Log writer lgwr writes entries from log buffer to redo the log files 日志读写器（ lgwr ）从日志缓冲区写入项目，以便重做日志文件。
As the mirror server redoes the log , the mirror database is continually rolled forward 当镜像服务器重做日志时，镜像数据库将继续前滚。
You can roll forward to do recovery as long as redo logs are not overwritten 如果重做日志没有被重新写入，您就可以通过前滚操作来执行恢复
Copy all data files , control files and online redo logs file to backup media 将所有数据文件、控制文件和联机重做日志文件复制到备份媒介。
This will result in enormous redo logs to be generated for online backup data files 对于联机备份数据文件，这将导致生成大量的重做日志。
Online redo logs are written round robin manner namely when the last log is full , it will overwrite log 1 联机重做日志以循环的方式进行写入，也就是当最后一个日志已满时，将重写日志1 。
To investigate what status is the missing redo log is - a . select from v logfile shows which group is invalid 要检查丢失的重做日志处于什么状态a . select * from v $ logfile展示了哪个组是无效的。
Checkpoint ckpt synchronizes the headers of data files and control files with the current redo log and checkpoint numbers 检查点（ ckpt ）将数据文件头和控制文件头与当前重做日志和检查点数同步。
All the redo logs after the alter database open resetlogs will render useless and will not be good for recovery Alter database open resetlogs之后，所有重做日志都将表现为无用，所以它不适合于恢复。
To investigate what status is the missing redo log is - select from v logfile shows which group and member is invalid 要检查丢失的重做日志处于什么状态select * from v $ logifle显示哪个组及哪个成员是无效的。
