Re: Automatic Segment Space Management

I was going to mention the paper - but don't take it as an extreme condemnation of ASSM. It's just an example of one detail of the implementation that still hasn't been corrected - and the example is doing something which you shouldn't do anyway. It does hint, however, that there may be other traps in the implementation waiting to catch the unwary.

As Greg says, there have been cases where using ASSM with RAC has resulted in an easy and comfortable reduction in contention on freelist blocks - but the price you pay is that instead of having a very small number of freelist blocks per segment, you end up with a couple of bitmap space management blocks per segment - and that could turn into a significant fraction of the blocks from your total db_cache_size.

More importantly, at the Miracle Masterclass in Copenhagen this year, Steve Adams listed a few outstanding (9.2) bugs with the way in which the bitmap space management blocks are maintained. In particular, it is very easy to 'leak' space and end up with bitmaps declaring that a data block is FULL when it isn't.

As an easy example try this:

    create table in ASS managed tablespace     insert 10,000 large rows into table
    rollback;
    dump all relevant bitmap blocks.

You will find that every single block that was used id marked as FULL. Rolling back does not reset the bitmaps correctly.

There are other, slightly more subtle, issues which might be a little more realistic in other cases - largely they revolve around rolling back or mixtures of deletes and inserts.

Regards

Jonathan Lewis
http://www.jlcomp.demon.co.uk

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-----Original Message-----
To: Multiple recipients of list ORACLE-L <ORACLE-L_at_fatcity.com> Date: 19 February 2003 20:12

>See Jonathan Lewis' paper on ASS Management:
>http://www.jlcomp.demon.co.uk/bustbits.html
>
>HTH,

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Author: Jonathan Lewis
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