Re: Bookish query Regarding Redo Log

arijitchatterjee123_at_yahoo.co.in (Arijit Chatterjee) wrote in message news:<ea01504d.0411042055.291dcd47_at_posting.google.com>...
> Respected Faculties,
> Again one more doubt has arised in my mind and
> I need some guidance from you people.I am explaining
> the doubt..
>
> I got these lines from Oracle Documentation
> ===================================================
> LGWR writes to online redo log files in a circular
> fashion. When the current online redo log file fills,
> LGWR begins writing to the next available online
> redo log file. When the last available online redo
> log file is filled, LGWR returns to the first online
> redo log file and writes to it, starting the cycle
> again. Figure 7-1 illustrates the circular writing
> of the online redo log file. The numbers next to
> each line indicate the sequence in which LGWR writes
> to each online redo log file.....
> ---------------------------------
> Sorry I couldn't draw the picture(Figure 7-1)
> ===================================================
> Now I have a question on this.
>
> Scenario
> ---------------------------------------------------
> Instance X useing thread 1, which contains two groups
> of online redo log files (groups 1, 2). Thread
> 1 is multiplexed, that is, each group has two copies
> (members) of the redo log files.
>
> So as per above definition when group 1 is filled up
> then LGWR start to write on group 2.But after filling
> up Group 2 it again swiches to Group 1.
>
> Now I have a doubt on this point..
> 1.Does it over write the existing contain of Redo log
> files.
> 2.If yes then how to identify that LGWR is now end of
> present Redo Log.If I need to archive the next Redo
> Log file do that..
> ----------------------------------------------------
> I tried my level best to explain the doubt.If you have
> some problem to understand the query please let me know.
>
> Regards
> Arijit Chatterjee

I think the DBA Administration manual chapter that covers this material answers the question asked rather directly.

If in archive log mode and the log has been successfully archived then the inactive online log is overlaid

In noarchive log mode the inactive online log is overlaid.

This is providing that the checkpoint has completed. With only two small logs there is a possibility of a delay occurring while the checkpoint completes. Normally there are at least three online redo logs and numbers in the range 3 to 6 are what commonly exist in the real world. Two is just the minimum number of online redo logs possible.

HTH -- Mark D Powell -- Received on Fri Nov 05 2004 - 09:04:38 CST