From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1750851AbYGIQ10 (ORCPT ); Wed, 9 Jul 2008 12:27:26 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751201AbYGIQ1S (ORCPT ); Wed, 9 Jul 2008 12:27:18 -0400 Received: from www.church-of-our-saviour.org ([69.25.196.31]:43695 "EHLO thunker.thunk.org" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751194AbYGIQ1R (ORCPT ); Wed, 9 Jul 2008 12:27:17 -0400 Date: Wed, 9 Jul 2008 12:27:15 -0400 From: Theodore Tso To: Jan Willem van den Brand Cc: linux-kernel@vger.kernel.org Subject: Re: [RFC] ext3/jbd, kernel 2.6.13, make ext3 mountable as ext2 when journal is empty. Message-ID: <20080709162715.GC11797@mit.edu> Mail-Followup-To: Theodore Tso , Jan Willem van den Brand , linux-kernel@vger.kernel.org References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.17+20080114 (2008-01-14) X-SA-Exim-Connect-IP: X-SA-Exim-Mail-From: tytso@mit.edu X-SA-Exim-Scanned: No (on thunker.thunk.org); SAEximRunCond expanded to false Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jul 09, 2008 at 05:24:25PM +0200, Jan Willem van den Brand wrote: > This patch makes ext3 mountable as ext2 even in case of a power > failure while mounted as ext3. We have tested it for kernel 2.6.13 but > it should be fairly easy to get it to work for other versions. What's the rationale for doing this. Why is it *useful* to have the filesystem be mountable as ext2 in case of a power failure? The whole point of ext3 is to be able to keep the filesystem consistent in case of a power failure. So the patch as given would never go into the kernel as-is, and in the general case it is totally counterproductive. Maybe it could go in as a optional behaviour enabled by a mount option, but that's assuming we can be convinced it's a good idea. > In case of a power failure, the INCOMPAT flag is not reset. Systems > that suffer from frequent power failure (e.g. SD-cards that are > unsafely removed) will often not be mountable as ext2. So what? Why can't you just run the journal and check the filesystem for consistency? If the user did a hot-eject while the SD-card was being written, even with your patch there is no guarantee that the card will even be readable. Some SD-cards go totally non-functional due to corruption at the flash translation layer when they are yanked in the middle of an update.... > Obviously, this solution will result in poor performance when many > small files are frequently closed after write but that is not the case > in our system (TomTom navigation device). How often does your TomTom device need to update files? A better (userspace-only) solution might be keep the filesystem mounted read-only, and when you need to write to the filesystem, turn on the LED (which hopefully will be a hint to the users to keep their grubby little paws off the eject button), remount it read/write, do your file writes, then remount it read/only, and turn off the LED. Regards, - Ted