From: Markus Schaber <schabi@logix-tt.com>
To: Michael Riess <mlriess@gmx.de>
Cc: pgsql-performance@postgresql.org
Subject: Re: Autovacuum / full vacuum
Date: Tue, 17 Jan 2006 18:25:06 +0100
Message-ID: <43CD2872.40509@logix-tt.com> (raw)
In-Reply-To: <dqiv77$2ijp$1@news.hub.org>
References: <dqigaf$1b98$1@news.hub.org>
<43CCC711.504@familyhealth.com.au>
<dqih4p$1clg$1@news.hub.org>
<43CCFA7E.1060307@zeut.net>
<dqiv77$2ijp$1@news.hub.org>
Hi, Michael,
Michael Riess wrote:
> But actually I never understood why the database system slows down at
> all when there is much unused space in the files. Are the unused pages
> cached by the system, or is there another reason for the impact on the
> performance?
No, they are not cached as such, but PostgreSQL caches whole pages, and
you don't have only empty pages, but also lots of partially empty pages,
so the signal/noise ratio is worse (means PostgreSQL has to fetch more
pages to get the same data).
Sequential scans etc. are also slower.
And some file systems get slower when files get bigger or there are more
files, but this effect should not really be noticeable here.
HTH,
Markus
--
Markus Schaber | Logical Tracking&Tracing International AG
Dipl. Inf. | Software Development GIS
Fight against software patents in EU! www.ffii.org www.nosoftwarepatents.org
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Reply to all the recipients using the --to and --cc options:
reply via email
To: pgsql-performance@postgresql.org
Cc: schabi@logix-tt.com, mlriess@gmx.de
Subject: Re: Autovacuum / full vacuum
In-Reply-To: <43CD2872.40509@logix-tt.com>
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
This inbox is served by DDX for PostgreSQL; see mirroring instructions
for how to clone and mirror all data and code used for this inbox