alter system switch logfile 和 alter system archive log current 的区别

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。(也可以是非归档情况下日志切换)

ALTER SYSTEM ARCHIVE LOG CURRENT 会对数据库中的所有实例执行日志切换。

详细解释：

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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。

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。