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

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Date:	Wed, 26 Aug 2009 06:39:14 -0400
From:	Ric Wheeler <rwheeler@...hat.com>
To:	Theodore Tso <tytso@....edu>, 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 08/25/2009 10:58 PM, Theodore Tso wrote:
> 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
>    

I agree that this is not an interesting (or likely) scenario, certainly 
when compared to the much more frequent failures that RAID will protect 
against which is why I object to the document as Pavel suggested. It 
will steer people away from using RAID and directly increase their 
chances of losing their data if they use just a single disk.

Ric
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