From: david@lang.hm Subject: Re: raid is dangerous but that's secret (was Re: [patch] ext2/3: document conditions when reliable operation is possible) Date: Thu, 3 Sep 2009 07:35:14 -0700 (PDT) Message-ID: References: <20090828064449.GA27528@elf.ucw.cz> <20090828120854.GA8153@mit.edu> <20090830075135.GA1874@ucw.cz> <4A9A88B6.9050902@redhat.com> <4A9A9034.8000703@msgid.tls.msk.ru> <20090830163513.GA25899@infradead.org> <4A9BCCEF.7010402@redhat.com> <20090831131626.GA17325@infradead.org> <4A9BCDFE.50008@rtr.ca> <20090831132139.GA5425@infradead.org> <4A9F230F.40707@redhat.com> <4A9FA5F2.9090704@redhat.com> Mime-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII; format=flowed Cc: Ric Wheeler , Christoph Hellwig , Mark Lord , Michael Tokarev , Pavel Machek , Theodore Tso , NeilBrown , Rob Landley , Florian Weimer , Goswin von Brederlow , kernel list , Andrew Morton , mtk.manpages@gmail.com, rdunlap@xenotime.net, linux-doc@vger.kernel.org, linux-ext4@vger.kernel.org, corbet@lwn.net To: Krzysztof Halasa Return-path: Received: from mail.lang.hm ([64.81.33.126]:59470 "EHLO bifrost.lang.hm" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752964AbZICOgO (ORCPT ); Thu, 3 Sep 2009 10:36:14 -0400 In-Reply-To: Sender: linux-ext4-owner@vger.kernel.org List-ID: On Thu, 3 Sep 2009, Krzysztof Halasa wrote: > Ric Wheeler writes: > >>>> Just to add some support to this, all of the external RAID arrays that >>>> I know of normally run with write cache disabled on the component >>>> drives. >>> >>> Do they use "off the shelf" SATA (or PATA) disks, and if so, which ones? >> >> Which drives various vendors ships changes with specific products. >> Usually, they ship drives that have carefully vetted firmware, etc. >> but they are close to the same drives you buy on the open market. > > But they aren't the same, are they? If they are not, the fact they can > run well with the write-through cache doesn't mean the off-the-shelf > ones can do as well. frequently they are exactly the same drives, with exactly the same firmware. you disable the write caches on the drives themselves, but you add a large write cache (with battery backup) in the raid card/chassis > Are they SATA (or PATA) at all? SCSI etc. are usually different > animals, though there are SCSI and SATA models which differ only in > electronics. it depends on what raid array you use, some use SATA, some use SAS/SCSI David Lang