data guard中的Online Redo Logs, Archived Redo Logs, and Standby Redo Logs

Online Redo Logs, Archived Redo Logs, and Standby Redo Logs

the most crucial structures for Data Guard recovery operations are online redo logs, archived redo logs, and standby redo logs. Redo data transmitted from the primary database is received by the 

remote file server (RFS) process on the standby system where the RFS process writes the redo data to archived log files or standby redo log files. Redo data can be applied either after the redo is written to the archived redo log file or standby redo log file, or, if real-time apply is enabled, directly from the standby redo log file as it is being filled.

oracle DG中最重要的结构就是三种日志。online redo logs, archived redo logs, and standby redo logs。redo 从主库传输到standby端，并且由RFS进程写到archived log files或者standby redo log files.redo 可以直接从standby redo log file进行应用（当开始实时应用模式的时候），也可以等到redo已经写到standby archived log进行归档后，再从standby archived log进行应用。

Online Redo Logs and Archived Redo Logs

The transmission of redo is integral to maintaining the transactional consistency of the primary and standby databases. Both online redo logs and archived redo logs are required in a Data Guard environment:

• Online redo logs

  Every instance of an Oracle primary database and logical standby database has an online redo log to protect the database in case of an instance failure. Physical standby databases do not use an online redo log, because physical standby databases are not opened for read/write I/O. Changes are not made to the physical standby database and new redo data is not generated.

• Archived redo logs

  An archived redo log is required because archiving is the method used to keep standby databases transactionally consistent with the primary database. Primary databases, and both physical and logical standby databases all use an archived redo log. Oracle databases are set up, by default, to run in ARCHIVELOG mode so that the archiver (ARCn) process automatically copies each filled online redo log file to one or more archived redo log files.

  Unlike physical standby databases, logical standby databases are open databases that generate redo data and have multiple log files, including online redo log files, archived redo log files, and standby redo log files (if configured).

Standby Redo Logs

A standby redo log is similar to an online redo log, except that a standby redo log is used to store redo data received from another database.

A standby redo log is required if you want to implement:

• The maximum protection and maximum availability levels of data protection (described in Section 1.4 and in more detail in Section 5.6)

• Real-time apply (described in Section 6.2)

• Cascaded destinations (described in Appendix E)

A standby redo log provides a number of advantages:

• Standby redo log files can reside on raw devices, which may be important if either or both the primary and standby databases reside in a Real Application Clusters environment.

• Standby redo log files can be multiplexed using multiple members, improving reliability over archived log files.

• During a failover, Data Guard can recover and apply more redo data from standby redo log files than from the archived log files alone.

• The archiver (ARCn) process or the log writer (LGWR) process on the primary database can transmit redo data directly to remote standby redo log files, potentially eliminating the need to register a partial archived log file (for example, to recover after a standby database crashes). See Chapter 5 for more information.