linux-ext4 - Re: [patch] ext2/3: document conditions when reliable operation is possible

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Message-ID: <20090824184421.GC28403@elf.ucw.cz>
Date:	Mon, 24 Aug 2009 20:44:21 +0200
From:	Pavel Machek <pavel@....cz>
To:	Greg Freemyer <greg.freemyer@...il.com>
Cc:	Goswin von Brederlow <goswin-v-b@....de>,
	Rob Landley <rob@...dley.net>,
	kernel list <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...l.org>, mtk.manpages@...il.com,
	tytso@....edu, rdunlap@...otime.net, linux-doc@...r.kernel.org,
	linux-ext4@...r.kernel.org
Subject: Re: [patch] ext2/3: document conditions when reliable operation is
	possible


> > +Either whole sector is correctly written or nothing is written during
> > +powerfail.
> > +
> > +       Because RAM tends to fail faster than rest of system during
> > +       powerfail, special hw killing DMA transfers may be necessary;
> > +       otherwise, disks may write garbage during powerfail.
> > +       This may be quite common on generic PC machines.
> > +
> > +       Note that atomic write is very hard to guarantee for RAID-4/5/6,
> > +       because it needs to write both changed data, and parity, to
> > +       different disks. (But it will only really show up in degraded mode).
> > +       UPS for RAID array should help.
> 
> Can someone clarify if this is true in raid-6 with just a single disk
> failure?  I don't see why it would be.
> 
> And if not can the above text be changed to reflect raid 4/5 with a
> single disk failure and raid 6 with a double disk failure are the
> modes that have atomicity problems.

I don't know enough about raid-6, but... I said "degraded mode" above,
and you can read it as double failure in raid-6 case ;-). I'll prefer
to avoid too many details here.

-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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