Re: [REVIEW] Re: Compression of full-page-writes

On 2014-12-12 11:15:46 -0500, Robert Haas wrote:
> On Fri, Dec 12, 2014 at 11:12 AM, Andres Freund <andres(at)anarazel(dot)de> wrote:
> > On 2014-12-12 11:08:52 -0500, Robert Haas wrote:
> >> Unless I'm missing something, this test is showing that FPW
> >> compression saves 298MB of WAL for 17.3 seconds of CPU time, as
> >> against master. And compressing the whole record saves a further 1MB
> >> of WAL for a further 13.39 seconds of CPU time. That makes
> >> compressing the whole record sound like a pretty terrible idea - even
> >> if you get more benefit by reducing the lower boundary, you're still
> >> burning a ton of extra CPU time for almost no gain on the larger
> >> records. Ouch!
> >
> > Well, that test pretty much doesn't have any large records besides FPWs
> > afaics. So it's unsurprising that it's not beneficial.
>
> "Not beneficial" is rather an understatement. It's actively harmful,
> and not by a small margin.

Sure, but that's just because it's too simplistic. I don't think it
makes sense to make any inference about the worthyness of the general
approach from the, nearly obvious, fact that compressing every tiny
record is a bad idea.

Greetings,

Andres Freund

--
Andres Freund http://www.2ndQuadrant.com/
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