Re: Concept Question --- Redo Log Recycle

"jane" <janeyiu_at_optonline.net> wrote in message news:NIlL7.28121$636.5030654_at_news02.optonline.net...
> I'm puzzled about the following scenario.
> This question is about the inter-workings of online redo log files,
> DBWn, CKPT...on 8i.
> Pls point out anything I said wrong below.
>
> NOARCHIVELOG mode, have a HUGE rollback segment,
> doing LOTS of modifications, SELECTs, but NO COMMITS.
>
> My question is:
> Online redo files will be reused (yes ?), thus CKPT will happen
> and dirty blocks with UNCOMMITED CHANGES in buffer will be
> flushed/written to disk (yes ?).

Since you mentioned no commits are happening, LGWR only writes to the online redo log when you tell it to commit. Here is the quote from the Oracle documentation:

"Redo records are buffered in a circular fashion in the redo log buffer of the SGA and are written to one of the online redo log files by the Oracle background process Log Writer (LGWR). Whenever a transaction is committed, LGWR writes the transaction's redo records from the redo log buffer of the SGA to an online redo log file, and a system change number (SCN) is assigned to identify the redo records for each committed transaction. Only once all redo records associated with a given transaction are safely on disk in the online logs is the user process notified that the transaction has been committed."

Thus, in your case, I presume the redo records are still in the redo log buffer of the SGA ( not yet recorded the actual online redo log FILES ).

>
> System crashes now.
>
> So what happens after instance is back up ?
> Since the redo logs have been reused....
> How exactly does Oracle roll-forward and undo the
> UNCOMMITED changes to the disk ?
> Read from RBS ?
>
> Pls be as specific as u possibly can.
> Really appreciated !
>
> jane
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Received on Fri Nov 23 2001 - 00:26:53 CST