Date: Wed, 27 Sep 2006 16:27:56 -0400
From: Bill Moran <wmoran@collaborativefusion.com>
To: Edoardo Ceccarelli <eddy@axa.it>
Cc: pgsql-performance@postgresql.org
Subject: Re: autovacuum on a -mostly- r/o table
Message-Id: <20060927162756.0a9df264.wmoran@collaborativefusion.com>
References: <451AA1FE.9050704@axa.it>
	<1159374712.46586.47.camel@home>
Organization: Collaborative Fusion
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In response to Edoardo Ceccarelli <eddy@axa.it>:

> Rod Taylor wrote:
> > On Wed, 2006-09-27 at 18:08 +0200, Edoardo Ceccarelli wrote:
> >   
> >> I have read that autovacuum cannot check to see pg load before
> >> launching 
> >> vacuum but is there any patch about it? that would sort out the
> >> problem 
> >> in a good and simple way. 
> >>     
> >
> > In some cases the solution to high load is to vacuum the tables being
> > hit the heaviest -- meaning that simply checking machine load isn't
> > enough to make that decision.
> >
> > In fact, that high load problem is exactly why autovacuum was created in
> > the first place.
> >   
> True,
> but autovacuum could check load -before- and -during- it's execution and 
> it could adjust himself automatically to perform more or less 
> aggressively depending on the difference between those two values.
> Maybe with a parameter like: maximum-autovacuum-load=0.2
> that would mean: "never load the machine more than 20% for the autovacuum"

This is pretty non-trivial.  How do you define 20% load?  20% of the
CPU?  Does that mean that it's OK for autovac to use 3% cpu and 100% of
your IO?  Ok, so we need to calculate an average of IO and CPU -- which
disks?  If your WAL logs are on one disk, and you've used tablespaces
to spread the rest of your DB across different partitions, it can
be pretty difficult to determine which IO parameters you want to take
into consideration.

-- 
Bill Moran
Collaborative Fusion Inc.

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