From: Eric Sandeen Subject: Re: Mentor for a GSoC application wanted (Online ext2/3 filesystem checker) Date: Sat, 19 Apr 2008 20:24:55 -0500 Message-ID: <480A9B67.2050200@redhat.com> References: <20080419012952.GE25797@mit.edu> <20080419185603.GA30449@mit.edu> <480A42F6.2030005@redhat.com> <20080419220432.GB30449@mit.edu> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Cc: Alexey Zaytsev , linux-ext4@vger.kernel.org, linux-fsdevel@vger.kernel.org, Rik van Riel To: Theodore Tso Return-path: Received: from mx1.redhat.com ([66.187.233.31]:38864 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750863AbYDTB1F (ORCPT ); Sat, 19 Apr 2008 21:27:05 -0400 In-Reply-To: <20080419220432.GB30449@mit.edu> Sender: linux-ext4-owner@vger.kernel.org List-ID: Theodore Tso wrote: > On Sat, Apr 19, 2008 at 02:07:34PM -0500, Eric Sandeen wrote: >> If you really just want to verify a snapshot of the fs at a point in >> time, surely there are simpler ways. If the device is on lvm, there's >> already a script floating around to do it in automated fasion. (I'd >> pondered the idea of introducing META_WRITE (to go with META_READ) and >> maybe lvm could do a "metadata-only" snapshot to be lighter weight?) > > That would be great, although I think the major issue is not > necessarily the performance problems of using an LVM snapshot on a > very busy filesystem well, backing space for the snapshot could be an issue too. Basically, if you're only using it for this purpose, why COW all the post-snapshot data if you just don't care... > (althouh I could imagine for some users this > might be an issue), but rather for filesystem devices that aren't > using LVM at all. (I've heard some complaints that LVM imposes a > performance penalty even if you aren't using a snapshot; has anyone > done any benchmarks of a filesystem with and without LVM to see > whether or not there really is a significant performance penalty; > whether or not there really is one, the perception is definitely out > there that it does.) I've heard from someone who did some testing about a minor penalty, but I can't point to any published test so I guess that's just more hearsay. It's intuitive that putting lvm on top of a block device might not be absolutely, 100% free, though.... Adds to stack, too. > If we could do a lightweight snapshot that didn't require an LVM, that > would be really great. But that's probably not an ext4 project, and > I'm not sure the it would be considered politically correct in the > LKML community. Yep; my original reply originally wished something about non-lvm snapshots but... while yes, it'd be nice for this purpose, ponies for everyone would be nice too... :) But I didn't mention it because... how do you do a generic non-lvm snapshot of, say, /dev/sda3 without some sort of volume manager...? If there's some clever idea that could be implemented cleanly, I'd be all ears. :) -Eric