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

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Message-ID: <20090826025849.GF32712@mit.edu>
Date:	Tue, 25 Aug 2009 22:58:49 -0400
From:	Theodore Tso <tytso@....edu>
To:	Ric Wheeler <rwheeler@...hat.com>
Cc:	Pavel Machek <pavel@....cz>, Florian Weimer <fweimer@....de>,
	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,
	rdunlap@...otime.net, linux-doc@...r.kernel.org,
	linux-ext4@...r.kernel.org, corbet@....net
Subject: Re: [patch] ext2/3: document conditions when reliable operation is
	possible

On Tue, Aug 25, 2009 at 09:15:00PM -0400, Ric Wheeler wrote:
>
> I agree with the whole write up outside of the above - degraded RAID  
> does meet this requirement unless you have a second (or third, counting  
> the split write) failure during the rebuild.

The argument is that if the degraded RAID array is running in this
state for a long time, and the power fails while the software RAID is
in the middle of writing out a stripe, such that the stripe isn't
completely written out, we could lose all of the data in that stripe.

In other words, a power failure in the middle of writing out a stripe
in a degraded RAID array counts as a second failure.

To me, this isn't a particularly interesting or newsworthy point,
since a competent system administrator who cares about his data and/or
his hardware will (a) have a UPS, and (b) be running with a hot spare
and/or will imediately replace a failed drive in a RAID array.

       	    	       	       	 	      - Ted
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