From: Ric Wheeler Subject: Re: [testcase] test your fs/storage stack (was Re: [patch] ext2/3: document conditions when reliable operation is possible) Date: Sat, 29 Aug 2009 07:28:21 -0400 Message-ID: <4A9910D5.4060208@redhat.com> References: <20090825235359.GJ4300@elf.ucw.cz> <4A947DA9.2080906@redhat.com> <20090826001645.GN4300@elf.ucw.cz> <4A948259.40007@redhat.com> <20090826010018.GA17684@mit.edu> <4A948C94.7040103@redhat.com> <20090826025849.GF32712@mit.edu> <4A9510D2.1090704@redhat.com> <20090826111208.GA26595@elf.ucw.cz> <20090829094919.GF1634@ucw.cz> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: david@lang.hm, Theodore Tso , Florian Weimer , Goswin von Brederlow , Rob Landley , 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: Pavel Machek Return-path: In-Reply-To: <20090829094919.GF1634@ucw.cz> Sender: linux-doc-owner@vger.kernel.org List-Id: linux-ext4.vger.kernel.org On 08/29/2009 05:49 AM, Pavel Machek wrote: > >>> So instead of fixing or at least documenting known software deficiency >>> in Linux MD stack, you'll try to surpress that information so that >>> people use more of raid5 setups? >>> >>> Perhaps the better documentation will push them to RAID1, or maybe >>> make them buy an UPS? >>> >> people aren't objecting to better documentation, they are objecting to >> misleading documentation. >> > Actually Ric is. He's trying hard to make RAID5 look better than it > really is. > > > I object to misleading and dangerous documentation that you have proposed. I spend a lot of time working in data integrity, talking and writing about it so I care deeply that we don't misinform people. In this thread, I put out a draft that is accurate several times and you have failed to respond to it. The big picture that you don't agree with is: (1) RAID (specifically MD RAID) will dramatically improve data integrity for real users. This is not a statement of opinion, this is a statement of fact that has been shown to be true in large scale deployments with commodity hardware. (2) RAID5 protects you against a single failure and your test case purposely injects a double failure. (3) How to configure MD reliably should be documented in MD documentation, not in each possible FS or raw device application (4) Data loss occurs in non-journalling file systems and journalling file systems when you suffer double failures or hot unplug storage, especially inexpensive FLASH parts. ric