Re: Online verification of checksums

Hi Fabien,

On Thu, Oct 25, 2018 at 10:16:03AM +0200, Fabien COELHO wrote:
> >New version 5 attached.
>
> Patch does not seem to apply anymore.

Thanks, rebased version attached.

> Moreover, ISTM that some discussions about behavioral changes are not fully
> settled.
>
> My current opinion is that when offline some errors are not admissible,
> whereas the same errors are admissible when online because they may be due
> to the ongoing database processing, so the behavior should not be strictly
> the same.

Indeed, the recently-added pg_verify_checksums testsuite adds a few
files with just 'foo' in them and with V5 of the patch,
pg_verify_checksums no longer bails out with an error on those.

I have now re-added the retry logic for partially-read pages, so that it
bails out if it reads a page partially twice. This makes the testsuite
work again.

I am not convinced we need to differentiate further between online and
offline operation, can you explain in more detail which other
differences are ok in online mode and why?

> This might suggest some option to tell the command that it should work in
> online or offline mode, so that it may be stricter in some cases. The
> default may be one of the option, eg the stricter offline mode, or maybe
> guessed at startup.

If we believe the operation should be different, the patch removes the
"is cluster online?" check (as it is no longer necessary), so we could
just replace the current error message with a global variable with the
result of that check and use it where needed (if any).

Michael

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