Received: from magus.postgresql.org (magus.postgresql.org [87.238.57.229]) by mail.postgresql.org (Postfix) with ESMTP id 46BF915F4A49 for ; Tue, 7 Feb 2012 23:13:41 -0400 (AST) Received: from moutng.kundenserver.de ([212.227.17.10]) by magus.postgresql.org with esmtp (Exim 4.72) (envelope-from ) id 1RuxyV-0002md-IV for pgsql-performance@postgresql.org; Wed, 08 Feb 2012 03:13:41 +0000 Received: from [192.168.2.118] (mail.highperformancepostgresql.com [71.179.240.8]) by mrelayeu.kundenserver.de (node=mrbap0) with ESMTP (Nemesis) id 0Lfpnu-1SJYQJ2awk-00pb23; Wed, 08 Feb 2012 04:13:25 +0100 Message-ID: <4F31E854.7080208@2ndQuadrant.com> Date: Tue, 07 Feb 2012 22:13:24 -0500 From: Greg Smith User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.2.24) Gecko/20111109 Thunderbird/3.1.16 MIME-Version: 1.0 To: pgsql-performance@postgresql.org Subject: Re: how to demonstrate the effect of direct I/O ? References: <4F2DBE5A.9090102@fuzzy.cz> In-Reply-To: <4F2DBE5A.9090102@fuzzy.cz> Content-Type: text/plain; charset=ISO-8859-2; format=flowed Content-Transfer-Encoding: 7bit X-Provags-ID: V02:K0:XZpYWTRvYEbS0Dk/q1mzi/n6zqbD/8SEIF+zwOckWIQ fLZiQhCdlNMXIHaEsFALfj5WxQdVx3UciHZJ5n+zsWLU5RknH+ Z1klvJWns+VyNOqgRyx+U+bT7b4N7KJoGoP8GHXF3rMbZyoQM7 140q0UVF9udQQxubwxa0pOfL93p0xe1Zo4bPTG4tGxocGW+nK8 7HqQ7Yb/uV6JDu+k6x431jFrUpUXDh6QRzNQNKXvWwgsqVnKc/ 98XGrYXbd65XDKiExW9c+ergfadla7qSFHINDsc7b7GQN3epxf 5AyqeSB7Cvj+QsNcGuzY/8Jyk4rXoeABqcbkcODsEGw+ue+pYE WLkkQBvIfxwYXYAwM7Uo= X-Pg-Spam-Score: -1.9 (-) X-Archive-Number: 201202/47 X-Sequence-Number: 46219 On 02/04/2012 06:25 PM, Tomas Vondra wrote: > What should I do to see the effect of direct I/O? Test something other than a mainstream Linux filesystem. The two times I've either measured an improvement myself for direct I/O were a) Veritas VxFS on Linux, which has some documented acceleration here and b) on Solaris. You won't find a compelling performance improvement listed at https://ext4.wiki.kernel.org/articles/c/l/a/Clarifying_Direct_IO%27s_Semantics_fd79.html and Linux has generally ignored direct I/O as something important to optimize for. -- Greg Smith 2ndQuadrant US greg@2ndQuadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.com