Re: Best/Only way to change RBS initial/minextents

Funny, I don't disagree with any of this. The conclusion at the end is exactly what I'd say if I could be bothered to type it all. I guess my 'take' on it is that it's a tricky thing to get right, and the costs therefore mount. The shorter version is thus that 'you are unlikely to get it right, so avoiding the costs is the better approach'.

Maybe I'm just pessimistic.

;-)
HJR "Jonathan Lewis" <jonathan_at_jlcomp.demon.co.uk> wrote in message news:angshg$dn$2$8300dec7_at_news.demon.co.uk...
>
> It's not often that I feel the need to disagree
> strongly with Howard, but I believe that setting
> Optimal (properly) is likely to be beneficial.
>
> Richard Foote wrote a reply on the newsgroup
> recently explaining why having the smallest
> possible volume of rollback can be an aid to
> performance by reducing the total DBWR load,
> so I won't repeat all the details.
>
> The concern that 'optimal' causes a massive
> dump to disc 'just when you don't want it to
> happen' is potentially deceptive.
>
> When a rollback segment shrinks, the dirty blocks
> in the discarded extents have to be written
> to disc (to cater for worst case crashes). This
> is true, and this is the 'massive performance hit'
> that you have complained about in the past.
>
> However, Oracle will dump the oldest extents
> to disc, and if the optimal has been set properly,
> these are the very extents which are likely
> to have been dribbled out to disc by the
> continuous checkpoint / MTTR algorithm
> that Oracle uses nowadays. After all, the
> I/O benefit you get from right-sizing is that
> you don't have excess extents that are constantly
> dribbling to disc.
>
>
> Clearly, if you set the optimal a little bit too small,
> then you are likely to be adding and dumping extents
> frequently, and the extra dumping you do is likely to
> be of extents that would not otherwise be written -
> and your argument has some merit.
>
> Otherwise it is the standard Oracle argument of
> which feature to use when:
> I can pay a small price thousands of times per hour
> or
> I can pay a large(r) price once per day
>
>
> Small rollback segments are good - if possible.
> If they are possible, then Optimal is good.
>
> The price may be unacceptable, in which case
> a 'manual optimal' may be the better approach.
>
>
> --
> Regards
>
> Jonathan Lewis
> http://www.jlcomp.demon.co.uk
>
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>
>
>
>
>
> Howard J. Rogers wrote in message ...
> >
> >
> >Don't set optimal. Please. It's crap for performance, and 1Mb is in any
> case
> >extremely small.
> >
>
>
>
Received on Thu Oct 03 2002 - 04:20:25 CDT