Re: the number of rollback segments

Interesting question:

I have seen 50 quoted somewhere too, but I can't remember where.

The other figures tend to suggest that you are likely to get contention if you have more than 4 concurrent transactions attacking a rollback segment at once, so the limit of 50 is implicitly suggesting that you have no more than 200 concurrent transactions in your database.

Since there are a minimum of 21 transaction slots per rollback segment (that's for 2K, perhaps someone else has the figures for larger block sizes to hand) it is of course possible to more than 200 concurrent
transactions.

Perhaps the limit is a trade-off point -

    more than 4 transactions per rbs => contention     more than 50 rbs => lots of CPU lost deciding

        which rollback segment is the next to be used.

Also, of course, it could be one of those figures that came out with small databases and v 5.1 and has never been reviewed too closely.

It would be interesting to try thrashing a system with (say) 6, 8, 10 concurrent transactions on a single rollback segment, and seeing how the rollback segment header contention started to climb. Has anyone got the spare time ?

--

Jonathan Lewis
Yet another Oracle-related web site: http://www.jlcomp.demon.co.uk

fumi wrote in message <7s5q5m$h6l$1_at_news.seed.net.tw>...
>Hi,
>
>Someone says that "The number of rollback segments should not be exceeding
>50."
>
>I can't find this rule in the "Administrator's Guide" and "Tunning" manuals
>and some other books.
>In the "Tunning" manual, it says the number of rollback segments is
> 4, if n<16
> 8, if 16<=n<32
> n/4, if 32<=n, where n is the number of current transactions.
>It doesn's say there is a upper bound of then number of rollback segments.
>
>Is this rule true or not?
>What will happen, and what's the disadvantage if we create more than 50 RBS
>in one database?
>
>
>
Received on Mon Sep 20 1999 - 13:49:47 CDT