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Re: Database backups

From: Howard J. Rogers <dba_at_hjrdba.com>
Date: Sat, 23 Mar 2002 07:54:29 +1100
Message-ID: <a7g5no$26s$1@lust.ihug.co.nz>


"daniel" <test_at_test.com> wrote in message news:a7g4mb$q7i$1_at_news5.svr.pol.co.uk...
> firstly i knew some smart arse would write such a reply,,,, my reply was
> trying to be generic!!!!!!
>

Generic is fine. Trying is fine. Failing to be generic, however, isn't.

> a cold backup is a consistent backup of a database that has shutdown
normal
> (minus online redo logs) thus negating the need to roll forward from such
a
> backup. yes in an archivelog db you could bring back a df from a cold
backup
> set and roll forward but my point was u would not normally roll forward
from
> a complete consistent cold backup even though you could do....
>

Rubbish. Just because you are in archivelog mode does not mandate that you do hot backups. Plenty of people do cold backups, and take archives. Archives gives you the ability to completely recover your database. Taking backups (hot or cold) gives you something which can be rolled forward. There's no other relationship between the two, and there's nothing "normal" or "abnormal" about either type of backup in archivelog mode.

In my experience, about 35-40% of people running in archivelog mode take cold backups. What you say is 'not normal' for them to do, they plan to do routinely.

So, whilst I knew the point you were trying to make, it's simply wrong.

> regarding log switch the user states it is an ebusiness environment
(oltp!)
> so we are probably putting tx's through it. well as we both know worst
case
> scenario is u lose your current online redo log, thus tx's that may have
not
> checkpointed (yes i know we can alter the frequency of the checkpoint) so
> online redo sized to switch every 15 mins means worst case scenario is we
> lose 15 mins of bussiness data....
>

So, why not checkpoint every second, 'cause that way you only lose 1 second of "bussiness [sic] data"? Because checkpoints have an overhead. And that overhead slows down oltp transactional activity. So to come out with a bald "make it 15 minutes" is just meaningless.

Checkpointing should be done at a rate that balances possible transaction loss/recovery time with the slowdown in performance that excessive checkpointing induces. The appropriate advice is to find some point on the spectrum that you feel comfortable with, not come out with some meaningless specific time interval.

And *that* is generic advice, whereas 'make it 15 mins' is highly specific, highly misleading, and a thoroughly dreadful piece of advice.

> so before we enter into a "my dad's bigger than your dad" argument there
are
> 15 billion approches to oracle backups so don't call it "dreadfull
advice",
> it's just another way of looking at it.
>

Actually the "dreadful advice" comment was made in relation to your assertion that you should aim "to log switch every 15 mins", and had nothing to do with how you do backups.

HJR
> reagrds,
>
> daniel...
>
> "Howard J. Rogers" <dba_at_hjrdba.com> wrote in message
> news:a7el2i$hek$1_at_lust.ihug.co.nz...
> > Comments below
> > HJR
> > --
> > ----------------------------------------------
> > Resources for Oracle: http://www.hjrdba.com
> > ===============================
> >
> >
> > "daniel" <test_at_test.com> wrote in message
> > news:a7deuk$417$1_at_newsg2.svr.pol.co.uk...
> > > as a rule of thumb u don't roll forward from a cold backup, (yes, yes
I
> > know
> > > you can but lets not get into bad habits)....
> > >
> >
> > That's simply not true, and it's not a bad habit either. Of course you
> can
> > roll forward from a cold backup. And taking cold backups is much easier
> > than the hot variety.
> >
> > > you can use just hot backups no probs and can recover quicker from
them
> > >
> > > pointers;
> > >
> > > have multiple ctl files spanning physical disks
> > > multiplex your online redo logs across physical disks
> > > aim to log switch every 15 mins
> >
> > Dreadful advice. If you want performance, log switch every 24 hours, in
> the
> > dead of night, when no-one gives a damn about the huge amount of I/O the
> > associated checkpoint will induce. If you want Instance Recovery in ten
> > seconds, log switch every second or so. Somewhere in between those two
> > extremes will be a happy medium for *you*.
> >
> > > every time you change the db structure "alter database backup
> controlfile
> > to
> > > trace"
> >
> > Equally dodgy advice, I think. Backup to trace should be routine.
Every
> > backup should include it.
> >
> > > after the hot backup take a copy of the ctlfile and "archive log
> current"
> > > and make sure your archive redo logs are protected stream them off
> > somewhere
> > > else every 15 mins
> >
> > Depends on your log switch rate, of course (see above!!)
> >
> > > also when you take the hot backup keep a copy locally if enough space
> also
> > > stream to tape and if recovery time is an issue copy to some network
> > storage
> > >
> >
> > Agreed. Keep as much on disk as possible.
> >
> > Regards
> > HJR
> >
> > > good luck
> > >
> > > daniel...
> > >
> > >
> > > "Dale DeRemer" <dderemer_at_agmc.org> wrote in message
> > > news:a7dcs8$q7u$1_at_malgudi.oar.net...
> > > We are new the the Oracle world. We want our ebusiness server to be
> 7x24.
> > > Never, ever down. Meaning... no cold backups. So, our question is
this:
> If
> > > we use hot backups, (RMAN), and never take a cold backup, will we be
> able
> > to
> > > recover from any failure. Additionally, what is the impact, or
> difference
> > in
> > > recovery time for a system with no cold backups, vs. one with a cold
> > backup
> > > done once a week, or once a month?The DB is 75GB and will grow to
about
> > > 100GB over the next year. It will be updated in batches from our
> > mainframe.
> > > Users will not update it. Thanks for your help.
> > >
> > >
> > >
> > >
> >
> >
>
>
Received on Fri Mar 22 2002 - 14:54:29 CST

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