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.

Yes, that is what it says. That is why you need to archive them elsewhere, so if you ever need to recover, you have logs to apply.

> 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..

If you have no archive logging, nobody cares. If you have archive logging but not automatic, your database will stop and wait for you to manually archive.

You should put together an installation so you can try these things, after you fumble about for a while and try recovery it becomes much more clear. Nobody runs just two groups with archive logging, you need at least one being used, one waiting to be used, and one being archived. Your database will stop if it cycles through them before the archiving is finished.

> ----------------------------------------------------
> 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

jg

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