V$LOGFILE：Contains information about the online redo log files and standby redo log files
V$LOGFILE.TYPE=ONLINE OR STANDBY
V$STANDBY_LOG：Contains log file information from the standby redo log files
RFS process：remote file server (RFS) process
The standby redo logs are populated with (被填充) redo information as fast as the primary redo logs, rather than waiting for the redo log to be archived and shipped(传送) to the standby database. This means that the standby redo log has more current information than the log apply mechanism because it took a "shortcut" and was written to the standby, bypassing the traditional archiving and FTP to the standby database.（这意味着备用重做日志比日志应用机制具有更多的当前信息，因为它采取了“快捷方式”，并被写入备用，绕过了传统的归档和FTP到备用数据库。）
The synchronous and asynchronous redo transport modes require that a redo
transport destination have a standby redo log. A standby redo log is used to store redo
received from another Oracle database. Standby redo logs are structurally（结构上） identical to
redo logs, and are created and managed using the same SQL statements used to create
and manage redo logs.
Redo received from another Oracle database via redo transport is written to the
current standby redo log group by an RFS foreground process.
（通过Redo传输从另一个Oracle数据库接收的Redo由RFS前台进程写入到当前备用Redo日志组）
When a log switch occurs on the redo source database, incoming redo is then written to the next standby redo log group, and the previously used standby redo log group is archived by an ARCn foreground process.
The process of sequentially filling and then archiving redo log file groups at a redo
source database is mirrored at each redo transport destination by the sequential filling
and archiving of standby redo log groups.
Each standby redo log file must be at least as large as the largest redo log file in the
redo log of the redo source database. For administrative ease, Oracle recommends that
all redo log files in the redo log at the redo source database and the standby redo log at
a redo transport destination be of the same size.
The standby redo log must have at least one more redo log group than the redo log at
the redo source database, for each redo thread at the redo source database. At the redo
source database, query the V$LOG view to determine how many redo log groups are in
the redo log at the redo source database and query the V$THREAD view to determine
how many redo threads exist at the redo source database.
Cases Where Redo Is Written Directly To an Archived Redo Log File
Redo received by a standby database is written directly to an archived redo log file if a
standby redo log group is not available or if the redo was sent to resolve a redo gap.
备库的归档日志是从 standby log 中写的，但是如果备库的standby log不可用，或者主库与备库有gap，这时候可以通过主库往备库写入的归档日志进行恢复与应用。
When this occurs, redo is written to the location specified by the LOCATION attribute of
one LOG_ARCHIVE_DEST_n parameter that is valid for archiving redo received from
another database. The LOG_ARCHIVE_DEST_n parameter that is used for this purpose is
determined when the standby database is mounted, and this choice is reevaluated
each time a LOG_ARCHIVE_DEST_n parameter is modified.
这里一般是 log_archive_dest_2 设定的往备库上传输的归档路径。
Apply Services
Apply redo data on the standby database to maintain transactional
synchronization with the primary database. Redo data can be applied either from
archived redo log files, or, if real-time apply is enabled, directly from the standby
redo log files as they are being filled, without requiring the redo data to be
archived first at the standby database.
applying the redo that it contains. However, you can enable real-time apply, which
allows apply services to apply the redo in the current standby redo log file as it is
being filled