Re: Hot standby 9.2.6 -> 9.2.6 PANIC: WAL contains references to invalid pages

On 2014-01-13 16:36:41 -0500, Tom Lane wrote:
> Heikki Linnakangas <hlinnakangas@vmware.com> writes:
> > Good point. Normally, we expect the checksum to match on all pages that
> > we read during WAL replay, because full page writes will initialize any
> > page that is modified to an untorn state, before it's ever read. But we
> > can't rely on that in the extra read that btree_xlog_vacuum() does.
>
> But it's not an "extra" read.  It's replicating a read that was done
> on the master in the btvacuumpage() scan.  AFAICS the only way to fail
> on the slave and not the master is if the slave has inconsistent data,
> in which case you're at hazard of failing anyway.

I tried to explain which scenario I see as dangerous nearby.

> >> Now, you could argue that that shouldn't be the case because we're only
> >> entering that codepath once STANDBY_SNAPSHOT_READY and you might be
> >> right...
>
> > I don't think that saves us. standbyMode can be STANDBY_SNAPSHOT_READY,
> > before we reach consistency. Adding a check for reachedConsistency,
> > though, ought to fix it.
>
> Huh?  Surely we're not letting queries in until we're consistent.

We don't, but STANDBY_SNAPSHOT_READY isn't the only variable controlling
that. It just determines whether we'd have the necessary visibility
information. The full check is:
     */
    if (standbyState == STANDBY_SNAPSHOT_READY &&
        !LocalHotStandbyActive &&
        reachedConsistency &&
        IsUnderPostmaster)
{
...
        xlogctl->SharedHotStandbyActive = true;
...
        SendPostmasterSignal(PMSIGNAL_BEGIN_HOT_STANDBY);
}

So we need to mimick that.

Greetings,

Andres Freund

--
 Andres Freund                       http://www.2ndQuadrant.com/
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