Re: Checkpointer split has broken things dramatically (was Re: DELETE vs TRUNCATE explanation)

On 17 July 2012 23:56, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> This implies that nobody has done pull-the-plug testing on either HEAD
> or 9.2 since the checkpointer split went in (2011-11-01), because even
> a modicum of such testing would surely have shown that we're failing to
> fsync a significant fraction of our write traffic.
>
> Furthermore, I would say that any performance testing done since then,
> if it wasn't looking at purely read-only scenarios, isn't worth the
> electrons it's written on. In particular, any performance gain that
> anybody might have attributed to the checkpointer splitup is very
> probably hogwash.
>
> This is not giving me a warm feeling about our testing practices.

The checkpointer slit-up was not justified as a performance
optimisation so much as a re-factoring effort that might have some
concomitant performance benefits. While I agree that it is regrettable
that this was allowed to go undetected for so long, I do not find it
especially surprising that some performance testing results post-split
didn't strike somebody as fool's gold. Much of the theory surrounding
checkpoint tuning, if followed, results in relatively little work
being done during the sync phase of a checkpoint, especially if an I/O
scheduler like deadline is used.

--
Peter Geoghegan http://www.2ndQuadrant.com/
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