Re: Why does Orcl generate REDO logs in NOARCHIVE mode?

If you pull the plug on the machine or the OS kernal crashes, then Oracle is able to retore any in doubt transactions to the point of failure. Without the redo logs, any crash or "shutdown abort" would require restoring to the last cold backup. The archive logs provide for a media failure recovery. A lost (delted) database file can be reovered with the archive logging. The redo logs provide for recovery from "smaller" failures.

Please note that by putting the redo logs in RAM disk, a power flicker or OS crash would put you into media failure mode (a database file would be lost). Recovery becomes more complex.

James

In article <36F8CD86.648A1AFE_at_capgemini.co.uk>,   Mike Burden <michael.burden_at_capgemini.co.uk> wrote:
> Agreed that redo logs aid recovery, but surely they are not essential. If you
> accept that in the event of a failure, you will restore to a cold backup, why
do
> you need the redo logs. If you don't back them up and you loose everything you
> can still go back to a cold back even without the redo logs. I have thought
> about putting redo and rollback segments into a RAM disk to prevent the disk
IO
> in certain situations (e.g. Development environment). I've already tried it
with
> Rollbacks in RAM which seem to work OK. If it all goes nasty you just restore
> and re-run. No roll forward necessary.
>
> It would be nice if you could turn either or both off and save on the
overhead.
> Hence three modes. This would be particularly useful for large overnight batch
> jobs where you do a cold backup at the end anyway. Because the job works
99.999%
> of the time you wish to take advantage of the fact by switching off redo and
> rollback and save on loads of IO.
>
> Just my thoughts. Anyone else care to comment. Are these features available in
> any other DBMS?
>
> I still think Oracle writes twice as much data to the redo logs than it needs
to
> because rollback info gets logged too. But that's another story.
>
> Jan-Marten Spit wrote:
>
> > Chakravarthy KM Nalamotu wrote in message <7d97kq$fh8_at_news.Hawaii.Edu>...
> > >Hello Oracle Users,

 [Snipped]
> > > Kittu.
> >
> > Separate these concepts:
> >
> > -Redo log
> > The redo log consist of two or more log files, and contains all changes to
> > data blocks in the buffer cache not synced with their datafiles. In case of
> > a database crash, the redo log is used to sync the datafiles with the
> > database state at the time of the crash. The redo log aids in recovering in
> > case of a disk failure. When a redo log file is full, a log switch occurs,
> > and the redo log is written to the next redo logfile. There are always two
> > or more redo logfiles.
> >
> > -Archive log
> > Since the redo log is circular, the redo log files are overwritten
> > eventually. In archive log mode, a trail of redo log files is kept. An
> > archive is created of a redo log file before the redo log overwrites it. The
> > archive log aids in instance failures, being able to roll forward the
> > database state from the last backup to a specific (check) point in time.
> > Only backups (while the database is fully operational) are also possible.
> >
> > In short, you need your redo logs files in all cases, because oracle needs
> > it to guarantee database consistency. The use of archive log mode is
> > optional. Benefits are recovering to point in time, and possibility for
> > online backups. When you only require offline backups (shut-backup-start),
> > and the tape from your last offline backup is good enough in case of a
> > failure, you do not need archive log mode. Note that the archive log grows
> > unless you do something about it, like cleaning after backup.
> >
> > Hope this helps,
> >
> > JM
>
>

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