linux-kernel - Re: [Patch] document ext3 requirements (was Re: [RFD] Incremental fsck)

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Message-ID: <20080115214325.GN155407@sgi.com>
Date:	Wed, 16 Jan 2008 08:43:25 +1100
From:	David Chinner <dgc@....com>
To:	Pavel Machek <pavel@....cz>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>, Theodore Tso <tytso@....edu>,
	Al Boldi <a1426z@...ab.com>,
	Valerie Henson <val.henson@...il.com>,
	Rik van Riel <riel@...hat.com>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [Patch] document ext3 requirements (was Re: [RFD] Incremental fsck)

On Tue, Jan 15, 2008 at 09:16:53PM +0100, Pavel Machek wrote:
> Hi!
> 
> > > What are ext3 expectations of disk (is there doc somewhere)? For
> > > example... if disk does not lie, but powerfail during write damages
> > > the sector -- is ext3 still going to work properly?
> > 
> > Nope. However the few disks that did this rapidly got firmware updates
> > because there are other OS's that can't cope.
> > 
> > > If disk does not lie, but powerfail during write may cause random
> > > numbers to be returned on read -- can fsck handle that?
> > 
> > most of the time. and fsck knows about writing sectors to remove read
> > errors in metadata blocks.
> > 
> > > What abou disk that kills 5 sectors around sector being written during
> > > powerfail; can ext3 survive that?
> > 
> > generally. Note btw that for added fun there is nothing that guarantees
> > the blocks around a block on the media are sequentially numbered. The
> > usually are but you never know.
> 
> Ok, should something like this be added to the documentation?
> 
> It would be cool to be able to include few examples (modern SATA disks
> support bariers so are safe, any IDE from 1989 is unsafe), but I do
> not know enough about hw...

ext3 is not the only filesystem that will have trouble due to
volatile write caches. We see problems often enough with XFS
due to volatile write caches that it's in our FAQ:

http://oss.sgi.com/projects/xfs/faq.html#wcache

Cheers,

Dave.
-- 
Dave Chinner
Principal Engineer
SGI Australian Software Group
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