Subject: Re: Heavy write activity on first vacuum of fresh TOAST
	data
From: Simon Riggs <simon@2ndquadrant.com>
To: Kevin Grittner <Kevin.Grittner@wicourts.gov>
Cc: pgsql-performance@postgresql.org
In-Reply-To: <47611B9D.EE98.0025.0@wicourts.gov>
References: <4760FF69.EE98.0025.0@wicourts.gov>
	<1197562283.4255.1829.camel@ebony.site>
	<47611B9D.EE98.0025.0@wicourts.gov>
Content-Type: text/plain
Date: Thu, 13 Dec 2007 18:12:44 +0000
Message-Id: <1197569564.4255.1855.camel@ebony.site>
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On Thu, 2007-12-13 at 11:46 -0600, Kevin Grittner wrote:
> >>> On Thu, Dec 13, 2007 at 10:11 AM, in message
> <1197562283.4255.1829.camel@ebony.site>, Simon Riggs <simon@2ndquadrant.com>
> wrote: 
> > On Thu, 2007-12-13 at 09:46 -0600, Kevin Grittner wrote:
> >  
> >> The data was inserted through a Java program using a prepared
> >> statement with no indexes on the table.  The primary key was then
> >> added, and now I've started a vacuum.  The new table wound up being
> >> the first big table vacuumed, and I noticed something odd.  Even
> >> though there have been no rollbacks, updates, or deletes on this
> >> table, the vacuum is writing as much as it is reading while dealing
> >> with the TOAST data.
> > 
> > Writing hint bits. Annoying isn't it? :-(
>  
> Is there anything in the documentation that mentions this pattern
> of activity?  Since I started clearing the WAL file tails before
> compression, it has surprised me how much WAL file activity there
> is from the nightly vacuum.  I had assumed that some part of this
> was freezing old tuples, but that didn't seem to exactly match the
> pattern of activity.  If the hint bit changes are written to the
> WAL, I think this explains it.

They're not.

-- 
  Simon Riggs
  2ndQuadrant  http://www.2ndQuadrant.com