Message-ID: <4D269899.6040901@2ndquadrant.com>
Date: Thu, 06 Jan 2011 23:37:45 -0500
From: Greg Smith <greg@2ndquadrant.com>
User-Agent: Thunderbird 2.0.0.24 (X11/20100317)
MIME-Version: 1.0
To: Josh Berkus <josh@agliodbs.com>
CC: Jeff Janes <jeff.janes@gmail.com>, pgsql-performance@postgresql.org
Subject: Re: Wrong docs on wal_buffers?
References: <4D24D7DE.6020507@agliodbs.com>
	<AANLkTi=YLCEGcwa-9hFYykRgTfUVDhZrwNGo7qaJZnRC@mail.gmail.com>
	<4D2610F3.8070407@agliodbs.com>
In-Reply-To: <4D2610F3.8070407@agliodbs.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit

Josh Berkus wrote:
> We talked about bumping it to 512kB or 1MB for 9.1.  Did that get in?
> Do I need to write that patch?
>   

If it defaulted to 3% of shared_buffers, min 64K & max 16MB for the auto 
setting, it would for the most part become an autotuned parameter.  That 
would make it 0.75 to 1MB at the standard anemic Linux default kernel 
parameters.  Maybe more than some would like, but dropping 
shared_buffers from 24MB to 23MB to keep this from being ridiculously 
undersized is probably a win.  That percentage would reach 16MB by the 
time shared_buffers was increased to 533MB, which also seems about right 
to me.  On a really bad setup (brief pause to flip off Apple) with only 
4MB to work with total, you'd end up with wal_buffers between 64 and 
128K, so very close to the status quo.

Code that up, and we could probably even remove the parameter as a 
tunable altogether.  Very few would see a downside relative to any 
sensible configuration under the current situation, and many people 
would notice better automagic performance with one less parameter to 
tweak.  Given the recent investigations about the serious downsides of 
tiny wal_buffers values on new Linux kernels when using open_datasync, a 
touch more aggression about this setting seems particularly appropriate 
to consider now.  That's been swapped out as the default, but it's still 
possible people will switch to it.

-- 
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