Standby Redo Logs (SRL) in Standby Database

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 from another database (primary database).Standby Redo Logs are only used if you have the LGWR as transport mode to Remote Standby Database. These are required to enable real time apply of redo data onto the standby.  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. So the loss of data in the event of a failover is minimized.

On the standby database Oracle Data Guard uses the Remote File Server (RFS) process to receive the redo records from the primary database, the Managed Recover Process (MRP) to apply redo information to the physical standby database.

When Real Time Apply, is enabled, MRP reads redo data directly from the current standby redo log file as it is being filled up by the RFS process.

When Real Time Apply is not enabled, the MRP reads from archive log and apply the redo data to the physical standby database.

When standby database doesn’t have SRL, the archive log files are shipped to the standby database from the primary and then the MRP reads it and applies the redo data to the physical standby

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 modes. (Physical Standby Database can run in one of three modes – Maximum Protection, Maximum Availability and Maximum Performance)

or

2) If you are using Real-Time Apply on Standby Database.

Or

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 the standby 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 mode regardless 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_    SERVICE=visr12_standby [ARCH | LGWR]

If neither the ARCH nor LGWR attribute is specified, the default is ARCH.

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’;

Q. How to add Standby Redo Log File Group to a Specific Group Number

SQL> alter database add standby logfile group 4 (

‘//log04a.dbf’,

‘//log04b.dbf’

) size 50m;

References:

http://onlineappsdba.com/index.php/2009/08/19/standby-redo-logs-srl-in-standby-database/

http://oracledbabhuvan.blogspot.com/2011/09/standby-redo-logs-srl.html