linux-kernel - Re: document ext3 requirements

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Message-ID: <20090105093817.GA27199@atrey.karlin.mff.cuni.cz>
Date:	Mon, 5 Jan 2009 10:38:17 +0100
From:	Pavel Machek <pavel@...e.cz>
To:	david@...g.hm
Cc:	Rob Landley <rob@...dley.net>,
	kernel list <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...l.org>, tytso@....edu,
	mtk.manpages@...il.com, rdunlap@...otime.net,
	linux-doc@...r.kernel.org
Subject: Re: document ext3 requirements


> >Sector writes are atomic (ATOMIC-SECTORS)
> >~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >
> >Either whole sector is correctly written or nothing is written during
> >powerfail.
> >
> >       Unfortuantely, none of the cheap USB/SD flash cards I seen do
> >       behave like this, and are unsuitable for all linux filesystems
> >       I know.
> >
> >               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, filesystem
> >               won't notice that data in the "sectors" _after_ the
> >               one your were trying to write to got trashed.
> 
> around, not after. the block you are reading could be in the middle or at 
> the end of an eraseblock.

Applied, thanks.

-- 
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