From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1750831AbWAVGno (ORCPT ); Sun, 22 Jan 2006 01:43:44 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1750760AbWAVGno (ORCPT ); Sun, 22 Jan 2006 01:43:44 -0500 Received: from rwcrmhc11.comcast.net ([204.127.198.35]:20460 "EHLO rwcrmhc11.comcast.net") by vger.kernel.org with ESMTP id S1750831AbWAVGno (ORCPT ); Sun, 22 Jan 2006 01:43:44 -0500 Message-ID: <43D3295E.8040702@comcast.net> Date: Sun, 22 Jan 2006 01:42:38 -0500 From: John Richard Moser User-Agent: Mozilla Thunderbird 1.0.7 (X11/20051013) X-Accept-Language: en-us, en MIME-Version: 1.0 To: linux-kernel@vger.kernel.org Subject: soft update vs journaling? X-Enigmail-Version: 0.92.1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 So I've been researching, because I thought this "Soft Update" thing that BSD uses was some weird freak-ass way to totally corrupt a file system if the power drops. Seems I was wrong; it's actually just the opposite, an alternate solution to journaling. So let's compare notes. I'm not quite clear on what the benefits versus costs of Soft Update versus Journaling are, so I'll run down what I got, and anyone who wants to give input can run down on what they got, and we can compare. Maybe someone will write a Soft Update system into Linux one day, far, far into the future; but I doubt it. It might, however, be interesting to compare ext2 + SU to ext3; and giving the chance to solve problems such as delayed delete (i.e. file system fills up while soft update has not yet executed a delete; try reacting by looking for a delete to suddenly actually execute) might also be cool. Soft Update appears to buffer and order meta-data writes in a dependency scheme that makes certain that inconsistencies can't happen. Apparently this means writing up directory entries before inodes, or something to that effect. I can't see how this would help in the middle of a buffer flush (half a dentry written? Partially deleted inode? Inode "deleted" but not freed from disk?), so maybe someone can fill me in. Journaling apparently means writing out meta-data to a log before transferring it to the file system. No matter what happens, a proper journal (for fun I've designed a transaction log format for low level filesystems; it's entirely possible to have interrupt at any bit recoverable) can always be checked over and either rolled back or rolled forward. This is easy to design. Soft Update appears to have the advantage of not needing multiple writes. There's no need for journal flushing and then disk flushing; you just flush the meta-data. Also, soft update systems mount instantly, because there's no journal to play back, and the file system is always consistent. It may be technically feasible to impliment soft update on any old file system; I'm unclear as to how exactly to make any soft-update work, so I can't say if this is absolutely possible (think for vfat, consistent at all times and still Win32 compatible; great for flash drives). Unfortunately, soft update can leave retarded situations where areas of disk are allocated still after a system failure during an inode delete. This won't cause inconsistencies in the on-disk structure, however; you can freely use the disk without causing even more damage. The system just has to sanity check stuff while running and clean up such damage as it sees it. Journaling appears to have the advantage that the data gets to disk faster. It also seems easier a concept to grasp (i.e. I understand it fully). It's old, tried, trusted, and durable. You also don't have to worry about having odd meta-data writes that leave deleted files around in certain circumstances, eating up space. Unfortunately, journaling uses a chunk of space. Imagine a journal on a USB flash stick of 128M; a typical ReiserFS journal is 32 megabytes! Sure it could be done in 8 or 4 or so; or (in one of my file system designs) a static 16KiB block could reference dynamicly allocated journal space, allowing the system to sacrifice performance and shrink the journal when more space is needed. Either way, slow media like floppies will suffer, HARD; and flash devices will see a lot of write/erase all over the journal area, causing wear on that spot. So, that's my understanding. Any comments? Enlighten me. - -- All content of all messages exchanged herein are left in the Public Domain, unless otherwise explicitly stated. Creative brains are a valuable, limited resource. They shouldn't be wasted on re-inventing the wheel when there are so many fascinating new problems waiting out there. -- Eric Steven Raymond -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.2 (GNU/Linux) Comment: Using GnuPG with Thunderbird - http://enigmail.mozdev.org iD8DBQFD0yldhDd4aOud5P8RAhzBAJwOvWpAYb+m3Zg8ugnvuY10K74jZgCeL69s y0172JATNX+q8jzrYGAJ/xc= =7Dcn -----END PGP SIGNATURE-----