Re: hung backends stuck in spinlock heavy endless loop

On Thu, Jan 22, 2015 at 1:50 PM, Merlin Moncure <mmoncure(at)gmail(dot)com> wrote:

>
> So far, the 'nasty' damage seems to generally if not always follow a
> checksum failure and the checksum failures are always numerically
> adjacent. For example:
>
> [cds2 12707 2015-01-22 12:51:11.032 CST 2754]WARNING: page
> verification failed, calculated checksum 9465 but expected 9477 at
> character 20
> [cds2 21202 2015-01-22 13:10:18.172 CST 3196]WARNING: page
> verification failed, calculated checksum 61889 but expected 61903 at
> character 20
> [cds2 29153 2015-01-22 14:49:04.831 CST 4803]WARNING: page
> verification failed, calculated checksum 27311 but expected 27316
>
> I'm not up on the intricacies of our checksum algorithm but this is
> making me suspicious that we are looking at a improperly flipped
> visibility bit via some obscure problem -- almost certainly with
> vacuum playing a role.

That very much sounds like the block is getting duplicated from one place
to another.

Even flipping one hint bit (aren't these index pages? Do they have hint
bits) should thoroughly scramble the checksum.

Because the checksum adds in the block number after the scrambling has been
done, copying a page to another nearby location will just move the
(expected) checksum a little bit.

Cheers,

Jeff