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Standby Redo log files tips

Oracle Database Tips by Donald Burleson

Question:  In Data Guard, why do we need standby redo log files?  Should the the primary redo log files be enough?

Answer:  A standby redo log resides on the standby database site.  The standby redo log file is similar to an online redo log, except that a standby redo log is used to store redo data that has been received from a primary database.

Oracle Data Guard used to have the onerous problem of loosing the last redo log.  If the primary instanced crashed, the "current" redo log (as written by the LGWR process) would need to be flushed (with a log switch) before the most recent changes could be applied to the standby database.  If you could not flush the current redo, data could be lost forever.

Also see these important notes on LNS log transport waits.

Standby Redo Logs

In Oracle 10g and beyond we see an exciting new approach to Data Guard management whereby we write the current redo log to a "standby redo log", allowing complete recovery in cases of catastrophic instance failure. 

Note:  The standby redo logs are populated with redo information as fast as the primary redo logs, rather than waiting for the redo log to be archived and shipped 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.

The Oracle documentation notes three Data Guard Protection Modes.  The Maximum protection mode offers redo synchronization:

Maximum Protection'this mode offers the highest level of data protection. Data is synchronously transmitted to the standby database from the primary database and transactions are not committed on the primary database unless the redo data is available on at least one standby database configured in this mode. If the last standby database configured in this mode becomes unavailable, processing stops on the primary database. This mode ensures no-data-loss.

Maximum Availability'this mode is similar to the maximum protection mode, including zero data loss. However, if a standby database becomes unavailable (for example, because of network connectivity problems), processing continues on the primary database. When the fault is corrected, the standby database is automatically resynchronized with the primary database.

Maximum Performance'this mode offers slightly less data protection on the primary database, but higher performance than maximum availability mode. In this mode, as the primary database processes transactions, redo data is asynchronously shipped to the standby database. The commit operation of the primary database does not wait for the standby database to acknowledge receipt of redo data before completing write operations on the primary database. If any standby destination becomes unavailable, processing continues on the primary database and there is little effect on primary database performance.

Asynchronous redo transmission

We see more about the standby redo log mode in the 10g r2 docs where a new feature called "Asynchronous Redo Transmission" uses a new background process called LNSn:

Asynchronous redo transmission using the log writer process (LGWR ASYNC) has been improved to reduce the performance impact on the primary database. During asynchronous redo transmission, the network server (LNSn) process transmits redo data out of the online redo log files on the primary database and no longer interacts directly with the log writer process.

This change in behavior allows the log writer process to write redo data to the current online redo log file and continue processing the next request without waiting for inter-process communication or network I/O to complete.

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