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

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 ...


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>Don't set optimal. Please. It's crap for performance, and 1Mb is in any

case


>extremely small.

>