X-Original-To: pgsql-performance-postgresql.org@localhost.postgresql.org Received: from localhost (av.hub.org [200.46.204.144]) by postgresql.org (Postfix) with ESMTP id 0368B9DC9B2 for ; Sat, 11 Mar 2006 04:17:10 -0400 (AST) Received: from postgresql.org ([200.46.204.71]) by localhost (av.hub.org [200.46.204.144]) (amavisd-new, port 10024) with ESMTP id 88247-03 for ; Sat, 11 Mar 2006 04:17:11 -0400 (AST) X-Greylist: from auto-whitelisted by SQLgrey- Received: from obelix.askesis.nl (laudanum.demon.nl [82.161.125.16]) by postgresql.org (Postfix) with ESMTP id 7EAE09DC84B for ; Sat, 11 Mar 2006 04:17:08 -0400 (AST) Received: obelix 172.31.0.1 from 172.31.1.4 172.31.1.4 via HTTP with MS-WebStorage 6.0.6249 Received: from panoramix by obelix; 11 Mar 2006 09:17:09 +0100 Subject: Re: x206-x225 From: Joost Kraaijeveld To: David Lang Cc: Richard Huxton , pgsql-performance@postgresql.org In-Reply-To: References: <441181C6.3040401@archonet.com> <1142063378.6634.13.camel@localhost> Content-Type: text/plain Content-Transfer-Encoding: 7bit Date: Sat, 11 Mar 2006 09:17:09 +0100 Message-Id: <1142065029.6634.23.camel@localhost> Mime-Version: 1.0 X-Mailer: Evolution 2.4.2.1 X-Virus-Scanned: by amavisd-new at hub.org X-Spam-Status: No, score=0.092 required=5 tests=[AWL=0.091, UNPARSEABLE_RELAY=0.001] X-Spam-Score: 0.092 X-Spam-Level: X-Archive-Number: 200603/149 X-Sequence-Number: 17586 On Fri, 2006-03-10 at 23:57 -0800, David Lang wrote: > On Sat, 11 Mar 2006, Joost Kraaijeveld wrote: > > > On Fri, 2006-03-10 at 13:40 +0000, Richard Huxton wrote: > >> Your ATA disk is lying about disk caching being turned off. Assuming > >> each insert is in a separate transaction, then it's not going to do > >> 10,000 / 6 = 1667 transactions/sec - that's faster than it's rotational > >> speed. > > Could you explain the calculation? Why should the number of transactions > > be related to the rotational speed of the disk, without saying anything > > about the number of bytes per rotation? > > each transaction requires a sync to the disk, a sync requires a real > write (which you then wait for), so you can only do one transaction per > rotation. Not according to a conversation I had with Western Digital about the write performance of my own SATA disks. What I understand from their explanation their disk are limited by the MB/sec and not by the number of writes/second, e.g. I could write 50 MB/sec *in 1 bit/write* on my disk. This would suggest that the maximum transactions of my disk (overhead of OS and PostgreSQL ignored) would be 50MB / (transaction size in MB) per second. Or am I missing something (what would not surprise me, as I do not understand the perforance of my system at all ;-))? -- Groeten, Joost Kraaijeveld Askesis B.V. Molukkenstraat 14 6524NB Nijmegen tel: 024-3888063 / 06-51855277 fax: 024-3608416 e-mail: J.Kraaijeveld@Askesis.nl web: www.askesis.nl