Re: Online verification of checksums

Hi,

Am Dienstag, den 19.03.2019, 11:22 -0400 schrieb Robert Haas:
> It's torn pages that I am concerned about - the server is writing and
> we are reading, and we get a mix of old and new content. We have been
> quite diligent about protecting ourselves from such risks elsewhere,
> and checksum verification should not be held to any lesser standard.

If we see a checksum failure on an otherwise correctly read block in
online mode, we retry the block on the theory that we might have read a
torn page. If the checksum verification still fails, we compare its LSN
to the LSN of the current checkpoint and don't mind if its newer. This
way, a torn page should not cause a false positive either way I think?.
If it is a genuine storage failure we will see it in the next
pg_checksums run as its LSN will be older than the checkpoint. The
basebackup checksum verification works in the same way.

I am happy to look into further option about how to make things better,
but I am not sure what the actual problem might be that you mention
above. I will see whether I can stress-test the patch a bit more but
I've already taxed the SSD on my company notebook quite a bit during the
development of this so will see whether I can get some real server
hardware somewhere.

Michael

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