alter system archive log current和alter system switch logfile

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

我們首先要明白兩個事實：
一個就是在logfile switch的時候會觸發arch程式，即通知歸檔程式說那個日誌可以歸檔了。
另一個是logfile switch的時候會觸發ckpt程式。而ckpt程式又會觸發dbwr程式。dbwr程式會把上一個redo中保護的儲藏在buffer cache中的dirty buffer全部寫回磁碟。然後，ckpt程式紀錄scn到controlfile中，這時會把redo log的狀態由active 變成inactive。

由此我們可以看出處於inactive狀態的redo log對於instance recovery是沒有用處了。但是它可能對於meda recovery還有用處，因為處於inactive狀態的redo log可能已經歸檔，也可能沒有歸檔。

出現 checkpoint not complete,unable to allocate new redo log file的錯誤，是因為lgwr要覆蓋一個其上檢查點過程沒有結束的日誌檔案，lgwr會等待dbwr完成，同時出現此錯誤。出現這個錯誤的原因，大概有幾點，一個是dbwr寫出速度太慢，一個是buffer cache中的髒資料太多。即事務太頻繁，另一個也可能是日誌太小，或日誌組太少導致的。