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Point-in-time recovery (PITR) is needed for some solutions to meet the RPO, but this can involve considerable extra cost and effort. Your RTO and RPO should guide you to an options that balance the cost of the backup process while meeting your requirements.
Indicative disk partition sizings are given here: Oracle planning, that account for different backup options. Note that these are only indicative. Predicting "undo" and "redo" space is often difficult, and observing the solution at work is the best way of measuring the required amount of space. It is important to not over-specify the server and each partition to avoid unecessary cost, and grow these if needed once the solution has been established.
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