Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757930AbZCYTxf (ORCPT ); Wed, 25 Mar 2009 15:53:35 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751949AbZCYTxX (ORCPT ); Wed, 25 Mar 2009 15:53:23 -0400 Received: from mx2.redhat.com ([66.187.237.31]:37538 "EHLO mx2.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750771AbZCYTxW (ORCPT ); Wed, 25 Mar 2009 15:53:22 -0400 Message-ID: <49CA8ADA.3040709@redhat.com> Date: Wed, 25 Mar 2009 15:49:46 -0400 From: Ric Wheeler User-Agent: Thunderbird 2.0.0.21 (X11/20090320) MIME-Version: 1.0 To: Jens Axboe CC: Jeff Garzik , Linus Torvalds , Theodore Tso , Ingo Molnar , Alan Cox , Arjan van de Ven , Andrew Morton , Peter Zijlstra , Nick Piggin , David Rees , Jesper Krogh , Linux Kernel Mailing List Subject: Re: Linux 2.6.29 References: <20090324093245.GA22483@elte.hu> <20090324101011.6555a0b9@lxorguk.ukuu.org.uk> <20090324103111.GA26691@elte.hu> <20090324132032.GK5814@mit.edu> <20090324184549.GE32307@mit.edu> <49C93AB0.6070300@garzik.org> <20090325093913.GJ27476@kernel.dk> <49CA86BD.6060205@garzik.org> <20090325194341.GB27476@kernel.dk> In-Reply-To: <20090325194341.GB27476@kernel.dk> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3965 Lines: 100 Jens Axboe wrote: > On Wed, Mar 25 2009, Jeff Garzik wrote: > >> Jens Axboe wrote: >> >>> On Tue, Mar 24 2009, Jeff Garzik wrote: >>> >>>> Linus Torvalds wrote: >>>> >>>>> But I really don't understand filesystem people who think that >>>>> "fsck" is the important part, regardless of whether the data is >>>>> valid or not. That's just stupid and _obviously_ bogus. >>>>> >>>> I think I can understand that point of view, at least: >>>> >>>> More customers complain about hours-long fsck times than they do >>>> about silent data corruption of non-fsync'd files. >>>> >>>> >>>> >>>>> The point is, if you write your metadata earlier (say, every 5 sec) >>>>> and the real data later (say, every 30 sec), you're actually MORE >>>>> LIKELY to see corrupt files than if you try to write them together. >>>>> >>>>> And if you write your data _first_, you're never going to see >>>>> corruption at all. >>>>> >>>> Amen. >>>> >>>> And, personal filesystem pet peeve: please encourage proper FLUSH >>>> CACHE use to give users the data guarantees they deserve. Linux's >>>> sync(2) and fsync(2) (and fdatasync, etc.) should poke the block >>>> layer to guarantee a media write. >>>> >>> fsync already does that, at least if you have barriers enabled on your >>> drive. >>> >> Erm, no, you don't enable barriers on your drive, they are not a >> hardware feature. You enable barriers via your filesystem. >> > > Thanks for the lesson Jeff, I'm obviously not aware how that stuff > works... > > >> Stating "fsync already does that" borders on false, because that assumes >> (a) the user has a fs that supports barriers >> (b) the user is actually aware of a 'barriers' mount option and what it >> means >> (c) the user has turned on an option normally defaulted to off. >> >> Or in other words, it pretty much never happens. >> > > That is true, except if you use xfs/ext4. And this discussion is fine, > as was the one a few months back that got ext4 to enable barriers by > default. If I had submitted patches to do that back in 2001/2 when the > barrier stuff was written, I would have been shot for introducing such a > slow down. After people found out that it just wasn't something silly, > then you have a way to enable it. > > I'd still wager that most people would rather have a 'good enough > fsync' on their desktops than incur the penalty of barriers or write > through caching. I know I do. > > >> Furthermore, a blatantly obvious place to flush data to media -- >> fsync(2), fdatasync(2) and sync_file_range(2) -- should cause the block >> layer to issue a FLUSH CACHE for __any__ filesystem. But that doesn't >> happen either. >> >> So, no, for 95% of Linux users, fsync does _not_ already do that. If >> you are lucky enough to use XFS or ext4, you're covered. That's it. >> > > The point is that you need to expose this choice somewhere, and that > 'somewhere' isn't manually editing fstab and enabling barriers or > fsync-for-real. And it should be easier. > > Another problem is that FLUSH_CACHE sucks. Really. And not just on > ext3/ordered, generally. Write a 50 byte file, fsync, flush cache and > wit for the world to finish. Pretty hard to teach people to use a nicer > fdatasync(), when the majority of the cost now becomes flushing the > cache of that 1TB drive you happen to have 8 partitions on. Good luck > with that. > > And, as I am sure that you do know, to add insult to injury, FLUSH_CACHE is per device (not file system). When you issue an fsync() on a disk with multiple partitions, you will flush the data for all of its partitions from the write cache.... ric -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/