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 4EDBA9DC876 for ; Tue, 28 Feb 2006 17:09:27 -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 22079-02 for ; Tue, 28 Feb 2006 17:09:28 -0400 (AST) X-Greylist: from auto-whitelisted by SQLgrey- Received: from noel.decibel.org (noel.decibel.org [67.100.216.10]) by postgresql.org (Postfix) with ESMTP id 2D1859DC869 for ; Tue, 28 Feb 2006 17:09:24 -0400 (AST) Received: by noel.decibel.org (Postfix, from userid 1001) id 97FB156468; Tue, 28 Feb 2006 15:09:26 -0600 (CST) Received: (hashcash-sendmail, from uid 1001); Tue, 28 Feb 2006 15:09:25 -0600 Date: Tue, 28 Feb 2006 15:09:25 -0600 From: "Jim C. Nasby" To: Javier Somoza Cc: Bruce Momjian , Tino Wildenhain , pgsql-performance@postgresql.org Subject: Re: fsync and battery-backed caches Message-ID: <20060228210925.GX82012@pervasive.com> References: <200602280106.k1S16DK16068@candle.pha.pa.us> <1141114775.1555.7.camel@pndsoft> <20060228172334.GV82012@pervasive.com> <1141147654.1555.46.camel@pndsoft> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1141147654.1555.46.camel@pndsoft> X-Operating-System: FreeBSD 6.0-RELEASE-p4 amd64 X-Distributed: Join the Effort! http://www.distributed.net User-Agent: Mutt/1.5.11 X-Hashcash: 1:20:060228:jsomoza@pandasoftware.es::+Rthp3+Yh3ew90zE:0000000000000 00000000000000000000000086wA X-Hashcash: 1:20:060228:pgman@candle.pha.pa.us::UVjVwg7k7GbFeToP:000000000000000 0000000000000000000000000FJw X-Hashcash: 1:20:060228:tino@wildenhain.de::AXGOC7fo9JyI+dYX:0000000000000000000 0000000000000000000000003gj/ X-Hashcash: 1:20:060228:pgsql-performance@postgresql.org::nbAiatcPdGzqkHk1:00000 0000000000000000000000000XT6 X-Virus-Scanned: by amavisd-new at hub.org X-Spam-Status: No, score=0.108 required=5 tests=[AWL=0.108] X-Spam-Score: 0.108 X-Spam-Level: X-Archive-Number: 200602/444 X-Sequence-Number: 17430 Ok, you absolutely can't guarantee you won't get partial page writes then. A UPS buys you no more data safety than being plugged directly into the wall. UPS's fail. People trip over cords. Breakers fail. Even if you have multiple power supplies on multiple circuits fed by different UPS's you can *still* have unexpected power failures. The 'master' for distributed.net had exactly that happen recently; the two breakers feeding it (from 2 seperate UPS's) failed simultaneously. In a nutshell, having a server on a UPS is notthing at all like having a BBU on the raid controller: commiting to the BBU is essentially the same as committing to the drives, unless the BBU runs out of power before the server has power restored, or fails in some similar fasion. But because there's many fewer parts involved, such a failure of the BBU is far less likely than a failure up-stream. So, if you want performance, get a controller with a BBU and allow it to cache writes. While you're at it, try and get one that will automatically disable write caching if the BBU fails for some reason. On Tue, Feb 28, 2006 at 06:27:34PM +0100, Javier Somoza wrote: > > Ups sorry. > > > > Actually, you can't assume that a BBU means you can safely disable > > full-page-writes. Depending on the controller, it's still possible to > > end up with partially written pages. > > > > BTW, if your mailer makes doing so convenient, it would be nice to trim > > down your .signature; note that it's about 3x longer than the email you > > actually sent. > > -- Jim C. Nasby, Sr. Engineering Consultant jnasby@pervasive.com Pervasive Software http://pervasive.com work: 512-231-6117 vcard: http://jim.nasby.net/pervasive.vcf cell: 512-569-9461