LGWR writes redo log (117)

LGWR writes synchronously to the active mirrored group of redo log files. If one of
the files in the group is damaged or unavailable, LGWR continues writing to other
files in the group and logs an error in the LGWR trace file and in the system alert log.
If all files in a group are damaged, or the group is unavailable because it has not been
archived, LGWR cannot continue to function.
When a user issues a COMMIT statement, LGWR puts a commit record in the redo log
buffer and writes it to disk immediately, along with the transaction’s redo entries. The
corresponding changes to data blocks are deferred until it is more efficient to write
them. This is called a fast commit mechanism. The atomic write of the redo entry
containing the transaction’s commit record is the single event that determines the
transaction has committed. Oracle returns a success code to the committing
transaction, although the data buffers have not yet been written to disk.

Note: Sometimes, if more buffer space is needed, LGWR writes redo
log entries before a transaction is committed. These entries become
permanent only if the transaction is later committed.

LGWR同步的寫入到映象的組日誌檔案
如果其中的一個損壞或者不可用 , LGWR寫入組內的另一日誌檔案 , 並記錄錯誤在LGWR跟蹤檔案和系統預警檔案中
如果組內的檔案都被損壞或者不可用 , LGWR則不能繼續工作
fast committed mechanism :當使用者提交commit語句時 , LGWR將提交記錄與事務條目立即寫入磁碟 ,
關聯的資料塊則被推遲直到更加有效時才寫入

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