Re: Patch for fail-back without fresh backup

On 2013-06-14 09:08:15 -0400, Tom Lane wrote:
> Heikki Linnakangas <hlinnakangas@vmware.com> writes:
> > Well, time will tell I guess. The biggest overhead with the checksums is 
> > exactly the WAL-logging of hint bits.
> 
> Refresh my memory as to why we need to WAL-log hints for checksumming?
> I just had my nose in the part of the checksum patch that tediously
> copies entire pages out of shared buffers to avoid possible instability
> of the hint bits while we checksum and write the page.

I am really rather uncomfortable with that piece of code, and I hacked
it up after Jeff Janes had reported a bug there (The one aborting WAL
replay to early...). So I am very happy that you are looking at it.

Jeff Davis and I were talking about whether the usage of
PGXAC->delayChkpt makes the whole thing sufficiently safe at pgcon - we
couldn't find any real danger but...

> Given that we're
> paying that cost, I don't see why we'd need to do any extra WAL-logging
> (above and beyond the log-when-freeze cost that we have to pay already).
> But I've not absorbed any caffeine yet today, so maybe I'm just missing
> it.

The usual torn page spiel I think. If we crash while only one half of
the page made it to disk we would get spurious checksum failures from
there on.

Greetings,

Andres Freund

-- Andres Freund                       http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training &
Services