From: Andres Freund <andres@anarazel.de>
To: Greg Smith <greg@2ndquadrant.com>
Subject: Re: Defaulting wal_sync_method to fdatasync on Linux for 9.1?
Date: Mon, 8 Nov 2010 00:45:23 +0100
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Cc: Marti Raudsepp <marti@juffo.org>,
 pgsql-performance@postgresql.org
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Message-Id: <201011080045.23947.andres@anarazel.de>

On Monday 08 November 2010 00:35:29 Greg Smith wrote:
> Marti Raudsepp wrote:
> > I will grant you that the details were wrong, but I stand by the
> > conclusion. I can state for a fact that PostgreSQL's default
> > wal_sync_method varies depending on the <fcntl.h> header.
> 
> Yes; it's supposed to, and that logic works fine on some other
> platforms.  The question is exactly what the new Linux O_DSYNC behavior
> is doing, in regards to whether it flushes drive caches out or not.
> Until you've quantified which of the cases do that--which is required
> for reliable operation of PostgreSQL--and which don't, you don't have
> any data that can be used to draw a conclusion from.  If some setups are
> faster because they write less reliably, that doesn't automatically make
> them the better choice.
I think thats FUD. Sorry.

Can you explain to me why fsync() may/should/could be *any* less reliable than 
O_DSYNC? On *any* platform. Or fdatasync() in the special way its used with 
pg, namely completely preallocated files.

I think the reasons why O_DSYNC is, especially, but not only, in combination 
with a small wal_buffers setting, slow in most circumstances are pretty clear.

Making a setting which is only supported on a small range of systems highest 
in the preferences list is even more doubtfull than the already strange choice 
of making O_DSYNC the default given the way it works (i.e. no reordering, 
synchronous writes in the bgwriter, synchronous writes on wal_buffers pressure 
etc).

Greetings,

Andres