Re: Online verification of checksums

On 2019-03-06 12:33:49 -0500, Robert Haas wrote:
> On Sat, Mar 2, 2019 at 5:45 AM Michael Banck <michael(dot)banck(at)credativ(dot)de> wrote:
> > Am Freitag, den 01.03.2019, 18:03 -0500 schrieb Robert Haas:
> > > On Tue, Sep 18, 2018 at 10:37 AM Michael Banck
> > > <michael(dot)banck(at)credativ(dot)de> wrote:
> > > > I have added a retry for this as well now, without a pg_sleep() as well.
> > > > This catches around 80% of the half-reads, but a few slip through. At
> > > > that point we bail out with exit(1), and the user can try again, which I
> > > > think is fine?
> > >
> > > Maybe I'm confused here, but catching 80% of torn pages doesn't sound
> > > robust at all.
> >
> > The chance that pg_verify_checksums hits a torn page (at least in my
> > tests, see below) is already pretty low, a couple of times per 1000
> > runs. Maybe 4 out 5 times, the page is read fine on retry and we march
> > on. Otherwise, we now just issue a warning and skip the file (or so was
> > the idea, see below), do you think that is not acceptable?
>
> Yeah. Consider a paranoid customer with 100 clusters who runs this
> every day on every cluster. They're going to see failures every day
> or three and go ballistic.

+1

> I suspect that better retry logic might help here. I mean, I would
> guess that 10 retries at 1 second intervals or something of that sort
> would be enough to virtually eliminate false positives while still
> allowing us to report persistent -- and thus real -- problems. But if
> even that is going to produce false positives with any measurable
> probability different from zero, then I think we have a problem,
> because I neither like a verification tool that ignores possible signs
> of trouble nor one that "cries wolf" when things are fine.

To me the right way seems to be to IO lock the page via PG after such a
failure, and then retry. Which should be relatively easily doable for
the basebackup case, but obviously harder for the pg_verify_checksums
case.

Greetings,

Andres Freund