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Home -> Community -> Usenet -> c.d.o.server -> Re: Do I really need more than 1 rollback segment?
Comments below
HJR
"Jonathan Lewis" <jonathan_at_jlcomp.demon.co.uk> wrote in message
news:1008285742.1773.0.nnrp-13.9e984b29_at_news.demon.co.uk...
>
[snip some good stuff]
>
> Stats from v$rollstat - virtually no difference.
> Stats from v$filestat -
> Small rbs 50 blocks written
> Large rbs 686 blocks written.
>
> If your rollback segments are small enough
> they will be recycled before Oracle has time
> to dump them to disc - and this can make
> a dramatic difference on the total I/O - which
> will affect the way DBWR can work even if the
> RBS is on a totally separate set of discs.
>
Makes sense. It seems to me that you are trading I/O against risk of 1555s (though I note your qualification below). Normally I would run a mile from extra I/O -but this one I have doubts about.
Furthermore, since DBWR does writes in the background, I'd be interested to know the actual performance impact of this particular bit of extra I/O. I guess you wouldn't agree with Steve's assertion that "It is not critical to optimize rollback segment writes because they are performed in the background by DBWn."? Or that "Rollback segment I/O is normatively write-intensive and largely logically sequential. If the rollback segment requirements are large enough, it is possible to obtain largely physically sequential I/O by placing each rollback segment in its own tablespace and on dedicated disks."?
As for Steve's statement that "There is no overhead in using large rollback segment extents instead of small ones. " -well, this can only be explained, given your figures, if he's assuming that the additional writes are insignificant in the scheme of things. Agree/Disagree?
I presume from all this that you would hate the 9i algorithm for undo segments, and particularly the undo_retention idea (on the grounds, I am assuming, that the need to retain undo for, say, three hours, would force such undo to be written to disk instead of being merely overwritten in the buffer cache???).
Would relish your thoughts.
Regards
HJR
> Having said that, I also point out in the seminar
> that whilst small is good, you have to size your
> total rollback to support your longest running
> read-consistent event.
>
>
> --
> Jonathan Lewis
> http://www.jlcomp.demon.co.uk
>
> Host to The Co-Operative Oracle Users' FAQ
> http://www.jlcomp.demon.co.uk/faq/ind_faq.html
>
> Author of:
> Practical Oracle 8i: Building Efficient Databases
>
> Screen saver or Life saver: http://www.ud.com
> Use spare CPU to assist in cancer research.
>
> Howard J. Rogers wrote in message
> <3c166064$0$559$afc38c87_at_news.optusnet.com.au>...
> >We could have a battle of the giants on our hand here. Steve Adams says
> >make 'em big, and who cares about a bit of wasted space: these things are
> >supposed to be on their own hard disk anyway. (I am paraphrasing like
> >crazy, natch).
> >
> >Personally, I go for the Steve Adams school of thought on this one. I
> can't
> >see any drawbacks (though I'm sure Jonathan will elaborate on the
> >'additional I/O' idea) of large segments. It's the NUMBER of them that's
> >the worry, to avoid contention issues.
> >
>
>
>
Received on Thu Dec 13 2001 - 22:27:04 CST
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