Oracle archive Log tips Oracle Application Server Tips by Burleson Consulting

The archive logs are files created by the Archive process (ARC) from the redo logs.  Once a redo log is full, the database performs a log switch and begins overwriting the next redo log.  The ARC process stats copying the changes from the full redo log to an archive log file. 

By default, the archive log files are located in the infrastructure?s $ORACLE_HOME/dbs/arch directory.  The ARC process creates one file for each redo log it copies (Figure 11-1).  The archive log files are the mechanism that oracle uses to recover the database from a backup.

Lets say that we took a backup of the metadata repository on Monday and on Tuesday an operator deleted one of the database?s data files.  We shutdown the database, restore the one missing data file and restart the database. 

Oracle will see that the SNC for the restored data files in incorrect and will determine which archive log file has the first change it needs to apply and will begin rolling the data file forward applying each change recorded in the archive logs.  The database will continue until the data file is current. 

When the ARC process is copying changes to archive logs, the database is in ARCHIVELOG mode.  When the installer creates the Metadata Repository database it is created in NOARCHIVELOG mode which means that to recover the database from a backup you must first place it into ARCHIVELOG mode.  To place the database in ARCHIVELOG mode using the default log file locations, you must shut it down, partially restart it, switch to ARCHIVELOG mode and finally open the database. 

[oracle@appsvr oracle]$ $ORACLE_HOME/bin/sqlplus ?/ as sysdba?
# Change the log_archive_start=true
SQL> alter database set log_archive_start=true scope both;
SQL> shutdown immediate
SQL> startup mount
SQL> alter database archivelog;
SQL> alter database open;

NOTE: The Metadata Repository database is created in NOARCHIVELOG mode.  You must change it to ARCHIVELOG mode to be able to recover and roll forward the database.  If the database is in NOARCHIVELOG mode you can only restore the backup file and all changes made since that time will be lost.

Now that you are in ARCHIVELOG mode, how long do you keep the archive logs.  The answer is, as long as you maintain a backup.  If you rotate backup tapes and maintain a weeks worth of tapes, then you need to maintain a weeks worth of archive logs. 

If you no longer have the backup, the archive logs for that backup are of no value.  Likewise, if you have the backup but have deleted the archive logs, the backup can only be used to restore to the backup?s point in time.  The database can not roll forward with out the archive logs.  Also, the database can not skip a missing archive log.  The database will roll forward until it needs the missing archive log and no further. 

Now that we have a basic understanding of how Oracle maintains transaction logs, we need to discuss the methods of backing up the database.  The key to having a valid backup is that each file is consistent and did not change during the backup.

Now that we know the mechanisms Oracle uses to protect data in the database we need to discuss the how those features are used in backup and recovery.

This is an excerpt from "Oracle 10g Application Server Administration Handbook" by Don Burleson and John Garmany.