Re: undo header congestion? (8.0.5)

"Bjørn Augestad" <boa_at_metasystems.no.spam.to.me> wrote in message news:2ofPb.30977$BD3.6794887_at_juliett.dax.net...
> Sybrand Bakker wrote:
>
> > On Tue, 20 Jan 2004 15:47:40 +0100, Bjorn Augestad
> > <Bjorn.Augestad_at_bokklubbene.no> wrote:
> >
> >
> >>Some additional info: instance 1 has two rollback segments, and instance
> >>2 has just one (+ system).
> >
> >
> >
> > The number of rollback segments should be set according to the rule of
> > four: per 4 users 1 rollback segments, with a minimum of 4 rollback
> > segments, and rounded up.
> > Consequently both instances have too few rollback segments, which is
> > confirmed by the number of waits on the undo header block.
> >
>
> Thank you for answering. :-)
>
> Would you(or anyone else) consider it a serious case of misconfiguration
> to set up an instance with just one/two rollback segments, assuming
> 25-50 users?
>
> Regards,
> Bjørn
>

No, because whoever told you the rule was 'per 4 users 1 rollback segment' has got it slightly, but horribly, wrong. The number of users is completely irrelevant. It's the number of concurrent transactions that is important. So instead of doing a select count(*) from v$session, you would be better off doing a select count(*) from v$transaction. Whatever that number is, divide by four and that's your starting position for an OK-ish number of rollback segments. If the number of 'proposed' segments is incredibly high, feel free to degrade the ratio a little, down to 1 maybe per 10 concurrent transactions.

But back to your question: 50 users, four or five of them concurrently doing DML? Sounds like 1 or 2 rollback segments would do just fine.

Regards
HJR

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