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If all files in a group are damaged, or the group is unavailable because it has not been
archived, LGWR cannot continue to function.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同步的写入到镜像的组日志文件
如果其中的一个损坏或者不可用 , LGWR写入组内的另一日志文件 , 并记录错误在LGWR跟踪文件和系统预警文件中
如果组内的文件都被损坏或者不可用 , LGWR则不能继续工作
fast committed mechanism :当用户提交commit语句时 , LGWR将提交记录与事务条目立即写入磁盘 ,
关联的数据块则被推迟直到更加有效时才写入

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