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Re: WAL FILES

From: Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us>
To: ohp(at)pyrenet(dot)fr
Cc: pgsql-hackers list <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: WAL FILES
Date: 2002-05-29 01:02:08
Message-ID: 200205290102.g4T128e24276@candle.pha.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
8 is the maximum unless WAL files have to be created _while_ the
checkpoint is taking place.

Current CVS SGML has:

   The number of 16MB segment files will always be at least
   <varname>WAL_FILES</varname> + 1, and will normally not exceed
   <varname>WAL_FILES</varname> + MAX(<varname>WAL_FILES</varname>,
   <varname>CHECKPOINT_SEGMENTS</varname>) + 1.

The real driver here is CHECKPOINT_SEGMENTS because WAL_FILES is going
away in 7.3 and will just be dynamically used.  The typical setup is
checkpoint_segments files.  I will also add better reporting so you can
know if your checkpoint_segments is too small, causing checkpoints too
frequently.

---------------------------------------------------------------------------

Olivier PRENANT wrote:
> Hi Bruce,
> 
> Thank you for your reply. It makes a lot of sense!
> However I don't really understand why we can't control the NUMBER of
> files.
> Are the 8 files I see a maximum usage when I reloaded the databases on the
> ne system or is it some sort of "plugged in value"?
> 
> Thank you for your explanation.
> On Mon, 27 May 2002, Bruce Momjian wrote:
> 
> > Date: Mon, 27 May 2002 17:17:58 -0400 (EDT)
> > From: Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us>
> > To: ohp(at)pyrenet(dot)fr
> > Cc: pgsql-hackers list <pgsql-hackers(at)postgresql(dot)org>
> > Subject: Re: [HACKERS] WAL FILES
> > 
> > Olivier PRENANT wrote:
> > > Hi every one.
> > > 
> > > I just moved (at last!) to 7.2.1. Works like a charm...
> > > I'm suprised though by the number of WAL files.
> > > 
> > > I have 8 files where postgresql.conf says WAL_FILES=4.
> > > 
> > > What did I miss ? (I have no outstanding transaction)
> > > 
> > > FWIW, t's on UW711.
> > 
> > No, you are fine.  The current GUC params are confusing. I did update
> > the documentation for 7.3, but I plan to reorganize those params to be
> > more meaningful.
> > 
> > Actually, I have in TODO:
> > 
> >   Remove wal_files postgresql.conf option because WAL files are now
> >   recycled 
> > 
> > because the param no longer controls what you think it controls.  In 7.1
> > WAL files where not recycled, so WAL_FILES was used to pre-allocate
> > files so there wasn't as much happening during checkpoint.  Now, with
> > recycling, there is no need.
> > 
> > 
> 
> -- 
> Olivier PRENANT         	Tel:	+33-5-61-50-97-00 (Work)
> Quartier d'Harraud Turrou           +33-5-61-50-97-01 (Fax)
> 31190 AUTERIVE                      +33-6-07-63-80-64 (GSM)
> FRANCE                      Email: ohp(at)pyrenet(dot)fr
> ------------------------------------------------------------------------------
> Make your life a dream, make your dream a reality. (St Exupery)
> 
> 

-- 
  Bruce Momjian                        |  http://candle.pha.pa.us
  pgman(at)candle(dot)pha(dot)pa(dot)us               |  (610) 853-3000
  +  If your life is a hard drive,     |  830 Blythe Avenue
  +  Christ can be your backup.        |  Drexel Hill, Pennsylvania 19026

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