Simply put, LOW calculates the checksum just for block headers, and MEDIUM for
the whole data block content. So, "full semantic checks" in the documentation
means "both headers and data".
This also explains why the performance with LOW doesn't depend on the block
size (unlike MEDIUM and FULL, which is proportional to the block size):
From: Mladen Gogala <gogala.mladen@xxxxxxxxx>
Sent: Freitag, 29. November 2019 16:21
To: Noveljic Nenad <nenad.noveljic@xxxxxxxxxxxx>; oracle-l@xxxxxxxxxxxxx
Subject: Re: db_block_checking
That is described here:
Basically, LOW performs only the header validations. MEDIUM performs the full
check (blood pressure, triglycerides, cholesterol, EKG) on all blocks except
the index blocks. Full checks the index blocks as well.
So, your assessment is correct: the worst that can happen with MEDIUM is to
have a corrupt block written to your index, which would render it unusable.
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