Message-ID: <4F14C101.60607@fuzzy.cz>
Date: Tue, 17 Jan 2012 01:29:53 +0100
From: Tomas Vondra <tv@fuzzy.cz>
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To: pgsql-performance@postgresql.org
Subject: Re: wal_level=archive gives better performance than minimal
 - why?
References: <4F0F6A02.3000403@fuzzy.cz> <4F14A649.5060501@2ndQuadrant.com>
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On 16.1.2012 23:35, Greg Smith wrote:
> On 01/12/2012 06:17 PM, Tomas Vondra wrote:
>> I've run a series fo pgbench benchmarks with the aim to see the effect
>> of moving the WAL logs to a separate drive, and one thing that really
>> surprised me is that the archive log level seems to give much better
>> performance than minimal log level.
> 
> How repeatable is this?  If you always run minimal first and then
> archive, that might be the actual cause of the difference.  In this
> situation I would normally run this 12 times, with this sort of pattern:
> 
> minimal
> minimal
> minimal
> archive
> archive
> archive
> minimal
> minimal
> minimal
> archive
> archive
> archive
> 
> To make sure the difference wasn't some variation on "gets slower after
> each run".  pgbench suffers a lot from problems in that class.

AFAIK it's well repeatable - the primary goal of the benchmark was to
see the benefir of moving the WAL to a separate device (with various WAL
levels and device types - SSD and HDD).

I plan to rerun the whole thing this week with a bit more details logged
to rule out basic configuration mistakes etc.

Each run is completely separate (rebuilt from scratch) and takes about 1
hour to complete. Each pgbench run consists of these steps

  1) rebuild the data from scratch
  2) 10-minute warmup (read-only run)
  3) 20-minute read-only run
  4) checkpoint
  5) 20-minute read-write run

and the results are very stable.

Tomas