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. (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 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_<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’;

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

SQL> alter database add standby logfile group 4 (
‘/<full_path_for_srl>/log04a.dbf’,
‘/<full_path_for_srl>/log04b.dbf’
) size 50m;

References/Related Document

Online Redo, Archived Redo and Standby Redo Logs
219344.1 Usage, Benefits and Limitations of Standby Redo Logs (SRL)

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10 comments

Frank says August 20, 2009

Great post. I wanted to let you know that when you build Standby Redo Logs using ASM and db_create_online_log_dest_N Oracle will automatically build TWO SRL. One in FRA and one in a Non-FRA datagroup. Oracle recommends that you remove the second one as you should only have 1 SRL per group and the # of groups should be 1 > current number of groups on your primary.
Thanks.

Pablo says August 25, 2010

Excelent post. I have a question about this issue…
1)i don’t have SRL’s in my Standby database but de Data Guard works fine(i’ve configured using RMAN) my DataGuar work’s in MAXIMUM PERFORMANCE, I have to create the standby redologs mandatory and I have to create in both instances?

dba says September 9, 2010

Can we use standby redo log to recover the standby database if the Primary database server is not accessible so that we can recover till the last commited transaction.

Thanks

Lenny Shearin says August 1, 2012

Your style is really unique in comparison to other people I’ve read stuff from. Thank you for posting when you’ve got the opportunity, Guess I will just bookmark this web site.

Bharath says April 26, 2013

Hi Atul,

Thanks for sharing information which is much needed to administer dataguard.

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Anu says January 27, 2015

Hi,
The standby redo log files accidently got deleted from OS level.
How can I recreate them. The DR site is otherwise working fine and primary archive logs are being applied properly.

Please reply asap.

Thanks
Anu

Siva says October 9, 2015

Nice Post.

I am having one doubt, the settings seems to be same for High Protection & Availability mode(Sync/LGWR/Affirm)..But how we can diffentiate this when configuring the standby protection modes?

Thanks,
Siva.

Prashanth K says December 22, 2016

Hi… I just had small doubt.. that, while creating “PRIMARY DATABASE”, how many “STANDBY DATABASE” can create ??? Is there’s any parameter is there can we set..?? kindly let me know.. I am newbie, learning Data guard technlogy

bala says January 2, 2017

Hi,
I have some clairfications. I have created physical standby DB with (real time apply) using RMAN duplicate database scripts. But there are some mismatch in v$logfile.
i have DATA,FRA groups in physical and standby db servers.
But DATA group is not available in standby server after RMAN copy.
Can you please help me what i am missing.

Below are the outputs in v$logfile

primaryDB

3 ONLINE +DATA/MORSB2PD/ONLINELOG/group_3.264.931111181 NO 0
3 ONLINE +FRA/MORSB2PD/ONLINELOG/group_3.259.931111181 YES 0
2 ONLINE +DATA/MORSB2PD/ONLINELOG/group_2.263.931111179 NO 0
2 ONLINE +FRA/MORSB2PD/ONLINELOG/group_2.258.931111181 YES 0
1 ONLINE +DATA/MORSB2PD/ONLINELOG/group_1.262.931111179 NO 0
1 ONLINE +FRA/MORSB2PD/ONLINELOG/group_1.257.931111179 YES 0
4 STANDBY +DATA/MORSB2PD/ONLINELOG/group_4.275.931267573 NO 0
4 STANDBY +FRA/MORSB2PD/ONLINELOG/group_4.447.931267573 NO 0
5 STANDBY +DATA/MORSB2PD/ONLINELOG/group_5.276.931267593 NO 0
5 STANDBY +FRA/MORSB2PD/ONLINELOG/group_5.448.931267593 NO 0
6 STANDBY +DATA/MORSB2PD/ONLINELOG/group_6.277.931267599 NO 0
6 STANDBY +FRA/MORSB2PD/ONLINELOG/group_6.449.931267599 NO 0
7 STANDBY +DATA/MORSB2PD/ONLINELOG/group_7.278.931267611 NO 0
7 STANDBY +FRA/MORSB2PD/ONLINELOG/group_7.450.931267611 NO 0

standbyDB

3 ONLINE +FRA/MORSB2S2/ONLINELOG/group_3.278.931837545 YES 0
4 STANDBY +FRA/MORSB2S2/ONLINELOG/group_4.279.931837545 YES 0
2 ONLINE +FRA/MORSB2S2/ONLINELOG/group_2.277.931837545 YES 0
5 STANDBY +FRA/MORSB2S2/ONLINELOG/group_5.280.931837545 YES 0
1 ONLINE +FRA/MORSB2S2/ONLINELOG/group_1.276.931837545 YES 0
6 STANDBY +FRA/MORSB2S2/ONLINELOG/group_6.281.931837545 YES 0
7 STANDBY +FRA/MORSB2S2/ONLINELOG/group_7.282.931837545 YES 0

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Standby Redo Logs (SRL) in Standby Database

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