linux-kernel - Re: document ext3 requirements

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Message-ID: <20090104222522.GA1913@elf.ucw.cz>
Date:	Sun, 4 Jan 2009 23:25:22 +0100
From:	Pavel Machek <pavel@...e.cz>
To:	Theodore Tso <tytso@....edu>, Rob Landley <rob@...dley.net>,
	kernel list <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...l.org>, mtk.manpages@...il.com,
	rdunlap@...otime.net, linux-doc@...r.kernel.org
Subject: Re: document ext3 requirements

On Sun 2009-01-04 17:06:34, Theodore Tso wrote:
> On Sun, Jan 04, 2009 at 01:49:49PM -0600, Rob Landley wrote:
> > 
> > Want to document the granularity issues with flash, while you're at it?
> > 
> > An inherent problem with using flash as a normal block device is that the 
> > flash erase size is bigger than most filesystem sector sizes.  So when you 
> > request a write, it may erase and rewrite the next 64k, 128k, or even a couple 
> > megabytes on the really _big_ ones.
> > 
> > If you lose power in the middle of that, ext3 won't notice that data in the 
> > "sectors" _after_ the one your were trying to write to got trashed.
> 
> True enough, although the newer SSD's will have this problem addressed
> (although at least initially, they are **far** more costly than the
> el-cheapo 32GB SD cards you can find at the checkout counter at Fry's
> alongside battery-powered shavers and trashy ipod speakers).
> 
> I will stress again, that most of this doesn't belong in
> Documentation/filesystems/ext3.txt, as most of this is *not*
> ext3-specific.

I've initially done the patch for ext3 because that's what I'm using
and becuase I felt responsible for documenting it after a huge thread.

At least barrier=1 seems to be ext3 specific, and perhaps logfs or
something can survive full eraseblocks disappearing. Anyway, i guess
we all agree that this needs to be documented _somewhere_, and that's
what I'm trying to do.
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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