Re: db_block_checking

Hello Nenad,
I think David Loinaz ( https://twitter.com/DavidLoinaz��) or Gabriel Alonso ( https://twitter.com/Gaalons0��) are the go-to-guys for this question as they deal / dealt with this kind of stuff all day :)  
David has also written a more general blog post about it here but not particular to your question about LOW / MEDIUM: https://davidloinaz.wordpress.com/2016/02/24/db_block_checksum-and-risk-perception/

Best Regards
Stefan Koehler

Independent Oracle performance consultant and researcher Website: http://www.soocs.de
Twitter: _at_OracleSK

> Noveljic Nenad <nenad.noveljic_at_vontobel.com> hat am 29. November 2019 um 10:49 geschrieben:
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> I’ve been always setting db_block_checking to FULL(TRUE). However, it wasn’t until recently, that I measured a significant impact on DML performance when db_block_checking is set to MEDIUM or FULL. Further, this degradation is clearly more severe with larger block sizes.
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> This prompted me to think of other options. Now, I need your help in understanding the risks associated with different settings.
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> First of all, I don’t want to use OFF – for obvious reasons.
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> Second, the difference between LOW and MEDIUM are “full semantic checks”, see Oracle documentation [ https://docs.oracle.com/en/database/oracle/oracle-database/19/refrn/DB_BLOCK_CHECKING.html#GUID-23700E5C-6BFC-48C2-9728-EB1F93F95DD6](https://docs.oracle.com/en/database/oracle/oracle-database/19/refrn/DB_BLOCK_CHECKING.html#GUID-23700E5C-6BFC-48C2-9728-EB1F93F95DD6)  .
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> What are these “full semantic checks” and what would be the risk of omitting them (i.e., setting db_block_checking to LOW)?
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> Finally, the only difference between MEDIUM and FULL – according to the documentation - is that the aforementioned “full semantic checks” aren’t performed for the index blocks. A little side note, though: I measured a significant DML performance difference between MEDIUM and FULL even on non-indexed tables.
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> What do you think could happen in case of an unnoticed index block corruption (that is, with MEDIUM)? Wrong results or some ORA-0600/ORA-07445, perhaps?
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> If you’re using anything less than FULL, could you explain your decision?
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> Best regards,
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> Nenad
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> https://nenadnoveljic.com/blog/

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