Re: Hot Standby and VACUUM FULL

On Sun, 2010-01-31 at 21:00 +0200, Heikki Linnakangas wrote:
> Simon Riggs wrote:
> > On Sat, 2010-01-30 at 15:17 -0500, Tom Lane wrote:
> >> Simon Riggs <simon(at)2ndQuadrant(dot)com> writes:
> >>> The last item on my list before close is making VACUUM FULL and Hot
> >>> Standby play nicely together.
> >>> The options to do this were and still are:
> >>> (1) Add WAL messages for non-transactional relcache invalidations
> >>> (2) Allow system relations to be cluster-ed/vacuum full-ed.
> >>> (1) was how we did it originally and I believe it worked without
> >>> problem. We saw the opportunity to do (2) and it has been worth
> >>> exploring.
> >> Refresh my memory on why (1) lets us avoid fixing (2)?
> >
> > (1) allows us to retain VACUUM FULL INPLACE for system relations, thus
> > avoiding the need to do (2). Non-transactional invalidations need to be
> > replayed in recovery for the same reason they exist on the primary.
> >
> >> It sounds like a kluge at best ...
> >
> > (2) isn't a necessary change right now. It is the best design going
> > forwards, but its burst radius stretches far beyond Hot Standby. There
> > is enough code in HS for us to support, so adding to it makes little
> > sense for me, in this release, since there is no functional benefit in
> > doing so.
>
> Well, it'll avoid having to support the kludges in HS required for
> VACUUM FULL INPLACE.
>
> I'm in favor of (2), unless some unforeseen hard problems come up with
> implementing the ideas on that discussed earlier. Yeah, that's pretty
> vague, but basically I think it's worth spending some more time doing
> (2), than doing (1). For one, if the plan is to do (2) in next release
> anyway, we might as well do it now and avoid having to support the
> HS+VACUUM FULL INPLACE combination in only 9.0 stable branch for years
> to come.

That's a good argument, but with respect, it isn't you who is writing
the code, nor will it be you that supports it, AIUI. Right now, HS is
isolated in many ways. If we introduce this change it will effect
everybody and that means I'll be investigating all manner of bug reports
that have zip to do with HS for a long time to come.

What I would say is that for 9.0 we can easily remove the INPLACE option
as an explicit request.

> I believe we had a pretty well-thought out plan on how to support system
> catalogs with the new VACUUM FULL.

I think calling it a "well thought out plan" is, err, overstating
things. We had what looked like a viable sketch of how to proceed and I
agreed to investigate that. Having done so, I'm saying I don't like what
I see going further and wish to backtrack to my known safe solution.

Overall, I don't see any benefit in pursuing that course any further. I
just see risk, on balance with (2), whereas (1) seems easier/faster,
less risk and more isolated.

--
Simon Riggs www.2ndQuadrant.com