Why we cannot take hot backup if database is in noarchive log mode

Hi,
I am aware that if database is in noarchive log mode, we cannot take hot backups and only cold backup is possible.
I would like to know the technical reason behind this restriction?
Thank You
Sarayu

Hot backups are fuzzy backups, inconsistent, in other words, since something is always happening in the database.  When you recover, you restore data files and then apply redo to make the transactions consistent.  You can do a complete recovery or recover to a point in time.  So where does the redo come from?  That's what we call archiving redo logs.  When the online redo gets full, it gets archived.
In the case of an instance crash, the redo is there in the online redo logs, so Oracle can recover automatically.  Anything beyond that, having to do with storage media, is a media recovery, and requires those archived logs.  So unless you have some other way to get your data back, always run in archivelog mode.
It is really important to understand the concepts.  Please read the docs.
http://docs.oracle.com/cd/E11882_01/server.112/e25789/cncptdba.htm#CNCPT031
http://docs.oracle.com/cd/E11882_01/backup.112/e10642/rcmintro.htm#i1005488
It may be worth your while to get a third party backup and recovery book too.

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