What is happening in the background when the database is in hot backup mode?
When you place a tablespace in backup mode for hot backups, Oracle does two things. First, Oracle freezes the datafile header, or more specifically, it freezes the SCN in the datafile header. Many people think that Oracle will not write to a datafile that is in backup mode. This is incorrect. Oracle will continue to write to that datafile, but Oracle will not update the SCN in the datafile header until that datafile is taken out of backup mode. Second, Oracle needs to make sure that any inconsistencies that occur when yor OS routines backup an active datafile. Oracle does this by logging information in the online redo logs, and thus the archived redo logs, that it needs to resolve these inconsistencies. Oracle takes an extra measure of not only logging the changes row, but logs the entire block that was changed to the online redo logs. For this reason, you may notice an increase in online redo log activity while you are performing your hot backups.
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