Hot Backup Question (Again!)

Folks,

Forgive me for asking again, I know someone recently asked this question....but (I wasn't paying complete attention)...

As I understand it, when a tablespace is in hot backup mode, the SCN for the datafile headers are not updated, entire data blocks are written to the redo logs as opposed to just the redo records, and when there is a commit, the lgwr writes the contents of the log buffer to the online redo logs and eventually, the data blocks of the datafiles are updated by dbwr, just as when a tablespace is not in hot backup mode. My point being, even with on-line hot backups, changed database blocks do get written to the datafiles, right?

So, having said that, I've read the following from a study guide:

'When a tablespace is in backup mode, the log sequence numbering is
temporarily halted and no more changes are made to it. Instead, transaction information is written to the redo log files. Once the tablespace is no longer in backup mode, all the information from the redo logs (including log sequence numbers) is applied to make it current.'

and also talked with someone who said the this:

'While a tablespace is in backup mode, it might as well be offline. Any
select statements for new data do come out of the datafiles, but anything changed or updated, goes to the redo log files. And of course for read consistency during concurrent transactions, the rollback segments keep the before image of any data. If there is data to be committed, it is committed to the redo logs. Then once the tablespace switches back to normal mode, the redo logs are applied. Basically, for new writes and changes, the redo logs are acting like temporary datafiles until recovery is done.'

None of which I agree with because it does not make complete sense.

Ok, so what am I missing?

Any enlightenment appreciated.

mkb

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