Hi Nenad!
That is described here:
https://docs.oracle.com/en/database/oracle/oracle-database/19/refrn/DB_BLOCK_CHECKING.html#GUID-23700E5C-6BFC-48C2-9728-EB1F93F95DD6
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.
Regards
On 11/29/19 9:52 AM, Noveljic Nenad wrote:
Thanks, Mladen.--
Yes, DB_BLOCK_CHECKING verifies the structure in memory. It's also worth
mentioning that this happens with each DML. As you mentioned, the block isn't
written and reread for verification purposes.
Would you happen to know the difference between LOW and MEDIUM?
So, with DB_BLOCK_CHECKING=MEDIUM the worst that can happen is that a corrupt
index block gets written to disk? In other words, there's a possibility of a
downtime between detecting this problem and rebuilding the index.
Best regards,
Nenad
https://nenadnoveljic.com/blog/
-----Original Message-----
From: oracle-l-bounce@xxxxxxxxxxxxx <oracle-l-bounce@xxxxxxxxxxxxx> On Behalf
Of Mladen Gogala
Sent: Freitag, 29. November 2019 15:21
To: oracle-l@xxxxxxxxxxxxx
Subject: Re: db_block_checking
Hi Nenad!
DB_BLOCK_CHECKING is a syntax check only which verifies the block structure in
memory. It's different from DB_BLOCK_CHECKSUM, which writes the full checksums
into block header. To my knowledge, Oracle doesn't do the write verification
with re-reading the block back to memory and recomputing the checksum because
of fairly dramatic performance impact.
I usually do DB_BLOCK_CHECKING on medium, unless the database is so critical
that I cannot afford to rebuild indexes.
Regards
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