Re: Heavy write activity on first vacuum of fresh TOAST data

On Thu, 2007-12-13 at 11:46 -0600, Kevin Grittner wrote:
> >>> On Thu, Dec 13, 2007 at 10:11 AM, in message
> <1197562283(dot)4255(dot)1829(dot)camel(at)ebony(dot)site>, Simon Riggs <simon(at)2ndquadrant(dot)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