From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from cuda.sgi.com (cuda3.sgi.com [192.48.176.15]) by oss.sgi.com (8.14.3/8.14.3/SuSE Linux 0.8) with ESMTP id n14FY879159535 for ; Wed, 4 Feb 2009 09:34:08 -0600 Received: from theco.de (localhost [127.0.0.1]) by cuda.sgi.com (Spam Firewall) with ESMTP id 7853618C1B62 for ; Wed, 4 Feb 2009 07:33:25 -0800 (PST) Received: from theco.de (scout.theco.de.mind.de [212.42.230.55]) by cuda.sgi.com with ESMTP id vc8xWwfknJLKQsGt for ; Wed, 04 Feb 2009 07:33:25 -0800 (PST) Date: Wed, 4 Feb 2009 16:33:22 +0100 From: Ralf Liebenow Subject: Re: xfs_force_shutdown after Raid crash Message-ID: <20090204153322.GC15454@theco.de> References: <498376CF.8020806@renderforce.de> <200902031140.08576@zmi.at> <20090203154907.GA21278@infradead.org> <200902040952.45440@zmi.at> <20090204122241.GL24173@disturbed> Mime-Version: 1.0 Content-Disposition: inline In-Reply-To: <20090204122241.GL24173@disturbed> Reply-To: ralf@theco.de List-Id: XFS Filesystem from SGI List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Sender: xfs-bounces@oss.sgi.com Errors-To: xfs-bounces@oss.sgi.com To: xfs@oss.sgi.com Hello ! Maybe this is a stupid question: Should Battery backed RAID controllers not always set their discs cache off= ? As I see it (in case of a power failure): - the discs are connectet to the main power, so if there is a power failure they're offline at that moment in time and their (write) cache = will be gone in that instance of time too = - if they are connected to a battery backed RAID cache, I assume that this cache will be written as soon as the system is online again (if the battery lasts that long) - if a RAID controller does not turn off the disks write cache, the contr= oller cannot know if previous writes have made it to the disk. A good RAID Controller would also use its cache to re-organise the disc = writes to minimize seek times doing somthing like intelligent command queuing. This would also mean, that any order of writes to a disk could have been changed by the controller. This would ultimately break any filesystem which does not explicitly fsyncing consistent checkpoints to the disk, which would make battery backed RAID Systems pretty useless ... would it ? = So .. a battery backed RAID controller should default to "no disk write= cache" should it ? Otherwise why should anyone want to use such expensive controllers ... it just does not make sense to have a battery backed cache on the controller, when things get inconsistent at a power outage ... It wouldn't have any purpuse ... I hope developers of battery backed RAID controllers are aware of that implication ... Greets Ralf > On Wed, Feb 04, 2009 at 09:52:45AM +0100, Michael Monnerie wrote: > > On Dienstag 03 Februar 2009 Christoph Hellwig wrote: > > > Yeah, that sounds correct. =A0Do you volunteer for the FAQ entry? > > > =A0xfs.org is a wiki so you could add it. =A0I'm happy to proof-read = it > > > if you want. > > = > > I don't know if it's good and correct, I just put this in the wiki, and = > > additionally changed 2 sections, please check the wiki log if it's = > > correct: > > = > > =3D=3D Q. What about the hard disk write cache? =3D=3D > > = > > The problem with hard disk write caches is that their contents are lost = > > in case of a power outage. With hard disk cache sizes of currently up t= o = > > 32MB that can be a lot of valuable information. > > = > > With a single hard disk and barriers turned on (on=3Ddefault), a powerf= ail = > > "only" looses data in the cache but at least does not destroy the = > > filesystem. > = > I'd drop this paragraph - powerfail can destroy filesystems even on > a single disk (e.g. root directory gets corrupted). > = > > With a RAID controller with battery backed cache, you should turn off = > > barriers, as recommended above. But then you *must* disable the hard = > > disk write cache in order to ensure to keep the filesystem intact after = > > a power failure. > = > I'd change this to say "*must* disable the individual hard disk > write caches" to make it clear that it is referencing the disks > behind the raid controller. I'd also say "The method for doing this > is different for each RAID controller. Please consult your RAID > controller documentation to determine how to change these settings." > = > Cheers, > = > Dave. > -- = > Dave Chinner > david@fromorbit.com > = > _______________________________________________ > xfs mailing list > xfs@oss.sgi.com > http://oss.sgi.com/mailman/listinfo/xfs > = -- = theCode AG = HRB 78053, Amtsgericht Charlottenbg USt-IdNr.: DE204114808 Vorstand: Ralf Liebenow, Michael Oesterreich, Peter Witzel Aufsichtsratsvorsitzender: Wolf von Jaduczynski Oranienstr. 10-11, 10997 Berlin [=D7] fon +49 30 617 897-0 fax -10 ralf@theCo.de http://www.theCo.de _______________________________________________ xfs mailing list xfs@oss.sgi.com http://oss.sgi.com/mailman/listinfo/xfs