scn 17156186
0 data blocks read, 0 data blocks written, 0 redo blocks read
Tue Jun 14 18:19:53 2011
LGWR: STARTING ARCH PROCESSES
ARC0: Archival started
LGWR: STARTING ARCH PROCESSES COMPLETE
ARC0 started with pid=16, OS id=7829
Tue Jun 14 18:19:53 2011
Thread 1 advanced to log sequence 1005 (thread open)
Thread 1 opened at log sequence 1005
Current log# 3 seq# 1005 mem# 0: /flashcard/oradata/G10R2/onlinelog/o1_mf_3_6v34jpmp_.log
Current log# 3 seq# 1005 mem# 1: /s01/flash_recovery_area/G10R2/onlinelog/o1_mf_3_6v34jpyn_.log
Successful open of redo thread 1
Tue Jun 14 18:19:53 2011
ARC0: Becoming the 'no FAL' ARCH
ARC0: Becoming the 'no SRL' ARCH
ARC0: Becoming the heartbeat ARCH
Tue Jun 14 18:19:53 2011
SMON: enabling cache recovery
Tue Jun 14 18:19:53 2011
db_recovery_file_dest_size of 204800 MB is 6.81% 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.
Tue Jun 14 18:19:54 2011
Successfully onlined Undo Tablespace 1.
Tue Jun 14 18:19:54 2011
SMON: enabling tx recovery
Database Characterset is UTF8
Opening with internal Resource Manager plan
where NUMA PG = 1, CPUs = 2
replication_dependency_tracking turned off (no async multimaster replication found)
Starting background process QMNC
QMNC started with pid=17, OS id=7831
Tue Jun 14 18:19:55 2011
Completed: alter database open
注意上述单实例Crash Recovery到数据库打开的整个过程:
alter database open
Beginning crash recovery of 1 threads
Started redo scan
Completed redo scan
Started redo application at
Completed redo application
Completed crash recovery at
SMON: enabling cache recovery
Successfully onlined Undo Tablespace 1
SMON: enabling tx recovery
Completed: alter database open
从上述步骤中我们可以看到三种恢复名词，即：
crash recovery
cache recovery
tx recovery
这和官方文档所描述的Crash Recovery概念是不一致的，我们现在来理清这几种recovery。
crash recov