From: Peter Schuller <peter.schuller@infidyne.com>
To: pgsql-performance@postgresql.org
Cc: Greg Smith <gsmith@gregsmith.com>
Subject: Re: shared_buffers in 8.3 w/ lots of RAM on dedicated PG machine
Date: Mon, 18 Feb 2008 08:39:46 +0100
Message-ID: <200802180839.46557.peter.schuller@infidyne.com> (raw)
In-Reply-To: <Pine.GSO.4.64.0802150916380.28161@westnet.com>
References: <20080215123528.GA16532@hyperion.scode.org>
<20080215135846.GA17327@hyperion.scode.org>
<Pine.GSO.4.64.0802150916380.28161@westnet.com>
> PostgreSQL only uses direct I/O for writing to the WAL; everything else
> goes through the regular OS buffer cache unless you force it to do
> otherwise at the OS level (like some Solaris setups do with
> forcedirectio). This is one reason it still make not make sense to give
> an extremely high percentage of RAM to PostgreSQL even with improvements
> in managing it.
Ok - thank you for the input (that goes for everyone).
> Another is that shared_buffers memory has to be
> reconciled with disk at every checkpoint, where OS buffers do not.
Hmm. Am I interpreting that correctly in that dirty buffers need to be flushed
to disk at checkpoints? That makes perfect sense - but why would that not be
the case with OS buffers? My understanding is that the point of the
checkpoint is to essentially obsolete old WAL data in order to recycle the
space, which would require flushing the data in question first (i.e.,
normally you just fsync the WAL, but when you want to recycle space you need
fsync() for the barrier and are then free to nuke the old WAL).
--
/ Peter Schuller
PGP userID: 0xE9758B7D or 'Peter Schuller <peter.schuller@infidyne.com>'
Key retrieval: Send an E-Mail to getpgpkey@scode.org
E-Mail: peter.schuller@infidyne.com Web: http://www.scode.orgAttachments:
signature.asc (application/pgp-signature, 186B) download
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: peter.schuller@infidyne.com, gsmith@gregsmith.com
Subject: Re: shared_buffers in 8.3 w/ lots of RAM on dedicated PG machine
In-Reply-To: <200802180839.46557.peter.schuller@infidyne.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