When you place a tablespace in BACKUP mode, Oracle guards the tablespace a little differently. It is possible that when backing up this tablespace, the datafile block that is currently being backed up undergoes a change due to some transaction. So the part of the block may be "old" and part may be "new" in your datafile backup. To guard against this, Oracle does not log just the transaction to the online redo logs. Instead, Oracle writes the entire block's contents to the online redo logs. This block normally contains more than just the one row of data affected by the transaction's change. Therefore, Oracle is writing more information to the online redo logs. When the tablespace is taken out of BACKUP mode, Oracle reverts to writing just the change vectors to the online redo logs.
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