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Date: Mon, 16 Mar 2009 22:48:25 +0100
From: Pavel Machek <pavel@ucw.cz>
To: Greg Freemyer <greg.freemyer@gmail.com>
Cc: kernel list <linux-kernel@vger.kernel.org>, Andrew Morton <akpm@osdl.org>,
       mtk.manpages@gmail.com, tytso@mit.edu, rdunlap@xenotime.net,
       linux-doc@vger.kernel.org, linux-ext4@vger.kernel.org
Subject: Re: ext2/3: document conditions when reliable operation is possible
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On Mon 2009-03-16 15:45:36, Greg Freemyer wrote:
> On Thu, Mar 12, 2009 at 5:21 AM, Pavel Machek <pavel@ucw.cz> wrote:
> <snip>
> > +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" _around_ the
> > + ? ? ? ? ? ? ? one your were trying to write to got trashed.
> 
> I had *assumed* that SSDs worked like:
> 
> 1) write request comes in
> 2) new unused erase block area marked to hold the new data
> 3) updated data written to the previously unused erase block
> 4) mapping updated to replace the old erase block with the new one
> 
> If it were done that way, a failure in the middle would just leave the
> SSD with the old data in it.

The really expensive ones (Intel SSD) apparently work like that, but I
never seen one of those. USB sticks and SD cards I tried behave like I
described above.
								Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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