On Fri, 2020-03-20 at 14:43 +0100, Laurenz Albe wrote:
> I.e. with the default settings we will perform a whole-index scan
> > (without visibility map or such) after every 10% growth of the
> > table. Which means that, even if the visibility map prevents repeated
> > tables accesses, increasing the rate of vacuuming for insert-only tables
> > can cause a lot more whole index scans. Which means that vacuuming an
> > insert-only workload frequently *will* increase the total amount of IO,
> > even if there is not a single dead tuple. Rather than just spreading the
> > same amount of IO over more vacuums.
> >
> > And both gin and gist just always do a full index scan, regardless of
> > vacuum_cleanup_index_scale_factor (either during a bulk delete, or
> > during the cleanup). Thus more frequent vacuuming for insert-only
> > tables can cause a *lot* of pain (even an approx quadratic increase of
> > IO? O(increased_frequency * peak_index_size)?) if you have large
> > indexes - which is very common for gin/gist.
>
> In the light of that, I agree that we should increase the scale_factor.
Here is version 10 of the patch, which uses a scale factor of 0.2.
Yours,
Laurenz Albe
