From: Eric Sandeen Subject: Re: xt4 - True Readonly mount [WAS - Re: [Bug 14354] Bad corruption with 2.6.32-rc1 and upwards] Date: Mon, 02 Nov 2009 17:02:05 -0600 Message-ID: <4AEF64ED.1040703@redhat.com> References: <87f94c370910300720s5ea3d780o45fcf32303820a3c@mail.gmail.com> <20091101054542.GP18464@mit.edu> <87f94c370911021359x19f57427o19e6f7bfcfadae21@mail.gmail.com> <42D1C316-13BF-401C-BB2F-A3DF62996A54@sun.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: Greg Freemyer , Theodore Tso , Ted Augustine , Alexey Fisher , linux-ext4@vger.kernel.org To: Andreas Dilger Return-path: Received: from mx1.redhat.com ([209.132.183.28]:55602 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757176AbZKBXCN (ORCPT ); Mon, 2 Nov 2009 18:02:13 -0500 In-Reply-To: <42D1C316-13BF-401C-BB2F-A3DF62996A54@sun.com> Sender: linux-ext4-owner@vger.kernel.org List-ID: Andreas Dilger wrote: > On 2009-11-02, at 14:59, Greg Freemyer wrote: >> 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 thought Takashi Sato was working on allowing a filesystem freeze > ioctl from userspace? This would hook into the filesystem-specific > freeze code so that when the ioctl() returns the on-disk filesystem > is fully consistent and does not even require journal replay. That's in and done; most recent xfsprogs' xfs_freeze utility will even freeze non-xfs filesystems now :) Otherwise a wrapper utility around the ioctl would be trivial to write. >> 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. these days a frozen xfs fs should be consistent & not need replay. -Eric >> ext4 may already support mounting of readonly clones, but if not it >> needs to before it will qualify as a data center ready filesystem.