From owner-freebsd-current@FreeBSD.ORG Sun Dec 16 17:22:34 2007 Return-Path: Delivered-To: freebsd-current@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34]) by hub.freebsd.org (Postfix) with ESMTP id 4356416A418; Sun, 16 Dec 2007 17:22:34 +0000 (UTC) (envelope-from ticso@cicely12.cicely.de) Received: from raven.bwct.de (raven.bwct.de [85.159.14.73]) by mx1.freebsd.org (Postfix) with ESMTP id 9C77813C4EF; Sun, 16 Dec 2007 17:22:33 +0000 (UTC) (envelope-from ticso@cicely12.cicely.de) Received: from cicely5.cicely.de ([10.1.1.7]) by raven.bwct.de (8.13.4/8.13.4) with ESMTP id lBGHMVrt061801; Sun, 16 Dec 2007 18:22:31 +0100 (CET) (envelope-from ticso@cicely12.cicely.de) Received: from cicely12.cicely.de (cicely12.cicely.de [10.1.1.14]) by cicely5.cicely.de (8.13.4/8.13.4) with ESMTP id lBGHMQSv030666 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=NO); Sun, 16 Dec 2007 18:22:26 +0100 (CET) (envelope-from ticso@cicely12.cicely.de) Received: from cicely12.cicely.de (localhost [127.0.0.1]) by cicely12.cicely.de (8.13.4/8.13.3) with ESMTP id lBGHMPKq052934; Sun, 16 Dec 2007 18:22:25 +0100 (CET) (envelope-from ticso@cicely12.cicely.de) Received: (from ticso@localhost) by cicely12.cicely.de (8.13.4/8.13.3/Submit) id lBGHMP0D052933; Sun, 16 Dec 2007 18:22:25 +0100 (CET) (envelope-from ticso) Date: Sun, 16 Dec 2007 18:22:25 +0100 From: Bernd Walter To: Darren Reed Message-ID: <20071216172225.GB51627@cicely12.cicely.de> References: <476343B4.8080208@FreeBSD.org> <86tzmk54tt.fsf@ds4.des.no> <476419CD.9070401@terranova.net> <20071216024259.GI48684@cicely12.cicely.de> <4764F282.7030706@freebsd.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4764F282.7030706@freebsd.org> X-Operating-System: FreeBSD cicely12.cicely.de 5.4-STABLE alpha User-Agent: Mutt/1.5.9i X-Spam-Status: No, score=-4.3 required=5.0 tests=ALL_TRUSTED=-1.8, AWL=0.144, BAYES_00=-2.599 autolearn=ham version=3.1.7 X-Spam-Checker-Version: SpamAssassin 3.1.7 (2006-10-05) on cicely12.cicely.de Cc: freebsd-current@freebsd.org, ticso@cicely.de, Ivan Voras Subject: Re: ZFS melting under postgres... X-BeenThere: freebsd-current@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list Reply-To: ticso@cicely.de List-Id: Discussions about the use of FreeBSD-current List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Sun, 16 Dec 2007 17:22:34 -0000 On Sun, Dec 16, 2007 at 08:40:18PM +1100, Darren Reed wrote: > Bernd Walter wrote: > ... > > One problem is with the data blocks beeing that big, when writing > > 512 Byte you effectifly do a read-modify-write of a larger physical > > block. > > This can be handled quite well with larger FS block. > > The much bigger problem is with power loss when writing such a > > maintenence block. > > You loose a very large area of logical blocks when this fails, > > since a 4k maintenence block contains the allocation for several hundert > > kB of logical data blocks. > > In other words - you possibly loose data blocks that were not written > > a long time and the database wouldn't expect a problem with that data. > > Even for ZIL it is very questionable if you loose a large data area, > > since the purpose is to have the data that was already sinced readable > > after a power loss. > ... > > ZFS doesn't suffer from this problem because the design > is to always write a new section of data rather than > over write "current" data. You missed the point: The filesystem doesn't overwrite written data, but the media does internaly to manage itself. You can loose data which hasn't beeen writen at all, since there is a large dependency chain with flash media. > So if you lose power in the middle of a write to a data > block, there is no damage to the old data. Yes there is, because that's the way flash media works. You write block x and if something goes wrong y is unreadable as well. And those dependency areas are very hughe. -- B.Walter http://www.bwct.de http://www.fizon.de bernd@bwct.de info@bwct.de support@fizon.de