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Home -> Community -> Usenet -> c.d.o.server -> Re: Do I really need more than 1 rollback segment?
It's mad, Hemant, because if OPTIMAL is set, when does the shrink actually
occur? When your transaction is seeking the next extent to write its
rollback to, that's when. Which means that your transaction has to take
time out from doing what it is supposed to do, and wait for the shrink to
take place. So it's sub-optimal for performance, in a nutshell.
Now clearly, if the rollback segment only balloons once a week, you're not going to particularly affected by that. But since growth of the rollback segment is not controllable (one user leaving open an uncommitted transaction will do it), the use of OPTIMAL also means that the shrinks are uncontrollable. You've been lucky, therefore, not to have shrinks happen left, right and centre.
Consistently sized rollback segments are easy to achieve without that overhead by simply arranging for a cron job (or a job scheduled in some other way) to issue the 'alter rollback segment shrink to X' command at a time and place of *your* choosing.
Regards
HJR
"Hemant K Chitale" <hkchital_at_singnet.com.sg> wrote in message
news:9ut672$lql$1_at_violet.singnet.com.sg...
> Why is setting optimal "equally mad" ? I've set OPTIMAL
> and haven't had a single ORA-1555 for more than a year.
> One Rollback Segment does grow as large as 400MB
> once a week when a particular job runs but shrinks back
> to 20MB -- it has to shrink so many extents only once a week.
> Other than that, yes, there are some shrinks etc but at least
> I have predictable rollback segment sizes.
>
> Hemant K Chitale
> http://hkchital.tripod.com
>
>
> "Howard J. Rogers" <dba_at_hjrdba.com> wrote in message
> news:3c110a2a$0$29051$afc38c87_at_news.optusnet.com.au...
> > I agree with your disagree! Setting rollback segments 'as small as you
> can
> > make them' is just plain daft, and setting optimal at all is equally
mad.
> > Optimal is there for those databases that don't have a full-time DBA,
and
> as
> > such it can be a useful 'light-weight management' tool. But in a
> > properly-managed production database, it's an utterly lousy idea.
> >
> > Regards
> > HJR
> > --
> > Resources for Oracle: http://www.hjrdba.com
> > ===============================
> >
> >
> > "godmann" <allanwtham_at_yahoo.com> wrote in message
> > news:95cd51c.0112070034.1fde8cee_at_posting.google.com...
> > > Hi Andrew
> > >
> > > >>Rule of thumb: rollback segments should be as small as you can make
> > > >>them, with lots of extents. You don't want them too small that
> they're
> > > >>constantly extending however. Oh, and set an optimal size for them
as
> > > >>well.
> > >
> > >
> > > Are you sure setting an optimal size is a good idea?? I kinda
disagree
> > >
> > > Allan W. Tham
> > > DBA
> >
> >
>
>
Received on Sat Dec 08 2001 - 15:59:11 CST
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