Message-ID: <4D509EE1.4070305@agliodbs.com>
Date: Mon, 07 Feb 2011 17:39:45 -0800
From: Josh Berkus <josh@agliodbs.com>
User-Agent: Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.5; en-US;
	rv:1.9.1b3pre) Gecko/20090223 Thunderbird/3.0b2
MIME-Version: 1.0
To: =?UTF-8?B?Q8OpZHJpYyBWaWxsZW1haW4=?=
 <cedric.villemain.debian@gmail.com>
CC: jd@commandprompt.com, pgsql-performance@postgresql.org
Subject: Re: Write-heavy pg_stats_collector on mostly idle server
References: <4D4C4E15.20401@agliodbs.com>
	<AANLkTinpWWrDHR9KJNebgtbvpXpL4UdQszD0GxF+FWWu@mail.gmail.com>
	<4D507920.2030909@agliodbs.com>
	<1297120356.18411.197.camel@jd-desktop>
	<AANLkTin5T+KartVKBjYRJuCNDEcTUAJkMW6Zna6uJi1Q@mail.gmail.com>
In-Reply-To: <AANLkTin5T+KartVKBjYRJuCNDEcTUAJkMW6Zna6uJi1Q@mail.gmail.com>
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 7bit


> Ooops.
> It looks like you are right, see ./src/backend/postmaster/pgstat.c
> 
> 3c2313f4 (Tom Lane           2008-11-03 01:17:08 +0000 2926)
>  if (last_statwrite < last_statrequest)
> 70d75697 (Magnus Hagander    2008-08-05 12:09:30 +0000 2927)
>          pgstat_write_statsfile(false);

This is a different issue.  This is happening because we have a bunch of
databases (25 to 35) and as a result autovacuum is requesting the stats
file rather frequently.  And autovacuum for whatever reason won't accept
a stats file more than 10ms old, so it pretty much rewrites the stats
file on every request.

At least, that's my reading of it after poking around and talking to
Gierth.

It seems like that 10ms window for autovac is way too small.

-- 
                                  -- Josh Berkus
                                     PostgreSQL Experts Inc.
                                     http://www.pgexperts.com