From owner-freebsd-fs@FreeBSD.ORG Fri Jan 1 22:39:54 2010 Return-Path: Delivered-To: freebsd-fs@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34]) by hub.freebsd.org (Postfix) with ESMTP id 369FD106566B for ; Fri, 1 Jan 2010 22:39:54 +0000 (UTC) (envelope-from ticso@cicely7.cicely.de) Received: from raven.bwct.de (raven.bwct.de [85.159.14.73]) by mx1.freebsd.org (Postfix) with ESMTP id 975728FC12 for ; Fri, 1 Jan 2010 22:39:51 +0000 (UTC) Received: from cicely5.cicely.de ([10.1.1.7]) by raven.bwct.de (8.13.4/8.13.4) with ESMTP id o01Mdb3r078195 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=OK); Fri, 1 Jan 2010 23:39:37 +0100 (CET) (envelope-from ticso@cicely7.cicely.de) Received: from cicely7.cicely.de (cicely7.cicely.de [10.1.1.9]) by cicely5.cicely.de (8.14.2/8.14.2) with ESMTP id o01MdTpc059356 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=NO); Fri, 1 Jan 2010 23:39:29 +0100 (CET) (envelope-from ticso@cicely7.cicely.de) Received: from cicely7.cicely.de (localhost [127.0.0.1]) by cicely7.cicely.de (8.14.2/8.14.2) with ESMTP id o01MdPeK023862; Fri, 1 Jan 2010 23:39:29 +0100 (CET) (envelope-from ticso@cicely7.cicely.de) Received: (from ticso@localhost) by cicely7.cicely.de (8.14.2/8.14.2/Submit) id o01Md82i023861; Fri, 1 Jan 2010 23:39:08 +0100 (CET) (envelope-from ticso) Date: Fri, 1 Jan 2010 23:39:08 +0100 From: Bernd Walter To: Bob Friesenhahn Message-ID: <20100101223907.GX43739@cicely7.cicely.de> References: <55389.88569.qm@web112405.mail.gq1.yahoo.com> <20100101204752.GW43739@cicely7.cicely.de> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Operating-System: FreeBSD cicely7.cicely.de 7.0-STABLE i386 User-Agent: Mutt/1.5.11 X-Spam-Status: No, score=-2.3 required=5.0 tests=ALL_TRUSTED=-1.8, AWL=-1.071, BAYES_00=-2.599, FH_DATE_PAST_20XX=3.188 autolearn=no version=3.2.5 X-Spam-Checker-Version: SpamAssassin 3.2.5 (2008-06-10) on spamd.cicely.de Cc: "freebsd-fs@freebsd.org" , "ticso@cicely.de" Subject: Re: ZFS RaidZ2 with 24 drives? X-BeenThere: freebsd-fs@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list Reply-To: ticso@cicely.de List-Id: Filesystems List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Fri, 01 Jan 2010 22:39:54 -0000 On Fri, Jan 01, 2010 at 03:45:19PM -0600, Bob Friesenhahn wrote: > On Fri, 1 Jan 2010, Bernd Walter wrote: > > > >Everyone do this if the board dies and needs replacement. > >Not willingly, but it happens. > >And what about zfs export - relocate disks to another machine - and > >zfs import - without halt? > >It is less safe if a cache flush won't flush its cache. > >The real purpose to have buffered cache is to handle asyncronity in > >RAID systems after power failure, but RAIDZ won't have this problem > >by design, at least if running with CRC enabled. > > A proper write-through cache should automatically commit itself (in > order) to backing store within a second or two. Other than cache > designs which are not "proper" (which we should not use) the main > concern is if the system loses power or crashes while it is producing > a significant write load so that there is uncomitted data in cache. There are many possible reasons why this won't happen. One of them is a simple write failure, which can't be reported back to the filesystem, because not even a cache flush fails. Yes - the risk might be tolerable for many people and I don't think it is very high. The main problem I see is that such controllers won't tell about their strategy, so you are left in the dark. > ZFS is not particularly more likely to lose user data, but it is much > more likely to detect and report loss since most other filesystems > don't even check, or even have a way to check. Agreed. And ZFS can win a lot from fast flushes. -- B.Walter http://www.bwct.de Modbus/TCP Ethernet I/O Baugruppen, ARM basierte FreeBSD Rechner uvm.