Database Reference
In-Depth Information
pUrpOSe OF OpeN reSetLOGS
sometimes, you're required to open your database with the OPEN RESETLOGS clause. you may do this when
recreating a control file, performing a restore and recovery with a backup control file, or performing an incomplete
recovery. When you open your database with the OPEN RESETLOGS clause, it either wipes out any existing online
redo log files or, if the files don't exist, recreates them. you can query the MEMBER column of V$LOGFILE to see
which files are involved in an OPEN RESETLOGS operation.
Why would you want to wipe out what's in the online redo logs? take the example of an incomplete recovery,
in which the database is deliberately opened to a point in time in the past. In this situation the sCn information
in the online redo logs contains transaction data that will never be recovered. oracle forces you to open the
database with OPEN RESETLOGS to purposely wipe out that information.
When you open your database with OPEN RESETLOGS , you create a new incarnation of your database and reset
the log sequence number back to 1. oracle requires a new incarnation so as to avoid accidentally using any old
archive redo logs (associated with a separate incarnation of the database), in the event that another restore and
recovery is required.
Summary
Some studies have indicated that airplane pilots who are over dependent on autopilot technology are less able to
cope with catastrophic in-flight problems than the pilots who have spent considerable time flying without autopilot
assistance. The over-autodependent pilots tend to forget key procedures when serious problems arise, whereas pilots
who aren't as dependent on autopilot are more adept at diagnosing and resolving stressful in-flight failures.
Similarly, DBAs who understand how to backup, restore, and recover a database manually, using user-managed
techniques, are more proficient at troubleshooting and resolving serious backup and recovery problems than DBAs
who only navigate backup and recovery technology via screens. This is why I included this chapter in the topic.
Understanding what happens at each step and why the step is required is vital for complete knowledge of the Oracle
backup and recovery architecture. This awareness translates into key troubleshooting skills when you're using Oracle
tools such as RMAN (backup and recovery), Enterprise Manager, and Data Guard (disaster recovery, high availability,
and replication).
The user-managed backup and recovery techniques covered in this chapter aren't taught or used much anymore.
Most DBAs are (and should be) using RMAN for their Oracle backup and recovery requirements. However, it's critical
for you to understand how cold backups and hot backups work. You may find yourself employed in a shop in which
old technology has been implemented and needing to restore and recover the database, troubleshoot, or assist in
migrating to RMAN. In these scenarios, you must fully understand the old backup technologies.
Now that you have an in-depth understanding of Oracle backup and recovery mechanics, you're ready to
investigate RMAN. The next several chapters examine how to configure and use RMAN for backup and recovery.
 
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