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Re: [ADMIN] shared_buffers and shmmax

From: Bruce Momjian <bruce(at)momjian(dot)us>
To: Alvaro Herrera <alvherre(at)commandprompt(dot)com>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, valiouk(at)yahoo(dot)co(dot)uk, pgsql-docs(at)postgresql(dot)org, dx k9 <bitsandbytes88(at)hotmail(dot)com>
Subject: Re: [ADMIN] shared_buffers and shmmax
Date: 2008-12-15 23:46:00
Message-ID: 200812152346.mBFNk0W28273@momjian.us (view raw or flat)
Thread:
Lists: pgsql-adminpgsql-docspgsql-hackers
Alvaro Herrera wrote:
> Bruce Momjian wrote:
> > Tom Lane wrote:
> > > [ redirecting to pgsql-docs ]
> > > 
> > > Valentin Bogdanov <valiouk(at)yahoo(dot)co(dot)uk> writes:
> > > >> From: dx k9 <bitsandbytes88(at)hotmail(dot)com>
> > > >> I'm trying to understand what the documentation means
> > > >> by bytes per increment, what is the increment supposed to
> > > >> be bytes, MB, or Kb.
> > > 
> > > > shared_buffers is in disk block size, typically 8K, at least that's what it is on Linux platforms. shmmax is quite simply in bytes.
> > > 
> > > The table the OP is looking at (table 17.2 in the 8.3 docs) predates
> > > the ability to specify shared_buffers in KB or MB instead of
> > > number-of-buffers.  I agree it's not entirely obvious that what it
> > > means is "multiply your setting in KB/MB by 8400/8192".  Anybody have
> > > an idea how to clarify things?
> > 
> > I have updated the table title to be clearer.
> 
> I don't find it any clearer ... I think the missing clue is that if you
> specify shared_buffers values in MB, you must divide the value by block
> size.

Well, the heading says "object" now so I thought it would suggest we are
talking about objects and not bytes.

> > ***************
> > *** 1119,1125 ****
> >   
> >         <row>
> >          <entry>Fixed space requirements</>
> > !        <entry>770 kB</entry>
> >         </row>
> >        </tbody>
> >       </tgroup>
> > --- 1119,1125 ----
> >   
> >         <row>
> >          <entry>Fixed space requirements</>
> > !        <entry>770 k</entry>
> >         </row>
> >        </tbody>
> >       </tgroup>
> 
> This change is wrong, why did you do it?

The heading says "bytes" so having the "B" was unnecessary and possibly
confusing.


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

  + If your life is a hard drive, Christ can be your backup. +

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