Standby Redo Logs (SRL) : is similar to Online Redo Log (ORL) and only difference between two is that Standby Redo Log is used to store redo data received fromanother database (primary database).
Standby Redo Logs are only used if you have the LGWR as transport mode to Remote Standby Database. (step to check transport mode below)
In what scenarios Standby Redo Logs are required ?
Standby Redo Log is required if
1) Your standby database is in maximum protection or maximum availabilitymodes. (Physical Standby Database can run in one of three modes – Maximum Protection, Maximum Availability and Maximum Performance)
2) If you are using Real-Time Apply on Standby Database.
3) If you are using Cascaded Destinations
Things good to know about SRL
i) Standby Redo Logs should be same size as of Online Redo Logs. (The RFS process will attach Standby Redo Logs only if they are of same size as of Online Redo Log)
ii) Although the standby redo log is only used when the database is running in thestandby role, Oracle recommends that you create a standby redo log on the primary database so that the primary database can switch over quickly to the standby role without the need for additional DBA intervention.
iii)Standby redo logs can be created even after the standby has been created. In this case create the SRL’s on the primary before the creation of SRL on the standby database. (Standby Redo Log is not mandatory for Primary Database but its good practice and useful in role conversion from Primary to Standby Database)
iv)It is a best practice/recommendation to maintain Standby Redo Logs (SRLs) on both the standby AND primary database when using LGWR transport moderegardless of protection mode (Maximum Protection/Performance/Availability).
Q. What is RFS process on Standby Database ?
RFS – Remote File System on standby database receives data from Primary Database and writes it to Disk.
Q. How to identify which transport mode (Archiver or Log Writer) you are using to ship ?
SQL> show parameter log_archive_dest_
log_archive_dest_<n> SERVICE=visr12_standby [ARCH | LGWR]
If neither the ARCH or LGWR attribute is specified, the default is ARCH.
More information about log_archive_dest_<n> here
Q. How to check if you are using Real-Time Apply ?
SQL> SELECT DEST_ID, RECOVERY_MODE FROM V$ARCHIVE_DEST_STATUS;
Q. How to identify standby redo logs ?
SQL> select * from v$standby_log;
Q. How to see members of standby redo log file
SQL> select * from v$logfile where type=’STANDBY’;