Re: Online verification of checksums

Hi.

Am Montag, den 17.09.2018, 20:45 -0400 schrieb Stephen Frost:
> > You're right it's not about the fsync, sorry for the confusion. My point
> > is that using the checkpoint LSN gives us a guarantee that write is no
> > longer in progress, and so we can't see a page torn because of it. And
> > if we see a partial write due to a new write, it's guaranteed to update
> > the page LSN (and we'll notice it).
>
> Right, no worries about the confusion, I hadn't been fully thinking
> through the LSN bit either and that what we really need is some external
> confirmation of a write having *completed* (not just started) and that
> makes a definite difference.
>
> > > Right, I'm in agreement with doing that and it's what is done in
> > > pgbasebackup and pgBackRest.
> >
> > OK. All I'm saying is pg_verify_checksums should probably do the same
> > thing, i.e. grab checkpoint LSN and roll with that.
>
> Agreed.

I've attached the patch I added to my branch to swap out the pg_sleep()
with a check against the checkpoint LSN on a recheck verification
failure.

Let me know if there are still issues with it. I'll send a new patch for
the whole online verification feature in a bit.

Michael

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