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

On Fri, Dec 12, 2014 at 05:19:42PM +0100, Andres Freund wrote:
> On 2014-12-12 11:15:46 -0500, Robert Haas wrote:
> > On Fri, Dec 12, 2014 at 11:12 AM, Andres Freund <andres@anarazel.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.

Well, it seems we need to see some actual cases where compression does
help before moving forward.  I thought Amit had some amazing numbers for
WAL compression --- has that changed?

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