Oracle records database changes in its redo logs.
When Data Guard is activated, these changes are shipped from the primary to the standby and applied there.
This keeps the two databases synchronized with a time lag dependent on how often the changes are shipped and how long it takes to ship them.
ODG Standby Database Types
Physical Standby
- Identical to primary
- Updated via redo apply
Logical Standby
- Can be different structure to primary
- Updated via sql statements
ODG – Available Modes
Mode
=Maximum protection (LGWR, SYNC, AFFIRM)
highest level of protection
no data loss
primary stops if redo data cannot be applied to the standby
=Maximum availability (LGWR, SYNC, AFFIRM)
high level of protection
minimal data loss
switches to maximum performance mode if standby unavailable
automated catch-up of gaps if redo was not able to be written
=Maximum performance (LGWR|ARCH, SYNC|ASYNC, AFFIRM|NOAFFIRM)
medium level of protection
some data loss
primary not affected by standby unavailability
General objectives of DR system :
- Reduce recovery time in case of a disaster
Recovery Point Objective (RPO) = no more than X minutes of data loss
Recovery Time Objective (RTO) = Y hours from declaration of disaster
where X and Y are chosen by Project Build Team
- Reduce dependence on tapes
With tape based DR, tapes need to be shipped to DR site. Tapes can be lost or damaged with problems identified too late. ODG will know when a redo log is missing or damaged at the point when it is to be applied to the standby not when the disaster has already occurred.
ODG Implementation Overview :
- Build Standby DB
- Setup network connection between primary and DR
- Refresh Standby with data from Primary DB (ship tapes or rsync over network)
- Synchronize databases (primary shutdown & apply redo logs or final rsync with primary in backup mode)
- Activate Oracle Data Guard
- Monitor (alert logs, network, primary db performance)
- Initial test - bring up standby to validate system
Configure required parameters on Primary and Secondary DB in init<SID>.ora or spfile as per Oracle online documentation - Data Guard
Concepts and Administration.
ODG Activation Overview
Once all ODG parameters are set, on the PRIMARY DB:
sqlplus / as sysdba
SQL> alter system set log_archive_dest_state_2=‘ENABLE';
SQL> show parameter log_archive_dest_state_2
Deactivate via:
sqlplus / as sysdba
SQL> alter system set log_archive_dest_state_2=‘DEFER';
SQL> show parameter log_archive_dest_state_2
ODG Troubleshooting
- Manual Monitoring options
Database alert logs
ST04N
- Automated Monitoring
Alerts from OS level
Solution Manger
Oracle tools
Database alert logs
Oracle Alert Logs can be found in $SAPDATA_HOME/saptrace/background on both Primary database and Standby database.View the logs to check for messages / errors:
- more alert_<SID>.log | grep <ORA- / error / archival / GAP>
- tail –f alert_<SID>.log
- less alert_<SID>.log
Standby Database Backups
- Place the DB in read only mode
- Take a file system backup
- Place the DB back into managed recovery mode
- Clean up old redo logs after 2 successful backups
Disaster Recovery: High Level Overview
ODG Support: Common Problems
The following problems can occur :
- Missing logs
- Logs not being applied
- Cannot allocate new log
- Redo log storage filesystem full / filling
Problem: logs missing / not received
Problem: Redo logs not being applied
Problem: Can not allocate log
Problem: Redo storage full / filling
