From: Greg Freemyer Subject: Re: xt4 - True Readonly mount [WAS - Re: [Bug 14354] Bad corruption with 2.6.32-rc1 and upwards] Date: Mon, 2 Nov 2009 16:59:18 -0500 Message-ID: <87f94c370911021359x19f57427o19e6f7bfcfadae21@mail.gmail.com> References: <87f94c370910300720s5ea3d780o45fcf32303820a3c@mail.gmail.com> <20091101054542.GP18464@mit.edu> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Cc: Ted Augustine , Alexey Fisher , linux-ext4@vger.kernel.org To: Theodore Tso Return-path: Received: from mail-iw0-f180.google.com ([209.85.223.180]:63979 "EHLO mail-iw0-f180.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756846AbZKBV7O (ORCPT ); Mon, 2 Nov 2009 16:59:14 -0500 Received: by iwn10 with SMTP id 10so3669945iwn.4 for ; Mon, 02 Nov 2009 13:59:18 -0800 (PST) In-Reply-To: <20091101054542.GP18464@mit.edu> Sender: linux-ext4-owner@vger.kernel.org List-ID: On Sun, Nov 1, 2009 at 12:45 AM, Theodore Tso wrote: > On Fri, Oct 30, 2009 at 10:20:35AM -0400, Greg Freemyer wrote: >> Ignoring computer forensics, with LVM snapshots, hardware raid array >> snapshots, etc. even in the presence of a dirty log, we need to be >> able to mount a drive in true read-only fashion fro many backup >> operations to function correctly. > > Can you go into more detail about "many backup operations"? One example is a hardware raid array that creates readonly snapshots or clones. (Lots of those exist in the real world). So the typical backup procedure is: ==== Queisce application (databases, etc. have utils to do this.) Queisce filesystem (xfs_freeze -f can be done from userspace. is there a ext4 util?) issue raid array command to create snapshot. release filesystem (xfs_freeze -u) release the app (util provided by app). Mount the snapshot readonly (true readonly with zero writes to the block device). Backup the readonly snapshot (to tape, etc.). === I believe XFS had 2 issues related to this process when first implemented in linux. 1) It required the UUID to be unique. Obviously in the above scenario it is not, so "mount -o nouuid" was added for xfs. 2) Journal replay was originally aways attempted in the above process, so the "mount -o norecovery" option was added to force a true readonly mount. ext4 may already support mounting of readonly clones, but if not it needs to before it will qualify as a data center ready filesystem. Greg -- Greg Freemyer Head of EDD Tape Extraction and Processing team Litigation Triage Solutions Specialist http://www.linkedin.com/in/gregfreemyer Preservation and Forensic processing of Exchange Repositories White Paper - The Norcross Group The Intersection of Evidence & Technology http://www.norcrossgroup.com