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From: Bruce Momjian <bruce@momjian.us>
To: Peter Geoghegan <pg@bowt.ie>
Cc: Andres Freund <andres@anarazel.de>
Cc: Jeff Davis <pgsql@j-davis.com>
Cc: Robert Haas <robertmhaas@gmail.com>
Cc: David Rowley <dgrowleyml@gmail.com>
Cc: Justin Pryzby <pryzby@telsasoft.com>
Cc: Melanie Plageman <melanieplageman@gmail.com>
Cc: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Cc: pgsql-hackers@postgresql.org <pgsql-hackers@postgresql.org>
Subject: Re: Default setting for enable_hashagg_disk
Date: Fri, 26 Jun 2020 19:56:22 -0400
Message-ID: <20200626235622.GE3337@momjian.us> (raw)
In-Reply-To: <CAH2-WzmK_hvWfUCabm5Fe+e-YF3oV0q8XpNA5vfyDO_0Hv9K2A@mail.gmail.com>
References: <20200624171203.GB17842@momjian.us>
	<20200624191900.t4sbfk236gndwoa3@alap3.anarazel.de>
	<20200624231810.GB13627@momjian.us>
	<20200624233837.GA21436@momjian.us>
	<CA+TgmoYi_03kpoukBzVrsVeNWY_BcLOFjUD8vTtt-fNmNy5NCA@mail.gmail.com>
	<20200625171756.GB12486@momjian.us>
	<8fd82a9e6c00b73191d18940b2da6a94a30477af.camel@j-davis.com>
	<20200625182512.GC12486@momjian.us>
	<20200625203629.7m6yvut7eqblgmfo@alap3.anarazel.de>
	<CAH2-WzmK_hvWfUCabm5Fe+e-YF3oV0q8XpNA5vfyDO_0Hv9K2A@mail.gmail.com>

On Fri, Jun 26, 2020 at 01:53:05PM -0700, Peter Geoghegan wrote:
> On Thu, Jun 25, 2020 at 1:36 PM Andres Freund <andres@anarazel.de> wrote:
> > 12      28164.865 ms
> >
> > fast ssd:
> > HEAD    92520.680 ms
> >
> > magnetic:
> > HEAD    183968.538 ms
> >
> > (no reads, there's plenty enough memory. Just writes because the age /
> > amount thresholds for dirty data are reached)
> >
> > In the magnetic case we're IO bottlenecked nearly the whole time.
> 
> I agree with almost everything you've said on this thread, but at the
> same time I question the emphasis on I/O here. You've shown that
> spinning rust is about twice as slow as a fast SSD here. Fair enough,
> but to me the real story is that spilling is clearly a lot slower in
> general, regardless of how fast the storage subsystem happens to be (I
> wonder how fast it is with a ramdisk). To me, it makes more sense to

This blog entry shows ORDER BY using ram disk, SSD, and magnetic:

	https://momjian.us/main/blogs/pgblog/2012.html#February_2_2012

It is from 2012, but I can re-run the test if you want.

-- 
  Bruce Momjian  <bruce@momjian.us>        https://momjian.us
  EnterpriseDB                             https://enterprisedb.com

  The usefulness of a cup is in its emptiness, Bruce Lee






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