Re: Patch: Write Amplification Reduction Method (WARM)

On Thu, Feb 23, 2017 at 11:53 PM, Bruce Momjian <bruce(at)momjian(dot)us> wrote:

> On Thu, Feb 23, 2017 at 03:03:39PM -0300, Alvaro Herrera wrote:
> > Bruce Momjian wrote:
> >
> > > As I remember, WARM only allows
> > > a single index-column change in the chain. Why are you seeing such a
> > > large performance improvement? I would have thought it would be that
> > > high if we allowed an unlimited number of index changes in the chain.
> >
> > The second update in a chain creates another non-warm-updated tuple, so
> > the third update can be a warm update again, and so on.
>
> Right, before this patch they would be two independent HOT chains. It
> still seems like an unexpectedly-high performance win. Are two
> independent HOT chains that much more expensive than joining them via
> WARM?

In these tests, there are zero HOT updates, since every update modifies
some index column. With WARM, we could reduce regular updates to half, even
when we allow only one WARM update per chain (chain really has a single
tuple for this discussion). IOW approximately half updates insert new index
entry in *every* index and half updates
insert new index entry *only* in affected index. That itself does a good
bit for performance.

So to answer your question: yes, joining two HOT chains via WARM is much
cheaper because it results in creating new index entries just for affected
indexes.

Thanks,
Pavan

--
Pavan Deolasee http://www.2ndQuadrant.com/
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