Comments below.
HJR
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Resources for Oracle: http://www.hjrdba.com
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"obakesan" <cjundieseastwd_at_powerup.com.au> wrote in message
news:eqmn8.11478$T4.89567_at_nnrp.gol.com...
>
> HiYa
>
> In article <a7hb54$9fg$1_at_lust.ihug.co.nz>, some kind human wrote:
> >Archivelog mode simply means that you can guarantee recoverability of all
> >transactions. If you can guarantee recoverability by some other
mechanism,
> >then no, you don't need archivelog mode.
>
> perhaps I misunderstood archivelog mode, I thought it was to mean that the
data
> in the redo log was written out by the archiver process, to files, rather
than
> being over written as the redologfiles filled ... maybe I dont quite get
when
> the redolog's are and are not used in the firstplace....
>
Well, your description here sounds fine. DML produces redo, and given the
finite number of online logs, fresh DML will overwrite the redo generated by
older DML. So yes, archiver can be switched on to copy a log when its full
so that the overwriting of the online logs doesn't mean we lose that redo.
> >
> >By that, I mean that these nightly loads must be coming from some source.
> >If you can back that source up, and are prepared to repeat a nightly load
if
> >something happens to the database, who cares about archives?!
>
> not me .... that was why I was asking.
>
What I was trying to say was that if you don't mind repeating loads
(potentially up to a weeks' worth), your weekly cold backups and the
preservation of the source of your nightly loads for that week would enable
you to restore the week-old files, and repeat the week's loads. At the end
of it, you'd have achieved exactly the same result as restoring an old file,
and applying archives to it. So, if you were prepared to do that, you
wouldn't need to take archives.
>
> > A lot of
> >sites, for example, will use SQL Loader to load millions of records from
a
> >text file. Since the text file can be burnt to CD, or put on tape, the
load
> >could be repeated if ever there was trouble on the database.
>
> my intention ... but I'd prever to only have to recover and then redo a
single
> load, as it will taketime and this is a reasonably mission critical
database.
>
Well, if you're doing nightly loads, and you want only to ever have to
reperform one of them, you'd need to take nightly backups. And as you know,
that backup would have to be cold if you weren't running in archivelog mode.
And if you *were* running in archivelog, the backup could be hot, but you
wouldn't need to have to reperform loads, because the load's redo would be
applied automatically from the archives as part of Oracle's recovery
process.
Just to complicate the picture, it's possible that your nightly loads are
very big. That to speed up the loading process you therefore run SQL Loader
in 'nologging' mode. At which point, the archives *won't* include your
load's redo, because 'nologging' means 'don't generate any redo for these
transactions'. So it's even possible that there's no point in being in
archivelog mode in the first place, if that's how you intend to perform your
loads. And noarchivelog means cold backups. So we come full circle:
depending on how you load your data, you might *have* to do cold backups.
>
> >
> >But: You can never, ever make a copy of any database file whilst the
> >database is running, without that copy being internally inconsistent.
And
> >only archive logs guarantee that you can make an internally inconsistent
> >datafile consistent.
>
>
> damn ... theres that consistent word again, I am going to have to get to
what
> that really means ... is it not the sequence of where Oracle is in all of
its
> threads? Cant I just trash the logfiles, and the control files, and
rebuild from
> a script from the trace file (command syntax escapes me now)??
>
> ok ... so its sounding like we need to use archivelog mode, or cold only.
>
It all depends on (1) the size of the data files involved and whether you
can cold copy the entire database in the time allowed by your nightly
maintenance window (2) how you perform your loads, nologging meaning that
being in archivelog mode is pretty pointless for you (3) how many loads you
are prepared to re-perform in the event of a failure.
> >So: your choice. If you can repeat the loads, and
> >don't mind doing so, don't bother with archives. But then all backups
will
> >require that you shut your database down before making a backup.
>
> I guess that since its not in use all the time, I could just do nightly
cold
> backups as long as the time taken to copy the files fited into my window
of
> available time ....
>
That's the crucial issue, of course. I don't think you've mentioned how
many records we're talking about. Or the size of the data files involved.
Hence the 'generalized' answers you've been getting.
<snip>
Regards
HJR
Received on Sun Mar 24 2002 - 14:09:06 CST
