From: Rob Landley Subject: Re: [patch] ext2/3: document conditions when reliable operation is possible Date: Tue, 25 Aug 2009 13:52:09 -0500 Message-ID: <200908251352.11959.rob@landley.net> References: <20090312092114.GC6949@elf.ucw.cz> <20090824195159.GD29763@elf.ucw.cz> <4A92F6FC.4060907@redhat.com> Mime-Version: 1.0 Content-Type: Text/Plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit Cc: Pavel Machek , Theodore Tso , 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 To: Ric Wheeler Return-path: In-Reply-To: <4A92F6FC.4060907@redhat.com> Content-Disposition: inline Sender: linux-doc-owner@vger.kernel.org List-Id: linux-ext4.vger.kernel.org On Monday 24 August 2009 15:24:28 Ric Wheeler wrote: > Pavel Machek wrote: > > Actually, ext2 should be able to survive that, no? Error writing -> > > remount ro -> fsck on next boot -> drive relocates the sectors. > > I think that the example and the response are both off base. If your > head ever touches the platter, you won't be reading from a huge part of > your drive ever again It's not quite that simple anymore. These days, most modern drives add an "overcoat", which is a vapor deposition layer of carbon (I.E. diamond) on top of the magnetic media, and then add a nanolayer of some kind of nonmagnetic lubricant on top of that. That protects the magnetic layer from physical contact with the head; it takes a pretty solid whack to chip through diamond and actually gouge your disk: http://www.datarecoverylink.com/understanding_magnetic_media.html You can also do fun things with various nitridies (carbon nitride, silicon nitride, titanium nitride) which are pretty darn tough too, although I dunno about their suitability to hard drives: http://www.physical-vapor-deposition.com/ So while it _is_ possible to whack your drive and scratch the platter, merely "touching" won't do it. (Laptops wouldn't be feasible if they couldn't cope with a little jostling while running.) In the case of repeated small whacks, your heads may actually go first. (I vaguely recall the little aerofoil wing thingy holding up the disk touches first, and can get ground down by repeated contact with the diamond layer (despite the lubricant, that just buys time) so it gets shorter and shorter and can't reliably keep the head above the disk rather than in contact with it. But I'm kind of stale myself here, not sure that's still current.) Here's a nice youtube video of a 2007 defcon talk from a hard drive recovery professional, "What's that Clicking Noise", series starts here: http://www.youtube.com/watch?v=vCapEFNZAJ0 And here's that guy's web page: http://www.myharddrivedied.com/presentations/index.html Rob -- Latency is more important than throughput. It's that simple. - Linus Torvalds