Message-ID: <4D3220E3.20804@agliodbs.com>
Date: Sat, 15 Jan 2011 14:34:11 -0800
From: Josh Berkus <josh@agliodbs.com>
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To: Greg Smith <greg@2ndquadrant.com>
CC: Tom Lane <tgl@sss.pgh.pa.us>, Robert Haas <robertmhaas@gmail.com>,
	Magnus Hagander <magnus@hagander.net>, pgsql-hackers@postgresql.org
Subject: Re: auto-sizing wal_buffers
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	<AANLkTinhU9EEPGM29S+KH=qK4t-mLhBBNAaLCGQSx-4y@mail.gmail.com>
	<20767.1294964709@sss.pgh.pa.us> <4D314406.3000703@2ndquadrant.com>
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On 1/14/11 10:51 PM, Greg Smith wrote:
> 
> !         Since the data is written out to disk at every transaction
> commit,
> !         the setting many only need to be be large enough to hold the
> amount
> !         of WAL data generated by one typical transaction.  Larger values,
> !         typically at least a few megabytes, can improve write performance
> !         on a busy server where many clients are committing at once.
> !         Extremely large settings are unlikely to provide additional
> benefit.

I think we can be more specific on that last sentence; is there even any
*theoretical* benefit to settings above 16MB, the size of a WAL segment?
 Certainly there have been no test results to show any.

If we don't know, keep it vague, but otherwise I suggest:

"Settings larger than the size of a single WAL segment (16MB by default)
are unlikely to produce any benefit."

-- 
                                  -- Josh Berkus
                                     PostgreSQL Experts Inc.
                                     http://www.pgexperts.com