If a physical standby database loses or corrupts an archived redo data or has an unresolvable archive gap, which is the result of archive logs getting missed on the primary database or the archives getting corrupted, then no valid backups exists for recovery.
If an archive exists on the primary database, troubleshoot the issue, and if the archive is not corrupted, the archive is automatically transferred. If an archive is corrupted at the operating system (OS) level and you do have a backup, then restore the database by using the archive log from the backup. If none of these solutions work, you can recover the standby database with an incremental system change number (SCN) for standby/Roll Forward.
This blog shows you how to use the Oracle® Recovery Manager (RMAN) incremental backup to roll the standby database forward faster than by using a redo log apply.
How does an incremental backup work?
The following image shows the incremental backup process:
All the changes between the SCN of the original image copy and the SCN of the incremental backup are applied to the image.
To use this method, create a backup on the primary database that starts at the standby database’s current SCN, which you can then use to roll forward the standby database.
Steps to roll forward a physical standby database
Use the following steps to roll forward a physical standby database.
Find the SCN gap
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Use the following code to record the SCN gap between the standby and the production databases:
SQL> select status,instance_name,database_role from v$database,v$instance; STATUS INSTANCE_NAME DATABASE_ROLE ------------ ---------------- ---------------- MOUNTED PROD PHYSICAL STANDBY SQL> SELECT to_char(CURRENT_SCN) FROM V$DATABASE; CURRENT_SCN ------------- 5997422841660 SQL> select min(fhscn) from x$kcvfh; CURRENT_SCN ------------- 5997422841643 -
Note the lower SCN from the preceding queries, and run the following code:
SQL> select status,instance_name,database_role from v$database,v$instance; STATUS INSTANCE_NAME DATABASE_ROLE ------------ ---------------- ---------------- OPEN PROD PRIMARY SQL> SELECT to_char(CURRENT_SCN) FROM V$DATABASE; CURRENT_SCN ------------- 5997428587053
Take an incremental SCN backup and create a standby controlfile backup
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Run the following code on the production server to take an SCN backup:
[[email protected]] $ rman target / RMAN>BACKUP INCREMENTAL FROM SCN 5997422841643 DATABASE FORMAT '/u01/orapi/stage/TEMP/DBDR_%U' tag 'ArchiveGap'; Starting backup at 25-MAR-18 channel ORA_DISK_1: starting full datafile backup set channel ORA_DISK_1: specifying datafile(s) in backup set including current control file in backup set channel ORA_DISK_1: starting piece 1 at 25-MAR-18 channel ORA_DISK_1: finished piece 1 at 25-MAR-18 piece handle=/u01/orapi/stage/TEMP/ DBDR123.bak tag= ArchiveGap comment=NONE channel ORA_DISK_1: backup set complete, elapsed time: 00:00:03 Finished backup at 25-MAR-18 -
Run the following code to create the standby controlfile backup:
RMAN> backup current controlfile for standby format ='/u01/orapi/stage/TEMP/standby_control.bctl'; -
Transfer the backups from the primary server to the standby.
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Run the following code to apply a incremental SCN backup on the standby database, which catalogs all the backup pieces and the controlfile:
[[email protected]] $ rman target / Recovery Manager: Release 11.2.0.1.0 - Production on Sun Mar 2515:51:02 2012 Copyright (c) 1982, 2009, Oracle and/or its affiliates. All rights reserved. connected to target database: PSTLI (DBID=431934829, not open) RMAN> catalog start with ‘/u01/archives/stage/temp/’; -
Run the following code to recover the standby database with the cataloged incremental backup pieces:
RMAN> RECOVER DATABASE NOREDO; -
Run the following code to shutdown the physical standby database, start it in the nomount stage, and restore the standby controlfile backup that you took from the primary database:
RMAN> shutdown immediate database dismounted Oracle instance shut down RMAN> startup nomount connected to target database (not started) Oracle instance started Total System Global Area 659730432 bytes Fixed Size 2216264 bytes Variable Size 398462648 bytes Database Buffers 255852544 bytes Redo Buffers 3198976 bytes RMAN> restore standby controlfile from ‘/u01/archives/stage/temp/standby_control.bctl’; Finished restore at 25-MAR-18 -
Shutdown the standby database and mount the standby database so that you can mount the standby database with the new controlfile that was restored in the previous step.
Now the standby database is synchronized with the primary database.
Conclusion
You can use RMAN incremental backups to synchronize a physical standby database
with the primary database. By using the RMAN BACKUP INCREMENTAL FROM SCN
command, you can create a backup on the primary database that starts at the
standby database’s current SCN, which you can then use to roll the standby
database forward in time.
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