ALTER SYSTEM SWITCH LOGFILE 意义与作用及与ALTER SYSTEM ARCHIVE LOG CURRENT区别

ALTER SYSTEM SWITCH LOGFILE 对单实例数据库或RAC中的当前实例执行日志切换。
ALTER SYSTEM ARCHIVE LOG CURRENT 会对数据库中的所有实例执行日志切换。 

手动切换日志组
日志组A：当前正在写
日志组B：可写
手动切换日志组后，Oracle开始往日志组B里写日志，并进行一次checkpoint，把日志组A里没有经过checkpoint的那部分日志对应的dirty buffer从buffer cache里写到数据文件里。
checkpoint完成之后
日志组A：可写
日志组B：当前正在写多个日志组是循环使用的，一般是前面一个日志组写满了，就开始用下一个日志组。最后一组写满了，又回过来写第一个日志组。
alter system switch logfile 是用手工的方式，让Oracle 直接使用下一个日志组，而不管当前使用的日志组是否写满。

在日志切换时 checkpoint 是自动引发的。
例如在做冷备份时要保存redo logs，所以要确保redo log和data file 和 control file 的 SCN 一致，最好手动切换一下redo log。
由 checkpoint 更新redo log、data file、controle file中的SCN

详细解释： 

-------------------------

我们都知道在自动归档的数据库中，这两个命令几乎产生的结果是一样的。来看看oracle的解释：

ALTER SYSTEM SWITCH LOGFILE ;
SWITCH LOGFILE Clause
The SWITCH LOGFILE clause lets you explicitly force Oracle to begin writing to a new redo log file group, regardless of whether the files in the current redo log file group are full. When you force a log switch, Oracle begins to perform a checkpoint but returns control to you immediately rather than when the checkpoint is complete. To use this clause, your instance must have the database open.
ALTER SYSTEM ARCHIVE LOG CURRENT ;
CURRENT Clause
Specify CURRENT to manually archive the current redo log file group of the specified thread, forcing a log switch. If you omit the THREAD parameter, then Oracle archives all redo log file groups from all enabled threads, including logs previous to current logs. You can specify CURRENT only when the database is open.
ALTER SYSTEM ARCHIVE LOG CURRENT NOSWITCH;
NOSWITCH
Specify NOSWITCH if you want to manually archive the current redo log file group without forcing a log switch. This setting is used primarily with standby databases to prevent data divergence when the primary database shuts down. Divergence implies the possibility of data loss in case of primary database failure.
You can use the NOSWITCH clause only when your instance has the database mounted but not open. If the database is open, then this operation closes the database automatically. You must then manually shut down the database before you can reopen it

我们首先要明白两个事实：
一个就是在logfile switch的时候会触发arch进程，即通知归档进程说那个日志可以归档了。
另一个是logfile switch的时候会触发ckpt进程。而ckpt进程又会触发dbwr进程。dbwr进程会把上一个redo中保护的储藏在buffer cache中的dirty buffer全部写回磁盘。然后，ckpt进程纪录scn到controlfile中，这时会把redo log的状态由active 变成inactive。
由此我们可以看出处于inactive状态的redo log对于instance recovery是没有用处了。但是它可能对于meda recovery还有用处，因为处于inactive状态的redo log可能已经归档，也可能没有归档。
出现 checkpoint not complete,unable to allocate new redo log file的错误，是因为lgwr要覆盖一个其上检查点过程没有结束的日志文件，lgwr会等待dbwr完成，同时出现此错误。出现这个错误的原因，大概有几点，一个是dbwr写出速度太慢，一个是buffer cache中的脏数据太多。即事务太频繁，另一个也可能是日志太小，或日志组太少导致的。
备份恢复中用到的几个操作：

alter system checkpoint;
alter system archive log all;
alter system archive log current

alter system switch logfile; 

1：alter system checkpoint; 

Specify CHECKPOINT to explicitly force Oracle to perform a checkpoint, ensuring that all changes made by committed transactions are written to datafiles on disk. You can specify this clause only when your instance has the database open. Oracle does not return control to you until the checkpoint is complete.

强迫oracle进行以次检查点，确保所有提交的事务的改变都被写到磁盘数据文件上。但此时数据库必须是打开的状态。

2： alter system archive log all;

手工归档所有的日志文件组 

3：alter system archive log current 

手工归档活动的日志文件组。
4： alter system switch logfile;
The SWITCH LOGFILE clause lets you explicitly force Oracle to begin writing to a new redo log file group, regardless of whether the files in the current redo log file group are full. When you force a log switch, Oracle begins to perform a checkpoint but returns control to you immediately rather than when the checkpoint is complete. To use this clause, your instance must have the database open.

开始写新的日志文件组。不管当前日志文件组是否满了。 

参考：http://blog.163.com/zengbo249tiancai@126/blog/static/108286622012101410102626/

            http://www.itpub.net/thread-287031-1-1.html