Oracle Database Redo Archives During Backup
When an online backup is triggered, the tablespaces are set to backup mode. During the time when the tablespace is in backup mode, any change to a table/index in that tablespace will cause the entire block to be written to the redo log. This causes the redo log to fill up faster during backup in comparison to the time when there is no online backup in progress.
Ensure that you take online backups during low usage time and keep a track of the backup duration.
If the backup process crashes, you should ensure that the tablespaces are not left in backup mode. You should manually alter the tablespaces to Active.