Re: Compression of full-page-writes

On Fri, Jan 2, 2015 at 05:55:52PM +0100, Andres Freund wrote:
> On 2015-01-02 11:52:42 -0500, Bruce Momjian wrote:
> > Why are we not seeing the 33% compression and 15% performance
> > improvement he saw? What am I missing here?
>
> To see performance improvements something needs to be the bottleneck. If
> WAL writes/flushes aren't that in the tested scenario, you won't see a
> performance benefit. Amdahl's law and all that.
>
> I don't understand your negativity about the topic.

I remember the initial post from Masao in August 2013 showing a
performance boost, so I assumed, while we had the concurrent WAL insert
performance improvement in 9.4, this was going to be our 9.5 WAL
improvement. While the WAL insert performance improvement required no
tuning and was never a negative, I now see the compression patch as
something that has negatives, so has to be set by the user, and only
wins in certain cases. I am disappointed, and am trying to figure out
how this became such a marginal win for 9.5. :-(

My negativity is not that I don't want it, but I want to understand why
it isn't better than I remembered. You are basically telling me it was
always a marginal win. :-( Boohoo!

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

+ Everyone has their own god. +