1. 如果要把redo的切换信息显示到alert日志中，需要设置个参数。
Oracle Database 11g Enterprise Edition Release 11.2.0.2.0 - 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options
SQL> show parameter check
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
db_block_checking string FALSE
db_block_checksum string TYPICAL
log_checkpoint_interval integer 0
log_checkpoint_timeout integer 1800
log_checkpoints_to_alert boolean FALSE
SQL> alter system set log_checkpoints_to_alert=true;
System altered.
2.查看redo的情况，current表示当前所属的redo日志组。
SQL> select group#,status from v$log;
GROUP# STATUS
---------- ----------------
1 INACTIVE
2 INACTIVE
3 INACTIVE
4 CURRENT
3.强制切换日志，使用switch logfile,切换后，current日志组切换到了第一组，刚刚的日志组4变成了active状态。 表示dbwr还没有把脏数据写入数据文件。
需要出发dbwr来写入，或者等待一会儿。
alert日志内容如下；
--alter system switch logfile;
Mon Mar 24 18:16:46 2014
Beginning log switch checkpoint up to RBA [0xb1d.2.10], SCN: 10583135604156
Thread 1 advanced to log sequence 2845 (LGWR switch)
Current log# 1 seq# 2845 mem# 0: /dbccbsPT1/oracle/PETCUS1/oracnt01/redolog_A1/redo/redo01A.log
Current log# 1 seq# 2845 mem# 1: /dbccbsPT1/oracle/PETCUS1/oracnt02/redolog_B1/redo/redo01B.log
Mon Mar 24 18:16:46 2014
Archived Log entry 2844 added for thread 1 sequence 2844 ID 0x4a0d6000 dest 1:
SQL> select group#,status from v$log;
GROUP# STATUS
---------- ----------------
1 CURRENT
2 INACTIVE
3 INACTIVE
4 ACTIVE
4.再强制切换一次日志，日志组继续切换，可以看到有两个日志组成了active,
alert日志内容如下；
--alter system switch logfile;
Mon Mar 24 18:17:35 2014
Beginning log switch checkpoint up to RBA [0xb1e.2.10], SCN: 10583135604180
Thread 1 advanced to log sequence 2846 (LGWR switch)
Current log# 3 seq# 2846 mem# 0: /dbccbsPT1/oracle/PETCUS1/oracnt01/redolog_A3/redo/redo03A.log
Current log# 3 seq# 2846 mem# 1: /dbccbsPT1/oracle/PETCUS1/oracnt02/redolog_B3/redo/redo03B.log
Mon Mar 24 18:17:35 2014
Archived Log entry 2845 added for thread 1 sequence 2845 ID 0x4a0d6000 dest 1:
SQL> select group#,status from v$log;
GROUP# STATUS
---------- ----------------
1 ACTIVE
2 INACTIVE
3 CURRENT
4 ACTIVE
5.如果进行全量检查点，强制dbwr写入数据文件，可以出发全量检查点。
alert日志内容如下：
--alter system checkpoint;
Mon Mar 24 18:18:48 2014
Beginning global checkpoint up to RBA [0xb1e.4c.10], SCN: 10583135604229
Completed checkpoint up to RBA [0xb1e.4c.10], SCN: 10583135604229
Completed checkpoint up to RBA [0xb1e.2.10], SCN: 10583135604180
Completed checkpoint up to RBA [0xb1d.2.10], SCN: 10583135604156
SQL> select group#,status from v$log;
GROUP# STATUS
---------- ----------------
1 INACTIVE
2 INACTIVE
3 CURRENT
4 INACTIVE
6.过了一会儿，可以看到日志中会出现增量检查点，增量检查点的优先级比全量的要低。
Mon Mar 24 18:19:22 2014
Incremental checkpoint up to RBA [0xb1e.4c.0], current log tail at RBA [0xb1e.5f.0]
7.触发日志归档
--alter system archive log current;
Mon Mar 24 18:20:51 2014
ALTER SYSTEM ARCHIVE LOG
Mon Mar 24 18:20:51 2014
Beginning log switch checkpoint up to RBA [0xb1f.2.10], SCN: 10583135604443
Thread 1 advanced to log sequence 2847 (LGWR switch)
Current log# 2 seq# 2847 mem# 0: /dbccbsPT1/oracle/PETCUS1/oracnt01/redolog_A2/redo/redo02A.log
Current log# 2 seq# 2847 mem# 1: /dbccbsPT1/oracle/PETCUS1/oracnt02/redolog_B2/redo/redo02B.log
Archived Log entry 2846 added for thread 1 sequence 2846 ID 0x4a0d6000 dest 1:
SQL> select group#,status from v$log;
GROUP# STATUS
---------- ----------------
1 INACTIVE
2 CURRENT
3 ACTIVE
4 INACTIVE
和alter system switch logfile的差别在于，对于日志组3，执行alter system archive log current和alter system switch logfile会有所不同
GROUP# THREAD# SEQUENCE# MEMBERS SIZE_MB ARC STATUS
---------- ---------- ---------- ---------- ---------- --- ----------------
1 1 685 2 2048 YES ACTIVE
2 1 686 2 2048 YES ACTIVE
3 1 687 2 2048 NO ACTIVE
4 1 688 2 2048 NO CURRENT