What is Standby Redo Logs?
Standby Redo Logs (SRL) is similar to Online Redo Log (ORL). Only difference between two is that Standby Redo Log is used to store redo data received from another database (primary database).
The SRL files are where the Remote File Server (RFS) process at your standby database writes the incoming redo so that it is persistent on disk for recovery. SRL files are important for better redo transport performance and data protection. SRL are MUST in Maximum Availability or Maximum Protection mode and OPTIONAL (but recommended) in Maximum Performance mode.If there are no Standby Redo Log (SRL) files, then at each log switch in the primary database,the RFS process on the standby database that is serving an asynchronous standby destination has to create an archive log of the right size. While the RFS is busy doing creating the archive log file, the LNS process at the primary database has to wait, getting further and further behind the LGWR (in case of Maximum Performance mode). That is why it recommended to have Standby Redo Log (SRL) files in Maximum Performance mode also.
We generally configure them on our primary database as well in preparation for a role transition b/w primary-standby.Also, do not multiplex SRLs. Since Data Guard will immediately request.
Why Standby Red Logs Needed?
Prior to 10G Oracle Data Guard used to have an issue of losing the last redo log during catastrophic instance failure. If the primary instance is crashed or lost, the “current” redo log (as written by the LGWR process) in primary database redo log file, halts at the primary site. Archive file did not get generated on primary for the data which is in Redo log file, this data is not applied on standby and there would be the data loss forever on standby in case of primary site is lost.
To avoid such situation 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. 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.
In what scenarios Standby Redo Logs are required ?
Standby Redo Log is required if
1) Your standby database is in maximum protection or maximum availability mode.
2) If you are using Real-Time Apply on Standby Database.
3) If you are using Cascaded Destinations.
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