"Translated from MoS article" Standby Redo Logs (SRL) uses, benefits and limitations

Standby Redo Logs (SRL) uses, benefits and limitations

From:
Usage, benefits and limitations of Standby Redo Logs (SRL) (document ID 219344.1)

Objective:
This article shows the purpose, benefits, and limitations of the standby Redo Logs (SRL) for Dataguard environments with SRL in Oracle 9i/10g/11g.

Range:
All DBAs and those wishing to implement 9i/10g/11g (9.x-11.x) Dataguard

1. What is Standby Redo Logs (SRL)---? When do I use an SRL?
Starting with Oracle 9i, you have the opportunity to add standby Redo Logs (SRL) Groups for online Redo Log Groups.
These standby Redo Logs (SRL) store the information received from the DG Master Library.
In the case of failover, there is standby Redo logs than no standby Redo logs will reduce data loss

At Oracle9i, only physical standby support SRL, starting with 10g, physical standby and Logical standby all support SRL.

The biggest advantage of SRL is that every entry written to online redologs on the main library is transferred to the standby and written to the SRL.
Therefore, the possibility of data loss is reduced on the standby.

Starting with 10g, real-time applications can be used on physical standby and Logical standby (real-time apply)
When using real-time apply, the redo from the SRL is applied to standby database, and redo apply waits until the archive log is established without using real-time apply.
Therefore, real-time Apply requires an SRL.

2. How do I build an SRL?
The SRL is an additional redo Log Groups. If you query v$logfile on the standby database, the output looks like this:

   Sql> select * from V$logfile;   group# STATUS  TYPE    MEMBER   --------------------------------------------------------        1          ONLINE C:\ORACLE\ORADATA\STANDBY\REDO01. LOG        2          ONLINE C:\ORACLE\ORADATA\STANDBY\REDO02. LOG        3          ONLINE C:\ORACLE\ORADATA\STANDBY\REDO03. LOG

Before adding an SRL to the standby, confirm the number of maximum Logfile Groups and Logfile members
If you don't remember these values, you can see them in the script for CREATE Controlfile. Use the following command to get the CREATE controlfile script:

   sql> ALTER DATABASE backup Controlfile to trace;

The above command will create a TRC file under the Udump directory.

Note the following related entry:

    Maxlogfiles   8    maxlogmembers 3

In this example, you can build up to 8 logfile groups, each groups can include up to 3 member.

Now we can add SRL to standby databae (of course, standby Databaes must be in Mount State)

   sql> ALTER DATABASE Add standby logfile Group 4 (' C:\ORACLE\ORADATA\STANDBY\STBY04. LOG ') SIZE 5M;

You can add member to a group:

   sql> ALTER DATABASE Add standby logfile member ' C:\ORACLE\ORADATA\STANDBY\STBY14. LOG ' to group 4;

Remember: If the RFS process writes data to SRL,SRL, the file size must be equal to the size of the current online redo log on the main library.

3. Limitations of SRL.
In the DG Environment of Oracle 9I/10G, the RFS process on standby database receives data from primary database and writes the data to disk (either SRL or archived Redo Logs)

If you use SRL for exams, you must make sure that these Srl are the same size as the online redo log.
If you have a different size of online redo log, you have to build the relevant SRL.
If the SRL differs from online redo log, the RFS process is not attached to the SRL.
It is recommended to the least one more than Standby Redo log Group as you have Online Redo Log Groups per Thread and Size.

The SRL is populated with the same content that is written on the online redo log on the main library.
Therefore, only the LGWR process can provide this information to the RFS process on the repository, so the SRL only works if the Log_archive_dest_n parameter setting of the main library is LGWR as the transmitter.
From 10.2.0 onwards, the arch process can be written to the SRL.

The rfs-process always tries to allocate the next available Standby Redolog,
So it's possible, that's encounter a Switch between only the Standby Redologs,
Although created lots more of them. There ' s no rota defined here as in Online Redologs. This is reported in Bug 2722195.

From 10.2.0 onwards, the arch process can be written to the SRL.

4. When using SRL, Log Apply service is different.
When an SRL is not used, the RFS process creates an archive log when the archive log is complete. The archive log is applied to the standby by the MRP (Managed Recovery process) session.
An open (not fully written) archive log cannot be applied on the standby and therefore cannot be used in failover situations, which results in a certain data loss

If you use the Srl,rfs process to write to the SRL, and when the log is switched, the repository's archiver process will archive the SRL to the archive log, and the MRP process applies the information to the repository.
At failover, you will be accessing the information that has been written to the SRL, so this information is not lost back.

Starting with 10g, you can use real-time apply for physical Standby and Logical Standby apply.
When using the real-time apply we directly apply Redo Data from Standby redologs.
If you do not use srl,real-time apply cannot apply a partially populated archive log to apply redo, the SRL is mandatory for real-time apply.

Note: In 12c, the default MRP will change to real-time Apply mode.

Default Standby Recovery in REAL time apply.
Sql>alter database recover managed standby database disconnect;
To-Start MRP in non real time apply mode use, (in 12c)
Sql>alter database recover managed standby database using archived logfile disconnect;

5. Standby Redologs and the Data Guard Broker
---------------------------------------------

If there is a active Data guard Broker Configuration on this Data guard environment the following Error can be raised in The Data Guard Broker Configuration If the setup Transport Method (logxptmode) is ' SYNC ' or ' ASYNC '
And there are no Standby redologs configured:

Warning:ora-16809:multiple warnings detected for the database

In the corresponding DRC. Log-files and the Database Status you can find:

Ora-16789:standby Redo logs Not configured

So please add corresponding Standby redologs to the Standby Database. As per default the Data Guard Broker tries to start Log apply Services in real-time apply Mode which are not possible witho UT Standby redologs.note that starting with the Oracle 11g ARCH Log Transport Method is deprecated.

Related DOCUMENTS
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Oracle9i Data Guard Concepts and administration
oracle10g Data Guard Concepts and administration
oracle11g Data Guard Concepts and administration

"Translated from MoS article" Standby Redo Logs (SRL) uses, benefits and limitations