Message-Id: <4BE3E205020000250003138F@gw.wicourts.gov>
Date: Fri, 07 May 2010 09:48:53 -0500
From: "Kevin Grittner" <Kevin.Grittner@wicourts.gov>
To: "Andrew Dunstan" <andrew@dunslane.net>
Cc: <pgsql-hackers@postgresql.org>
Subject: Re: no universally correct setting for fsync
References: <4BE3D3930200002500031382@gw.wicourts.gov>
	<4BE425F7.30804@dunslane.net>
In-Reply-To: <4BE425F7.30804@dunslane.net>
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Andrew Dunstan <andrew@dunslane.net> wrote:
 
> I think the critical question is really whether you are prepared
> to lose your database.
 
Precisely; and the docs don't make that at all clear.  They mention
the possibility of database corruption, but downplay it:
 
| When fsync is disabled, the operating system is allowed to do its
| best in buffering, ordering, and delaying writes. This can result
| in significantly improved performance. However, if the system
| crashes, the results of the last few committed transactions might
| be lost in part or whole. In the worst case, unrecoverable data
| corruption might occur.

> [valid use case for fsync=off]
> 
> So I think its true that there is no universally right answer.
> Maybe the criteria mentioned in the last para need tweaking some,
> though.
 
I think it goes beyond "tweaking" -- I think we should have a bald
statement like "don't turn this off unless you're OK with losing the
entire contents of the database cluster."  A brief listing of some
cases where that is OK might be illustrative.
 
I never meant to suggest any statement in that section is factually
wrong; it's just all too rosy, leading people to believe it's no big
deal to turn it off.
 
-Kevin