Message-ID: <4CCDCAA3.4050805@2ndquadrant.com>
Date: Sun, 31 Oct 2010 15:59:31 -0400
From: Greg Smith <greg@2ndquadrant.com>
User-Agent: Thunderbird 2.0.0.24 (X11/20100317)
MIME-Version: 1.0
To: Marti Raudsepp <marti@juffo.org>
CC: pgsql-performance@postgresql.org
Subject: Re: Defaulting wal_sync_method to fdatasync on Linux for
 9.1?
References: <AANLkTi=n3zxiLNFhbpiN=JPQPwB=BhXf9iepw1KOEs8U@mail.gmail.com>
In-Reply-To: <AANLkTi=n3zxiLNFhbpiN=JPQPwB=BhXf9iepw1KOEs8U@mail.gmail.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit

Marti Raudsepp wrote:
> Unless fdatasync is unsafe, I'd very much want to see it as the
> default for 9.1 on Linux (I don't know about other platforms).  I
> can't see any reasons why each write would need to be sync-ed if I
> don't commit that often. Increasing wal_buffers probably has the same
> effect wrt data safety.
>   

Writes only are sync'd out when you do a commit, or the database does a 
checkpoint.

This issue is a performance difference introduced by a recent change to 
Linux.  open_datasync support was just added to Linux itself very 
recently.  It may be more safe than fdatasync on your platform.  As new 
code it may have bugs so that it doesn't really work at all under heavy 
load.  No one has really run those tests yet.  See 
http://wiki.postgresql.org/wiki/Reliable_Writes for some background, and 
welcome to the fun of being an early adopter.  The warnings in the 
tuning guide are there for a reason--you're in untested territory now.  
I haven't finished validating whether I consider 2.6.32 safe for 
production use or not yet, and 2.6.36 is a solid year away from being on 
my list for even considering it as a production database kernel.  You 
should proceed presuming that all writes are unreliable until proven 
otherwise.

-- 
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