Re: Patch for fail-back without fresh backup

On Fri, Jun 14, 2013 at 4:12 PM, Heikki Linnakangas <hlinnakangas(at)vmware(dot)com
> wrote:

> Robert Haas pointed out in that thread that it has a problem with hint
> bits that are not WAL-logged,
>

I liked that tool a lot until Robert pointed out the above problem. I
thought this is a show stopper because I can't really see any way to
circumvent it unless we enable checksums or explicitly WAL log hint bits.

> but it will still work if you also enable the new checksums feature, which
> forces hint bit updates to be WAL-logged.
>

Are we expecting a lot of people to run their clusters with checksums on ?
Sorry, I haven't followed the checksum discussions and don't know how much
overhead it causes. But if the general expectation is that checksums will
be turned on most often, I agree pg_rewind is probably good enough.

> Perhaps we could add a GUC to enable hint bits to be WAL-logged,
> regardless of checksums, to make pg_rewind work.
>
>
Wouldn't that be too costly ? I mean, in the worst case every hint bit on a
page may get updated separately. If each such update is WAL logged, we are
looking for a lot more unnecessary WAL traffic.

> I think that's a more flexible approach to solve this problem. It doesn't
> require an online feedback loop from the standby to master, for starters.
>
>
I agree. That's a big advantage of pg_rewind. Unfortunately, it can't work
with 9.3 and below because of the hint bits issue, otherwise it would have
been even more cool.

Thanks,
Pavan

--
Pavan Deolasee
http://www.linkedin.com/in/pavandeolasee