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From: Greg Smith <greg@2ndquadrant.com>
To: Scott Carey <scott@richrelevance.com>
Cc: Marti Raudsepp <marti@juffo.org>
Cc: Andres Freund <andres@anarazel.de>
Cc: pgsql-performance@postgresql.org <pgsql-performance@postgresql.org>
Subject: Re: Defaulting wal_sync_method to fdatasync on Linux for 9.1?
Date: Mon, 08 Nov 2010 17:12:57 -0500
Message-ID: <4CD875E9.30207@2ndquadrant.com> (raw)
In-Reply-To: <9D7DB2F2-683B-480C-B781-8D282A280E35@richrelevance.com>
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	<201011010010.28724.andres@anarazel.de>
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	<AANLkTinEAmKDz7x356bgX=-6x7zpjK3bB0hYAmKvYHB9@mail.gmail.com>
	<4CD47FCD.6010106@2ndquadrant.com>
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	<9D7DB2F2-683B-480C-B781-8D282A280E35@richrelevance.com>

Scott Carey wrote:
> Im my opinion, the burden of proof lies with those contending that the default value should _change_ from fdatasync to O_DSYNC on linux.  If the default changes, all power-fail testing and other reliability tests done prior on a hardware configuration may become invalid without users even knowing.
>   

This seems to be ignoring the fact that unless you either added a 
non-volatile cache or specifically turned off all write caching on your 
drives, the results of all power-fail testing done on earlier versions 
of Linux was that it failed.  The default configuration of PostgreSQL on 
Linux has been that any user who has a simple SATA drive gets unsafe 
writes, unless they go out of their way to prevent them.

Whatever newer kernels do by default cannot be worse.  The open question 
is whether it's still broken, in which case we might as well favor the 
known buggy behavior rather than the new one, or whether everything has 
improved enough to no longer be unsafe with the new defaults.

-- 
Greg Smith   2ndQuadrant US    greg@2ndQuadrant.com   Baltimore, MD
PostgreSQL Training, Services and Support        www.2ndQuadrant.us
"PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books




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