Re: Online verification of checksums

On Wed, Oct 21, 2020 at 12:00:23PM +0200, Michael Banck wrote:
> The check was ported (or the concept of it adapted) from pgBackRest if I
> remember correctly.

Okay, I did not know that.

>> As things stand, I'd like to think that it would be much more useful
>> to remove this check and to have one or two extra retries (the current
>> code only has one). I don't like much the possibility of false
>> positives for such critical checks, but as we need to live with what
>> has been released, that looks like a good move for stable branches.
>
> Sounds good to me. I think some were advocating for locking the page
> before re-reading. When I looked at it, the level of abstraction that
> pg_basebackup has (just a list of files chopped up into blocks, no
> notion of relations I think) made that non-trivial, but maybe still
> possible for v14 and beyond.

That's a API layer I was looking at here:
https://www.postgresql.org/message-id/flat/CAOBaU_aVvMjQn=ge5qPiJOPMmOj5=ii3st5Q0Y+WuLML5sR17w(at)mail(dot)gmail(dot)com

My guess is that we should be able to make use of that for base
backups as well, but I also think that I'd rather let v13 go with more
retries without depending on a new API layer, removing of the LSN
check altogether. Thinking of it, that's actually roughly what I
posted here, but without the PageGetLSN() bit in the refactored code.
So I see a pretty good argument to address the stable branches with
that, and study for the future a better API to govern them all:
https://www.postgresql.org/message-id/20201020062432.GA30362@paquier.xyz
--
Michael