If you use a retention policy of a recovery window of X days, and the backup script is executed once each Y days, then if X>=Y, then disk quota is determined by the following formula:

Disk Quota = Size of 1 copy of the database +
size of ((X * ceil(X/Y)) +1) days of archived redo logs

Enable Flashback Database on both the primary and standby database so that the old primary database can be easily reinstated as a new standby database following a failover. If there is a failure during the switchover process, then it can easily be reversed when Flashback Database is enabled.

5.3.4.1 Estimating Disk Space Requirements for Flashback Database Logs

The V$FLASHBACK_DATABASE_LOG view can help you estimate how much space to add to your flash recovery area for flashback logs. After you have enabled logging for Flashback Database and set a flashback retention target, allow the database to run under a normal workload for a while, to generate a representative sample of flashback logs. Then run the following query:

SQL> SELECT ESTIMATED_FLASHBACK_SIZE FROM V$FLASHBACK_DATABASE_LOG;


The result is an estimate of the disk space needed to meet the current flashback retention target, based on the database workload since Flashback Database was enabled. Add the amount of disk space specified in $FLASHBACK_DATABASE_LOG.ESTIMATED_FLASHBACK_SIZE to your flash recovery area size, to hold the expected database flashback logs.